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Nanotechnology Reviews

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Volume 3, Issue 5

Issues

High-resolution electron microscopy and electron diffraction of perovskite-type superconducting copper oxides

Takeo Oku
  • Corresponding author
  • Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan
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Published Online: 2014-06-19 | DOI: https://doi.org/10.1515/ntrev-2014-0003

Abstract

High-resolution electron microscopy and electron diffraction are quite useful for structural characterization of perovskite-type superconducting copper oxides on the atomic scale. Valuable information on the crystal structures and the microstructures could be obtained from observed high-resolution images if severe conditions such as thinner crystals and definite defocus values are satisfied. The structure images and electron diffraction patterns include information not only on accurate atomic coordinates of cations but also on ordered arrangements of oxygen atoms and oxygen vacancies. Crystal structures of the various perovskite-type copper oxides were analyzed from the observed structure images. Modulated structures, defects, intergowth, surfaces, and interfaces were also investigated.

Keywords: copper oxide; perovskite; structure; superconductor; TEM

1 Introduction

Transmission electron microscopy (TEM) using high-resolution observation and electron diffraction has been applied for microstructural analysis for various advanced materials. Superconductors with high critical temperatures (Tc) based on the perovskite-type structure are suitable subjects for high-resolution electron microscopy (HREM) and electron diffraction, and many TEM studies of these materials have been carried out. In the early stage of high-Tc superconductor studies [1–6], high-resolution images of Y- [7], Bi- [8, 9], and Tl-based [10] superconductors have individual cations that are represented as separated dark spots [11, 12].

In the present paper, studies on the perovskite-type superconducting copper oxides by high-resolution electron microscopy and electron diffraction were reviewed. High-resolution structure images and selected electron diffraction patterns give us valuable information for crystal structure analysis [13, 14]. The main contents of the present paper are as follows. In the first part, the necessary experimental conditions to observe the high-resolution structure images, which can be used for crystal structure analysis were discussed, and information obtained from the structure images was mentioned. In the main part, crystal structures and microstructures of Tl-, Pb-, Ln-, Y- and Bi-system superconductors were presented, which were determined from observed high-resolution structure images and electron diffraction patterns. In the final part, defects, intergrowth, interfaces and surfaces of the perovskite-type superconducting copper oxides are touched on. The high-resolution structure images presented in the present paper were obtained with a 400-kV electron microscope (JEM-4000EX) having a point-to-point resolution of 0.17 nm, and some lattice images and electron diffraction patterns were taken with a 200-kV microscope (JEM-200CX) having a resolution of 0.23 nm.

2 High-resolution electron microscopy

HREM images are phase contrast due to interference of electron waves, and direct information on atomic arrangements in crystals can be obtained. HREM images are sensitive to defocus values, crystal thickness, crystal tilting, and other parameters.

HREM images of TlBa2Ca3Cu4O11 superconductor [15], which has the almost highest transition temperature of 123 K are shown in Figure 1. Although these three images were taken at the same magnification for the same sample, the image contrast is very different due to the imaging conditions. Figure 1A is a one-dimensional lattice image taken perpendicular to the c-axis of the crystal by 200 kV TEM. Tl and Ba layers show dark contrast, and Ca layers together with oxygen vacancies (Ov) show white lines. Figure 1B is a two-dimensional HREM image taken along the a-axis and perpendicular to the c-axis by 200 kV TEM. Although oxygen vacancy positions are imaged as bright dots due to dynamical diffraction effect, the image does not show the direct atomic arrangements. Figure 1C is a structure image taken along the a-axis and perpendicular to the c-axis by 400 kV TEM. This HREM image is imaged by the interference of multielectron waves, and the potential in the crystal is clearly imaged. When the atomic arrangements of the superconducting oxides are discussed, structure images, which show potential clearly in the crystal, are needed, as shown in Figure 1C. On the other hand, lattice images like Figure 1A and B are suitable for HREM observation in the unit cell scale such as defects, interface, and modulated structures.

HREM images of TlBa2Ca3Cu4O11 (Tc=123 K). (A) One-dimensional lattice image taken by 200 kV TEM. (B) Two-dimensional HREM image taken by 200 kV TEM. (C) Structure image taken by 400 kV TEM with the incident beam parallel to the a-axis, together with a projected structure model. Brighter spots indicated by Ov correspond to the oxygen vacant positions.
Figure 1

HREM images of TlBa2Ca3Cu4O11 (Tc=123 K). (A) One-dimensional lattice image taken by 200 kV TEM. (B) Two-dimensional HREM image taken by 200 kV TEM. (C) Structure image taken by 400 kV TEM with the incident beam parallel to the a-axis, together with a projected structure model. Brighter spots indicated by Ov correspond to the oxygen vacant positions.

Necessary conditions to observe the structure images, which can be used to obtain information of crystal structures in the present work, are investigated by calculating high-resolution images with the multislice method. For the image calculations, a large computer (ACOS-2020) was used. Basic parameters used for the image calculations are as follows: accelerating voltage=400 kV, radius of the objective aperture=5.9 nm-1, spherical aberration Cs=1.0 mm, spread of focus Δ=8 nm, semi-angle of convergence α=0.55 mrad, crystal thickness t=1.9 nm, and defocus value of the objective lens Δf=-49 nm (Scherzer defocus).

Figure 2 shows simulated images based on a structural model of TlBa2Ca3Cu4O11 with the incident beam parallel to the a-axis. Structure parameters in the calculations were used with values determined by X-ray diffraction [16]. In Figure 2A, the images calculated with various thicknesses under the Scherzer defocus condition (Δf=-45 nm) are shown, whereas in Figure 2B, the images are calculated as a function of defocus value for a crystal thickness of 1.92 nm. In Figure 2A, the images of thinner crystals than 2 nm faithfully represent the projection of crystal structure. The darkness or size of dark spots corresponding to Tl, Ba, Cu, and Ca positions can be identified to be nearly proportional to their atomic numbers. This type of high-resolution structure images are mainly used in the present paper hereafter. On the other hand, in the images of thicker than about 3 nm, the cation positions cannot be identified as dark spots. The calculations show that only images taken at nearly the Scherzer defocus (Δf=-45 nm) and from thinner crystals than 2 nm can be used for structure analysis. In Figure 2B, it is interesting to note that the image contrasts at Δf=15 nm and -75 nm are almost reversed to that of the Scherzer condition, and the cation positions appear as bright spots. As these types of wrong images have been occasionally reported in some papers, one should keep it in mind.

Calculated images of TlBa2Ca3Cu4O11 with the a-axis incidence as functions of (A) crystal thickness and (B) defocus value.
Figure 2

Calculated images of TlBa2Ca3Cu4O11 with the a-axis incidence as functions of (A) crystal thickness and (B) defocus value.

Unfortunately, oxygen atoms cannot be represented as dark spots in structure images taken with the present microscope. However, information on oxygen atom positions is contained in the structure images. Figure 3 is calculated images of TlBa2Ca3Cu4Ox, and two types of models with oxygen vacancies and with oxygen atoms on the Ca layers are shown in Figure 3A and B, respectively. Image calculations were performed under the condition of a defocus value of -45 nm and crystal thickness of 1.92 nm. Intensity profiles along the lines through Tl and Cu atoms parallel to the c-axis are also shown at the right side of the calculated images. By the comparison between Figure 3A and B, one can notice that the oxygen vacant positions indicated by Ov show brighter contrast than the oxygen positions, and the intensity of the vacant positions shows higher peaks in the intensity profile. In order to confirm the above results of image calculations, an observed structure image of TlBa2Ca3Cu4O11 in Figure 1C is examined [14]. In the observed image, the oxygen vacant positions indicated by Ov show brighter contrast than the other bright regions corresponding to oxygen positions. Therefore, the image clearly shows the existence of ordering of oxygen vacancies on the Ca layers.

Models and calculated images of TlBa2Ca3Cu4O11 with oxygen vacancies on the Ca layers (A), and those of TlBa2Ca3Cu4O14 with oxygen atoms on the Ca layers (B). Intensity profiles along the lines through Tl and Cu atoms parallel to the c-axis are also shown at right side.
Figure 3

Models and calculated images of TlBa2Ca3Cu4O11 with oxygen vacancies on the Ca layers (A), and those of TlBa2Ca3Cu4O14 with oxygen atoms on the Ca layers (B). Intensity profiles along the lines through Tl and Cu atoms parallel to the c-axis are also shown at right side.

3 Crystal structures of Tl-based copper oxides

Bulk samples of Tl-based copper oxides were prepared by reacting Tl2O3, CaO, BaO2, CuO, and BaCuO2 [15, 17–22]. The samples of Tl2Ba2CuO6 and TlBa2CaCu2O7 were reheated in the range of 500–850°C, and then quenched in liquid nitrogen in order to control the oxygen content and to obtain the highest Tc. The Tl2Ba2CuO6, Tl2Ba2CaCu2O8, and TlBa2CaCu2O7 samples showed the superconductivity at transition temperatures. Samples for high-resolution observation were prepared by dispersing crushed materials on holey carbon films [23].

Figure 4A and B are high-resolution structure images of Tl2Ba2CuO6 (Tc=80 K) taken with the incident beam parallel to the [100] and [110] directions, respectively. To observe the atomic arrangements clearly, a crystallographic image processing was carried out. The digital images were masked and fast Fourier transformed. The reciprocal lattice was indexed according to a unit cell, and the lattice parameters were determined using the positions of the strongest peaks in the analysis. The local background was subtracted, and the amplitudes and phases of the peaks were corrected using symmetrization [24, 25]. Before correcting the phases, the phase origin was determined by investigating the origin shift that gave the best accordance with the phase conditions for the two-dimensional plane group. Averaged symmetrized images were reconstructed from the corrected Fourier transforms. Crystallographic symmetrization based on the two-dimensional space group was used for the reconstruction of the Fourier transform, as shown in Figure 4C and D, respectively. Tl, Ba, and Cu atoms are clearly observed, and the darkness and size of the dark spots corresponding to the metal atom positions can be identified to be nearly proportional to their atomic numbers.

High-resolution structure images of Tl2Ba2CuO6 (Tc=80 K) taken with the incident beam parallel to the (A) [100] and (B) [110] directions. (C, D) HREM images after crystallographic image processing of (A) and (B), respectively.
Figure 4

High-resolution structure images of Tl2Ba2CuO6 (Tc=80 K) taken with the incident beam parallel to the (A) [100] and (B) [110] directions. (C, D) HREM images after crystallographic image processing of (A) and (B), respectively.

A high-resolution structure image of Tl2Ba2CaCu2O8 (Tc=114 K) is shown in Figure 5A, which was taken with the incident beam parallel to the a-axis. Figure 5B is a HREM image after crystallographic image processing of Figure 4A, and metal atom positions are clearly observed. In addition to the metal atom positions, oxygen vacancies are clearly observed as bright white dots on the Ca layers.

High-resolution structure image of Tl2Ba2CaCu2O8 (Tc=114 K) taken with the incident beam parallel to the a-axis. (B) HREM image after crystallographic image processing of (A).
Figure 5

High-resolution structure image of Tl2Ba2CaCu2O8 (Tc=114 K) taken with the incident beam parallel to the a-axis. (B) HREM image after crystallographic image processing of (A).

Figure 6 has high-resolution structure images of TlBa2CaCu2O7, Tl2Ba2Cu3O10, TlBa2Ca4Cu5O13, and Tl2Ba2Ca3Cu4O12, taken with the incident beam parallel to the a-axis, together with projected structure models [26]. Metal atoms positions are clearly observed, and brighter spots indicated by asterisks correspond to the oxygen vacant positions. It is well known that the Tl-based superconductors have various layered structures, TlBa2Can-1CunO2n+3 (n=1–5) with single Tl layers and Tl2Ba2Can-lCunO2n+4 (n=1–4) with double Tl layers. Their structure models were proposed, as shown in Figure 7. In the TlBa2Can-lCunO2n+3 (n=1–5) system, structure parameters of TlBa2CaCu2O7 (Tl-1212) [27], TlBa2Ca2Cu3O9 (Tl-1223) [16], and TlBa2Ca3Cu4O11 (Tl-1234) [16] have been determined by X-ray diffraction. On the other hand, in the Tl2Ba2Can-lCunO2n+4 (n=1–4) system, Tl2Ba2CuO6 (Tl-2201) [28], Tl2Ba2CaCu2O8 (Tl-2212) [28], and Tl2Ba2Ca2Cu3O10 (Tl-2223) [28] have been examined in detail by X-ray diffraction. Although crystal structures of TlBa2Ca4Cu5O13 [15] and Tl2Ba2Ca3Cu4O12 [26] structures were proposed, their structure parameters were not determined from diffraction analysis because they are always formed as the intergrowth with other structures. Here, the structure models of TlBa2CaCu2O7, Tl2Ba2CuO6, Tl2Ba2CaCu2O8, and Tl2Ba2Ca2Cu3O10 proposed by X-ray diffraction was examined, and next, the structure parameters of TlBa2Ca4Cu5O13 and Tl2Ba2Ca3Cu4O12 were determined from observed high-resolution structure images.

High-resolution structure images of (A) TlBa2CaCu2O7 (Tc=110 K), (B) Tl2Ba2Ca2Cu3O10 (Tc=122 K), (C) TlBa2Ca4Cu5O13, and (D) Tl2Ba2Ca3Cu4O12 taken with the incident beam parallel to the a-axis, together with a projected structure model. Brighter spots indicated by asterisks correspond to the oxygen vacant positions.
Figure 6

High-resolution structure images of (A) TlBa2CaCu2O7 (Tc=110 K), (B) Tl2Ba2Ca2Cu3O10 (Tc=122 K), (C) TlBa2Ca4Cu5O13, and (D) Tl2Ba2Ca3Cu4O12 taken with the incident beam parallel to the a-axis, together with a projected structure model. Brighter spots indicated by asterisks correspond to the oxygen vacant positions.

Structure models of TlBa2Can-1CunO2n+3 (n=1–5) and Tl2Ba2Can-1CunO2n+4 (n=1–4).
Figure 7

Structure models of TlBa2Can-1CunO2n+3 (n=1–5) and Tl2Ba2Can-1CunO2n+4 (n=1–4).

Figures 4, 5, and 6A and B are high-resolution structure images of Tl2Ba2CuO6, Tl2Ba2CaCu2O8, TlBa2CaCu2O7, and Tl2Ba2Ca2Cu3O10, taken with the incident beam parallel to the a-axis. In the images, a good correspondence between the arrangement of dark spots in the image and that of cations in the projected structure model proposed by X-ray diffraction is clearly observed. The larger black spots correspond to the Tl atoms and Ba atoms, and Cu and Ca atoms with smaller atomic numbers are represented as small dark spots. Oxygen atom positions are located on bright regions between the dark spots of the cations. In addition, positions of the dark spots corresponding to cations faithfully reflect the coordinates of the cations, determined with X-ray diffraction, as listed in Table 1. The z coordinates of cations, measured from the observed images, are listed in Table 1, together with values determined by diffraction methods [27, 28]. Good correspondences between the z coordinates of cations were determined from the observed high-resolution images and from diffraction methods. From Table 1, it can be seen that the dark spot positions in the structure images, reflect the real positions of cations within an error of 0.01 nm, which may be in a measured error. This result shows that the cation positions can be determined with the precision of 0.01 nm from the structure images. The oxygen vacant positions, which are located on the Ca layers, can be distinguished as brighter regions from the other bright regions corresponding to the oxygen sites.

Table 1

Structural parameters (z coordinates) of cations in TlBa2CaCu2O7 and Tl2Ba2CuO6 determined from the structure images of Figures 6A and 4C, and diffraction methods.

Figures 6C and D are high-resolution structure images of TlBa2Ca4Cu5O13 and Tl2Ba2Ca3Cu4O12, respectively. They were observed as the intergrowth with other structures having smaller numbers of Cu layers. The z coordinates of cations determined from the images are shown in Tables 2 and 3, respectively. Oxygen vacancies on the Ca layers can be observed in the images. In Tables 2 and 3, the z coordinates of oxygen atoms in Tl-O and Cu-O layers were assumed to be the same as those of cations, and oxygen positions in the Ba layer were assumed to be shifted by 0.07 nm along the c-axis from the results of other Tl-based superconductors [16, 27–29]. Tl-1201 structures and other substitution-type structures had also been reported for the Tl-based superconductors [30–34].

Table 2

Structural parameters of cations in TlBa2Ca4Cu5O13 determined from the high-resolution image of Figure 6C.

Table 3

Structural parameters of cations in Tl2Ba2Ca3Cu4O12 determined from the high-resolution image of Figure 6D.

4 Modulated superstructures of Tl-based copper oxides

In addition to the basic layer structures of Tl-based copper oxides, modulated structures accompanied with the satellite spots have also been observed [35–41]. Figure 8 is electron diffraction patterns of Tl2Ba2CuO6 taken along the various directions of the crystal [42]. In addition to the fundamental reflections, sharp satellite spots are observed in Figure 8A, B, and C, which indicate the modulated superstructure. The electron diffraction pattern of Figure 8C can be obtained from Figure 8D by rotation of 18° along the c-axis. There is no satellite spot in Figure 8D, and the satellite spots along the [3 1 0] appear by the 18°-rotation along the c-axis as observed in Figure 8C. The Tl2Ba2CuO6 has both tetragonal and orthorhombic structures, and the modulated structure is observed in the orthorhombic phase. The fundamental structure with the modulated structure is distorted a little, and the indices are those of an orthorhombic unit cell (a=0.545 nm, b=0.549 nm, c=2.318 nm) as observed in the electron diffraction pattern of Figure 8A. The fundamental lattice has a twin structure with a twin plane of {110} as observed in Figure 8B. The superstructure reflections are observed along the <1 3 0>, and the modulated wave vector was determined as q=[±0.07 0.22 1]=1/6.2 <1 3 0>, i.e., the modulation is incommensurate.

Electron diffraction patterns of Tl2Ba2CuO6 taken with the incident beam parallel to the (A) [001], (B) [001], (C) [1̅30], and (D) [11̅0] directions. The electron diffraction pattern (B) showing the modulated structure and its twinning.
Figure 8

Electron diffraction patterns of Tl2Ba2CuO6 taken with the incident beam parallel to the (A) [001], (B) [001], (C) [1̅30], and (D) [11̅0] directions. The electron diffraction pattern (B) showing the modulated structure and its twinning.

Figure 9A is a HREM image of Tl2Ba2CuO6 taken along the c-axis. An enlarged image of Figure 9A is shown in Figure 9B. In addition to the fundamental lattice fringes, dark and bright contrasts with a distance of ∼1.2 nm and their twin relations can be seen. This HREM images show the direction of the modulated structure is near [130]. Although twinning of the modulated structure appears on both {110} and near {100} planes, the twinning of fundamental lattice appears only on {110} planes, as indicated by the arrows in Figure 9A and B. Figure 9C is a high-resolution image of Tl2Ba2CuO6 2201 taken with the [1̅30] incidence. Dark and bright contrasts with a distance of ∼2.4 nm are observed, and the modulated region (a) and the nonmodulated region (b) are clearly distinguishable. By careful observation of the modulation contrast, changes in both the darkness and position of Tl atoms can be seen, as indicated by arrows in the region (a) of Figure 9C. This indicates that the origin of modulated structure would exist on the Tl-O planes.

HREM image of Tl2Ba2CuO6 taken along c-axis. (B) Enlarged image of (A). (C) HREM image of with the incident beam parallel to the [31̅0] direction.
Figure 9

HREM image of Tl2Ba2CuO6 taken along c-axis. (B) Enlarged image of (A). (C) HREM image of with the incident beam parallel to the [31̅0] direction.

From detailed composition analysis of the Tl-2201 phase, the orthorhombic with modulated structure and the tetragonal without the modulation had the composition of Tl1.7Ba2CuO5.7 and Tl1.6Ba2CuO5.6, respectively. The modulated structure has 0.3 oxygen and 0.3 Tl deficiencies per unit cell. The electron diffraction and high-resolution observation showed the 6.2 times superstructure along the [130] direction. These results indicated that the modulation would be due to the atomic ordering of oxygen and Tl in the Tl-O layers along the [130] with a period of 6.2 times. If the oxygen and Tl deficiencies are assumed along the [130] direction, the deficiencies are calculated as 0.36 per the unit cell, which agree well with the composition analysis. Therefore, the modulated superstructure is believed to be due to Tl and oxygen vacancies in the Tl-O layers along the [130] with a period of 6.2 times (∼2.4 nm). On the other hand, the tetragonal phase had more atomic deficiencies randomly and did not show the modulated structure.

Electron diffraction patterns of Tl2BaSrCuO6 taken with the incident beam parallel to the [001] and [010] directions are shown in Figure 10A and B, respectively. When Sr atoms are doped at the Ba sites, the fundamental structure has a tetragonal structure. Satellite reflections due to a modulated structure are observed, which are weak and diffuse compared to the Tl-2201 phase. In addition, the modulation wave vector was changed as q=<1/6 0 1>.

Electron diffraction patterns of Tl2BaSrCuO6 taken with the incident beam parallel to the (A) [001] and (B) [010] directions.  (C) Electron diffraction pattern and (D) HREM image of Tl2Ba2CaCu2O8 taken with the incident beam parallel to the [010] direction. Electron diffraction patterns of (E) TlBa2CaCu2O7 and (F) TlBa2Ca2Cu3O9 taken along the c-axis.
Figure 10

Electron diffraction patterns of Tl2BaSrCuO6 taken with the incident beam parallel to the (A) [001] and (B) [010] directions. (C) Electron diffraction pattern and (D) HREM image of Tl2Ba2CaCu2O8 taken with the incident beam parallel to the [010] direction. Electron diffraction patterns of (E) TlBa2CaCu2O7 and (F) TlBa2Ca2Cu3O9 taken along the c-axis.

For Tl2Ba2CaCu2O8 (Tl-2212) [35, 36] and Tl2Ba2Ca2Cu3O10 (Tl-2223) [35], weak diffuse scatterings were also observed, and the modulation wave vector is determined to be q=<l/6 0 1> as observed in Figure 10C. This modulation shows two-dimensional character, and the symmetry of the fundamental lattice remains tetragonal (a=0.385 nm, c=2.92 nm). A HREM image corresponding to Figure 10C is shown in Figure 10D. Modulation contrast with a distance of ∼2.3 nm is observed in the Tl-O layer along the a-axis. Models for the modulated superstructures were reported as follows: short-range ordering due to displacements of Tl and O in the Tl-O planes [36], extra oxygen in the Tl-O planes [35], a partial substitution of Tl3+ by Tl+ [35, 38], the mutual substitution of Tl and Ca atoms [10, 41]. The compositional analysis of the present samples showed that the Tl-2212 and Tl-2223 phases had compositions of Tl1.7Ba2Ca1.3Cu2O8 and Tl1.7Ba2Ca2.3Cu3O10, respectively. This implies that the excess 0.3 Ca atoms are doped at the Tl sites per the unit cell. The electron diffraction and HREM observation showed 6 times superstructure along the a-axis. If the Tl atoms are substituted by Ca atoms with a period of 6 times along the axis, the substitution atoms are 0.33 per unit cell, which agreed well with the measured composition of the samples. Therefore, the modulated superstructure is believed to be due to the Tl substitution by Ca atoms along the a-axis with a period of 6 times (∼2.3 nm).

Figure 10E and F are electron diffraction patterns of TlBa2CaCu2O7 (Tl-1212) and TlBa2Ca2Cu3O9 (Tl-1223) taken along the c-axis, respectively. Weak, diffuse satellite scatterings are observed as indicated by arrows, and the observed modulation wave vector is approximately q=<0.28 0 0.5> [42]. Almost the same incommensurate diffuse scattering was observed for the TlBa2Ca3Cu4O11 (Tl-1234) phase. As the modulation shows a two-dimensional character, the symmetry of the fundamental lattice remains tetragonal. The diffuse scattering becomes stronger as the oxygen loss is increased, and also the Tc increases. Oxygen deficiencies δ of Tl-based superconductors calculated from electron diffractions of Figure 10 and iodimetric measurements of oxygen contents are summarized in Table 4. From the compositional analysis for the Tl-1212, 1223, and 1234 phases, the atomic ratios of Tl:Ba:Ca:Cu were determined to be 1:2:1:2, 1:2:2:3, and 1:2:3:4 (stoichiometry), respectively. However, 0.29 oxygen atoms are deficient per unit cell, as summarized in Table 4, which implies that the oxygen vacant positions are the same for these structures. In the present work, oxygen atoms in the Tl-O layers would be deficient, and the measured modulation from the electron diffraction patterns are summarized in Table 4. As listed in Table 4, assumed oxygen vacancies in the Tl-O layers measured by electron diffraction agreed well with the measured oxygen vacancies by iodimetric measurements. Therefore, it is believed that the modulation superstructure would be due to oxygen vacancy ordering in the Tl-O layer along the a-axis with a period of 3.6 times (=0.28-1) and 2 times along the c-axis. The period of 3.6 times is incommensurate, which implies the mixture of 3- and 4-times superstructures. In fact, modulations with periods of 3.1∼4.2 times (=0.32-1∼0.24-1) are observed for electron diffraction patterns, as listed in Table 4.

Table 4

Oxygen deficiencies δ of Tl-based superconductors calculated from electron diffractions of Figure 10 and iodimetric measurements of oxygen contents.

5 Crystal structures of Pb-based copper oxides

Pb-based superconductors and related oxides have been discovered and investigated [5], and various new types of crystal structures of PbBaSr(Y,Ca)Cu3Oy (y=7–8.4) and Pb2(Ba,Sr)2(Ln,Ce)2Cu3Oy (Ln: Lanthanoid, y=9–10.4) [43–45] were determined by high-resolution electron microscopy, with the aid of electron and X-ray diffraction, and quantitative analysis of compositions of cations and oxygen. Some results of the determination of crystal structures are shown here. Samples were prepared from a mixture of PbO, BaO2, Sr2CuO3, Y2O3, Eu2O3, CeO2, and CuO by solid state reaction. After heating at 830°C in a 1% O2-N2 mixed gas, the specimens were slowly cooled in a furnace, quenched into liquid nitrogen or annealed in a flowing O2 gas at 400°C. Electron probe microanalysis and iodide titration method are used for quantitative analysis of the element.

Figure 11A and B are high-resolution structure images of PbBaSrYCu3O7 [43] and Pb2Sr2Y0.5Ca0.5Cu3O8 taken with the incident beam parallel to the a-axis. The PbBaSrYCu3O7 shows superconductivity at 65 K by Ca substitution for Y sites [44], and the Pb2Sr2YCu3O8 also showed superconductivity at 75 K by Ca substitution for Y sites [46]. HREM images after crystallographic image processing of Figure 11A and B are shown in Figure 11C and D, respectively. PC, BS, YC, and Ov represent (Pb, Cu), (Ba, Sr), (Y, Ca), and oxygen vacancy, respectively, and the image directly shows an arrangement of cations. The indicated atomic arrangements were proposed from the structure image, X-ray diffraction, and energy-dispersive spectroscopy (EDS) analysis. In Figure 11C, zigzag arrays consisting of larger black spots correspond to (Pb, Cu) layers, and on both sides of the (Pb, Cu) layers, there are layers of a mixture of Ba and Sr atoms [43]. In this image, Cu and Y atoms, with smaller atomic numbers, are represented as small dark spots. Oxygen atom positions are located at bright regions between the cation sites of the dark spots. In addition, the oxygen vacant positions on the Y layers, indicated by Ov, can be distinguished as brighter regions from the other bright regions corresponding to the oxygen-occupied sites in Figure 11C and D.

HREM images of (A) PbBaSrYCu3O7 and (B) Pb2Sr2Y0.5Ca0.5Cu3O8 taken with the incident beam parallel to the a-axis. (C, D) HREM images after crystallographic image processing of (A, B), respectively.
Figure 11

HREM images of (A) PbBaSrYCu3O7 and (B) Pb2Sr2Y0.5Ca0.5Cu3O8 taken with the incident beam parallel to the a-axis. (C, D) HREM images after crystallographic image processing of (A, B), respectively.

Darkness of the spots in the double (Pb,Cu) layers in Figure 11A suggests that the zigzag double layers contain some light element, Cu in addition to Pb. A random mixture of Pb and Cu in the double Pb layers is unlikely because of their different ionic radii and coordination schemes. Careful examination of contrast for the double Pb layers in a thin part near the crystal edges, as indicated by arrows in Figure 11A, suggests the pairwise distribution of Pb and Cu layers [47]. Some of these spots are large and dark, while some others are small and faint, which would correspond to Pb and Cu, respectively. The observed HREM image indicates that small domains of Pb and Cu layers alternate every several unit cells. This short-range ordering seems to be two-dimensional in the Pb and Cu layers, and Pb and Cu layers are not distinguishable due to an averaging effect in a thicker part of the specimen.

Atomic coordinates of PbBaSrYCu3O7 determined from the high-resolution structure image of Figure 10C are shown in Table 5. The z coordinates of oxygen atoms in (Pb, Cu) and Cu-O layers were assumed to be the same as those of cations, and oxygen positions in (Ba, Sr) layers were assumed to be shifted by 0.03 nm along the c-axis from the results of other Pb-based oxides [5].

Table 5

Structural parameters of cations and oxygen atoms in PbBaSrYCu3O7, determined from the high-resolution structure image of Figure 11C.

Figure 12A and B are electron diffraction patterns of PbBaSrY0.7Ca0.3Cu3O7 taken with the incident beam parallel to the [001] and [110] directions, respectively. Satellite reflections at 1/2 1/2 0 are observed, and weak reflections at 1/4 1/4 0 are also observed in Figure 12A, which indicates a modulated superstructure with a modulation wave vector of q=1/4<1 1 0>, and the modulation would be due to oxygen ordering. Weak streaks at 1/2 1/2 0 along the c-axis are also observed in Figure 12B, which indicates stacking faults of the superstructure along the c-axis. The similar modulated superstructure with a modulation wave vector of q=1/4<1 1 0> was also observed in PbBa0.7Sr1.3EuCeCu3O9 [45].

Electron diffraction patterns of PbBaSrY0.7Ca0.3Cu3O7 taken with the incident beam parallel to the (A) [001] and (B) [110] directions.
Figure 12

Electron diffraction patterns of PbBaSrY0.7Ca0.3Cu3O7 taken with the incident beam parallel to the (A) [001] and (B) [110] directions.

A high-resolution structure image of PbBa0.7Sr1.3EuCeCu3O9, which was taken with the incident beam parallel to the a-axis, is shown in Figure 13A. Here, the foil thickness increases from the top to the bottom in the picture. The image contrast of the thin region directly represents the projected potential. The structure model of PbBa0.7Sr1.3EuCeCu3O9 was determined from the structure image by the aid of X-ray diffraction. The structure has a characteristic layer structure formed by alternate stacking of (Eu, Ce) double layers and (Pb, Cu) double layers, which are separated by (Ba, Sr) and Cu layers. In Figure 13A, (Eu, Ce) atoms are represented as the largest dark spots.

HREM images of (A) PbBa0.7Sr1.3EuCeCu3O9, (B) PbBa0.7Sr1.3YCe2Cu3O11, and (C) Pb2Sr2(Y,Ce)5Cu3O16 taken with the incident beam parallel to the a-axis, together with projected structure models. Calculated images of two models of (D) PbBa0.7Sr1.3EuCeCu3O9 and (E) Pb2Sr2(Y,Ce)5Cu3O16 with the a-axis incident, together with projected structure models.
Figure 13

HREM images of (A) PbBa0.7Sr1.3EuCeCu3O9, (B) PbBa0.7Sr1.3YCe2Cu3O11, and (C) Pb2Sr2(Y,Ce)5Cu3O16 taken with the incident beam parallel to the a-axis, together with projected structure models. Calculated images of two models of (D) PbBa0.7Sr1.3EuCeCu3O9 and (E) Pb2Sr2(Y,Ce)5Cu3O16 with the a-axis incident, together with projected structure models.

Figure 13B is a high-resolution structure image of PbBa0.7Sr1.3YCe2Cu3O11, which was taken with the incident beam parallel to the a-axis, together with a structure model. A good correspondence between the arrangement of dark spots in the image and that of cations in the projected structure model of PbBa0.7Sr1.3YCe2Cu3O11 is clearly observed. The image shows a layer structure with three (Y, Ce) layers between Cu-O layers. The atomic coordinates were directly determined from the observed high-resolution images as summarized in Table 6. Structure models of Pb(Ba,Sr)2(Ln,Ce)nCu3O5+2n (n=1–5) determined by the present work are summarized in Figure 14A.

Table 6

Structural parameters of cations and oxygen atoms in PbBa0.7Sr1.3YCe2Cu3O11, which were determined from the high-resolution image of Figure 13B.

Structure models of (A) Pb(Ba,Sr)2(Ln,Ce)nCu3O5+2n (n=1–5) and (B) Pb2Sr2(Ln,Ce)nCu3O6+2n (n=1–5) determined from high-resolution electron microscopy.
Figure 14

Structure models of (A) Pb(Ba,Sr)2(Ln,Ce)nCu3O5+2n (n=1–5) and (B) Pb2Sr2(Ln,Ce)nCu3O6+2n (n=1–5) determined from high-resolution electron microscopy.

In addition to the Pb(Ba,Sr)2(Ln,Ce)nCu3O5+2n (n=1–5) with double (Pb, Cu) layers, Pb2Sr2(Ln,Ce)nCu3O6+2n (n=1–7) with triple (Pb,Cu) layers were investigated by high-resolution electron microscopy. Figure 13C is a HREM image of the Pb2Sr2YCe4Cu3O16 [48] taken with the incident beam parallel to the a-axis, together with a determined structure model, as listed in Table 7. This structure can be characterized as a layer structure formed by stacking of fivefold (Y, Ce) fluorite layers between (PbO-Cu-PbO) blocks. Structure models of Pb2Sr2(Ln, Ce)nCu3O6+2n (n=1–5) determined by the present work are summarized in Figure 14B. In addition, Pb single-layer structures were synthesized and reported [49–53], and new types of structures with (Ce, Y) fluorite layers were also reported [54, 55].

Table 7

Structural parameters of cations and oxygen atoms in Pb2Sr2YCe4Cu3O16, which were determined from the high-resolution image of Figure 13C. Oxygen sites were assumed.

The observed image of Figure 13A was examined in detail with the aid of computer simulations. Multislice calculations were carried out with the z coordinates obtained from high-resolution image. Two types of oxygen positions (left: tetrahedral sites, right: octahedral sites) between (Eu, Ce) layers were assumed, as shown in Figure 13D. The calculated image of Figure 13D with oxygen positions at tetrahedral sites, under the condition of a defocus of -35 nm and a crystal thickness of 1.93 nm, is in good agreement with the observed image of Figure 13A, compared with Figure 13B. This result shows that the positions of oxygen atoms can be determined by comparing calculated images with observed images. The detailed structure of PbBa0.7Sr1.3EuCeCu3O9 was investigated by imaging plates. The observed image of PbBa0.7Sr1.3YCe2Cu3O11 in Figure 13B was also investigated by comparing with computer simulations. The z coordinates obtained from the high-resolution image of Figure 13B were used, and two types of oxygen positions at tetrahedral sites (left) and octahedral sites (right) between (Y, Ce) layers were assumed. The calculated image of oxygen positions at tetrahedral sites in Figure 11E agrees well with the observed image of Figure 13B.

6 Structures of lanthanoid-based copper oxides

Various types of lanthanoid-based copper oxides have been reported [56–58], and electron-doped Nd2-xCexCuO4 superconductors were discovered [59–61]. In order to clarify the microstructures, single crystals of Ln2CuO4 prepared with various heat treatments are investigated by means of high-resolution electron microscopy and electron diffraction.

Single crystals of Ln2CuO4 (Ln=Pr, Nd, Sm) were grown by the traveling-solvent-floating-zone technique using an infrared-heating furnace [62, 63]. To reduce oxygen content, parts of the Ln2CuO4 (Ln=Pr, Nd, Sm) samples were annealed at 1100°C for 18 h in air and quenched in liquid nitrogen. The rests of the samples were annealed at 400°C in air for 38–140 h to saturate oxygen content in the crystals. SmLa0.75Sr0.25CuO4 was also synthesized from a mixture of La2O3, Sm2O3, CuO, and SrCO3 [64]. Mixed powder was first calcined at 950°C in air for 10 h, then pressed into pellets, and finally sintered at 1130°C in air for 15 h. The pellets were quenched to room temperature in air and subsequently annealed at 550°C in the atmosphere with various oxygen pressures.

A high-resolution image of Sm2CuO4 taken with the [100] incidence is shown in Figure 15A. The specimen thickness increases from the top to the bottom in this HREM image. The observed image shows the projected arrangement of metal atoms. There are two kinds of dark spots in the image. The larger ones in the zigzag arrays correspond to the Sm atoms. Cu atoms, with a smaller atomic number, are represented as less dark spots between the double Sm layers. Figure 15B is a HREM image of SmLa0.75Sr0.25CuO4 taken along the a-axis. Fairly large separation between two lines of La ions is clearly observed in the HREM image, which is in accord with X-ray diffraction analysis [64].

HREM images of (A) Sm2CuO4 and (B) SmLa0.75Sr0.25CuO4 taken along the a-axis. (C) Structure models and calculated images of the (D) Sm2CuO4 and (E) SmLa0.75Sr0.25CuO3.95, respectively.
Figure 15

HREM images of (A) Sm2CuO4 and (B) SmLa0.75Sr0.25CuO4 taken along the a-axis. (C) Structure models and calculated images of the (D) Sm2CuO4 and (E) SmLa0.75Sr0.25CuO3.95, respectively.

Based on the crystal structure models of Figure 15C, image calculations on the Sm2CuO4 and SmLa0.75Sr0.25CuO4 were carried out to confirm the structures, as shown in Figure 15D and E, respectively. The images of Figure 15D and E calculated at a Scherzer defocus of -45 nm and a crystal thickness of 1.2∼2.7 nm, agree well with the observed image contrast for both thin and thick regions. For the SmLa0.75Sr0.25CuO4 structure, the difference of oxygen atom positions in the Sm and La-Sr layers are clearly observed both in the observed and calculated image, which indicates the HREM image includes information both on metal and oxygen atoms in the crystal.

Modulated superstructures are also observed in the lanthanoid-based copper oxides [65–68]. The domains of superlattice with sizes of 5∼60 nm in diameter were observed, and the smaller domains (5∼10 nm) are observed around a large one and seem to grow into larger ones (40∼60 nm). A representative superstructure domain in Nd2CuO4 is shown in Figure 16A, and B is an electron diffraction pattern of Figure 16A. Sharp satellite reflections with a wave vector q=<1/4 1/4 0> are observed in Figure 16B. In Figure 16A, repeated dark and bright contrasts separated at a distance of 1.1 nm (≃2√2×a) are observed in the [110] direction. The high-resolution image and the diffraction pattern reveal that the basic lattice spacing of the superlattice lengthen as much as 101.5% and 100.3% in the [110] and [11̅0] directions, respectively, compared with the fundamental lattice. Therefore, the contrast due to strain field is observed around the domain. Two-directional superlattice domain is also observed as shown in a HREM image in Figure 16C, and an electron diffraction pattern of the superlattice domains along two directions is shown in Figure 16D, which also indicates a modulation wave vector of q=<1/4 1/4 0>. It can be considered that such domain structure is due to non-uniformity of oxygen content in the specimens.

(A) HREM image and (B) electron diffraction pattern of a single domain of the superlattice in Nd2CuO4, taken with the [001]  incidence. (C) HREM image and (D) electron diffraction pattern of superlattice domains along two directions.
Figure 16

(A) HREM image and (B) electron diffraction pattern of a single domain of the superlattice in Nd2CuO4, taken with the [001] incidence. (C) HREM image and (D) electron diffraction pattern of superlattice domains along two directions.

Various types of modulated superstructures were observed in the Ln2CuO4, as listed in Table 8. Figure 17A–D are electron diffraction patterns of Nd2CuO4, Pr2CuO4, Pr1.85Ce0.15CuO4, and Sm2CuO4, taken along the [11̅0] direction. For the Pr2CuO4, Pr1.85Ce0.15CuO4, and Sm2CuO4 crystals, satellite reflections at 0.24 0.24 0.72, 1/3 1/3 0, and 1/2 1/2 1 are observed, as shown in Figure 17B, C, and D, respectively. Figures 17E and F are electron diffraction patterns of Pr2CuO4 taken along the [010] and [1̅11] directions, respectively, which also indicates weak diffuse scattering and sharp satellite reflections at 1/2 0 1/2 and 1/4 1/4 1/2, respectively. A superstructure have been observed and characterized for Nd2-xCexCuO4 by a wave vector q=<1/4 1/4 0> [62] and was suggested that the modulation is due to ordering of oxygen vacancy and/or Ce. However, the satellite reflections were not observed in the diffraction patterns of Nd2CuO4 and Pr2CuO4 quenched from 1100°C. The result indicates that the appearance of superstructures is sensitive to the oxygen content and unrelated to ordering of Ce atoms. As neutron diffraction study shows the deficiency of oxygen in Cu-O planes, it can be supposed that the superlattices are due to ordering of oxygen atoms in the Cu-O planes.

Table 8

Summary of modulated structures of Ln2CuO4.

Electron diffraction patterns of (A) Nd2CuO4, (B) Pr2CuO4, (C) Pr1.85Ce0.15CuO4, and (D) Sm2CuO4, taken with the [11̅0] incidence. Electron diffraction patterns of Pr2CuO4 taken along the (E) [010] and (F) [1̅11] directions.
Figure 17

Electron diffraction patterns of (A) Nd2CuO4, (B) Pr2CuO4, (C) Pr1.85Ce0.15CuO4, and (D) Sm2CuO4, taken with the [11̅0] incidence. Electron diffraction patterns of Pr2CuO4 taken along the (E) [010] and (F) [1̅11] directions.

7 Structures of Y-based copper oxides with high Jc

Superconducting thin films can be used for electronic devices and other applications [69–73]. Chemically, vapor deposited (CVD) YBa2Cu3O7 thin films with high critical current density (Jc) of 6.5×104 A/cm2 at 77.3 K and 27 T were investigated [74, 75]. The films were deposited on the SrTiO3(100) substrate. β-diketonate complexes (2, 2, 6, 6-tetramethy 1-3, 5-heptanedionate chelated Y3+, Ba2+, and Cu2+) were used as the three vapor sources [76]. The films were deposited in a mixed gas atmosphere or Ar/O2=3/1 at 10 Torr on the substrate, which was heated at 900°C close to the melting point of YBa2Cu3O7-y. The post-annealing was carried out in the O2 atmosphere at 1 atm. To view the cross-section of the CVD film, two pieces of cut substrate and film were pasted together, film to film, then cut again perpendicular to the film and polished in an Ar ion sputtering mill.

Figure 18A is a low-magnification image obtained with the incident beam perpendicular to the substrate. Relatively large grains of 20∼100 nm in size are observed in the image. Many observed precipitates showed Moiré fringes, which indicates that the precipitates are semicoherent with the matrix and have very similar crystal lattice. A cross-sectional high-resolution image is shown in Figure 18B. Relatively large precipitates ∼30 nm in size are embedded parallel to the c-planes. These precipitates could act as the flux-pinning centers, which would result in the high Jc. Many defect regions are formed parallel to c-planes, as shown in Figure 18C. The defect regions consist of extra layers and ∼20 nm in diameter with deformed regions adjacent to them. Those faulted regions may also act as the flux-pinning centers. Figure 18D is a cross-sectional HREM image of a grain in the CVD film, and most of the grains have a preferred c-axis orientation perpendicular to the SrTiO3(100) substrate, as indicated by region 1. On the other hand, the c-axis is oriented parallel to the SrTiO3(100) substrate in a region 2. An enlarged HREM image at the grain boundary interface is shown in Figure 18E, and the c-axis of the YBa2Cu3O7 structure is perpendicular to one another. A structure model and a structure image of YBa2Cu3O7 are shown in Figure 18F and G, respectively. Undeformed YBa2Cu3O7-x domains, interleaved by the faulted regions, are ∼50 nm in size. The electron diffraction pattern taken with the incident beam parallel to [010] did not change when the specimen was tilted by 5°, which is appreciably wider than the usual crystals. Such morphology indicates that the sample was grown quickly at a temperature just below the melting point or YBa2Cu3O7-x where the homogeneous nucleation process followed by the preferential crystal growth in the c-planes occurs.

(A) Low magnification image of CVD-YBa2Cu3O7 film taken with the electron beam perpendicular to the substrate. Round precipitates with Moiré fringes on them are observed. (B) Cross-sectional HREM image of CVD-YBa2Cu3O7 film taken with the electron beam parallel to the a-axis. (C) Stacking faults in the film. HREM image of (D) a grain and (E) grain boundary of the film. (F) Structure model and (G) structure image of YBa2Cu3O7 together with a projected structure model.
Figure 18

(A) Low magnification image of CVD-YBa2Cu3O7 film taken with the electron beam perpendicular to the substrate. Round precipitates with Moiré fringes on them are observed. (B) Cross-sectional HREM image of CVD-YBa2Cu3O7 film taken with the electron beam parallel to the a-axis. (C) Stacking faults in the film. HREM image of (D) a grain and (E) grain boundary of the film. (F) Structure model and (G) structure image of YBa2Cu3O7 together with a projected structure model.

Bulk types of YBa2Cu3O7-x superconductors with high Jc were also investigated. Oxide powders or Y, Ba, and Cu were mixed with the composition of YBa2Cu3O7-x phase. The mixture was calcined at 900°C for 24 h and then reground. After this process was repeated twice, a small amount of additive oxide powder with a perovskite structure BaZrO3 was added, 3 mol%, to the YBa2Cu3O7-x powder [77]. Again, grinding and calcining were repeated, and the pellets were sintered at 950°C for 24 h and then cooled slowly. The bulk sample showed high critical current density of 2.0×105 A/cm2 at 77.3 K and 0.1 T.

Figure 19A is a TEM image of an YBa2Cu3O7-x bulk specimen with BaZrO3 additive, where two types of fine particles with spherical and irregular shapes are lying in the YBa2Cu3O7-x matrix. Their minimum size was ∼20 nm, and the average size was roughly estimated to be ∼100 nm, and they were confirmed to be BaZrO3 phase from electron diffraction. Figure 19B is a HREM lattice image near the grain boundary between the BaZrO3 phase and the YBa2Cu3O7-x matrix, where both lattice planes generating a lattice fringe are assigned as indicated in the figure. The lattice plane continues across the interface, but the extra half plane is inserted in every 20 fringes within the BaZrO3 phase, while the interval for the extra half plane is theoretically suggested to be every 14 fringes. Therefore, the interface in this region was suggested to be semi-coherent. The mechanism of fine dispersion of ABO3 oxides in the matrix is not yet fully understood. It is suggested that the grain growth takes place during sintering, and ABO3 phase is dragged into the growing grain as the same species as the matrix because both phases have a similar crystal structure and have a semi-coherent interface as mentioned above. Several types of pinning centers like Y2BaCuO5 [78], CuO [79], SnO2 [80], ZrO2 [81], and irradiation-induced defects [82] have been reported to be effective for the YBa2Cu3O7-x superconductor [83–87]. A perovskite-type oxide, ABO3, seems to be a promising candidate as pinning center for the superconducting copper oxides because the crystal structure is similar to the host superconducting phase. A remarkable flux-pinning effect could be expected by introducing the oxide. Homogeneous distribution of non-superconducting fine particles is very effective, when they distribute in the same dimension with the fluxoid lattice. In addition, the matrix is distorted around the particles with different unit volume by the coherent interface with the matrix, and a significant change of the superconducting property is expected around the coherent particles.

(A) TEM image of 3 mol%-BaZrO3 dispersed YBa2Cu3O6+x. (B) HREM image at the BaZrO3 nanoparticles and YBa2Cu3O6+x matrix interface.
Figure 19

(A) TEM image of 3 mol%-BaZrO3 dispersed YBa2Cu3O6+x. (B) HREM image at the BaZrO3 nanoparticles and YBa2Cu3O6+x matrix interface.

8 Oxygen ordering in YBa2Cu3O7-x

The crystal structure of YBa2Cu3O7 is based on a triple perovskite structure and is characterized by the ordering of oxygen vacancies, as shown in Figure 18F, that is, the oxygen positions on the Y atom layer and between two Ba atoms are vacant. In addition, oxygen orderings in the Cu-O basal planes were observed [7]. Bulk samples of YBa2Cu3O7-x superconductors were prepared by mixing BaCO3, Y2O3, and CuO powders with the composition of YBa2Cu3O7-x phase. The mixture pellets were calcined at 930°C for 12 h in air and then cooled slowly in a furnace. After crushing the pellets to form powders, the process was repeated once more. The obtained pellets were reheated at 500∼900°C and quenched into liquid nitrogen, and subsequently annealed at 500∼300°C in the vacuum seal. The pellets were also annealed in a flowing N2 gas at various temperatures to control the oxygen contents. The oxygen contents were investigated by iodimetric measurements and mass change.

As the consequence of the tetragonal-to-orthorhombic phase transition of YBa2Cu3O7-x at ∼600°C [88, 89], twin boundaries are often observed. Figures 20A and B are TEM image and lattice image of YBa2Cu3O7-x taken with the incident beam parallel to the c-axis. Twin boundaries (TB) are coherent and indicated by arrows. The twin boundaries show distinct contrast in the TEM image, and the existence of boundaries is evident from kinks of the lattice fringes. In the oxygen-deficient YBa2Cu3O7-x compounds, oxygen vacancy ordering was observed. Figure 20C and D are electron diffraction patterns of YBa2Cu3O6.68 taken with the incident beam parallel to the c-axis and b-axis, respectively. The electron diffraction pattern shows orthorhombic structure with a- and b-axis and a twin structure with a {110} twin plane. Both electron diffraction patterns in Figure 20C and D shows diffuse satellite reflections at 1/2 0 0 along the a-axis. This indicates the existence of modulated superstructure with a modulation wave vector q=<1/2 0 0>, which would be due to ordering of oxygen vacancies on the basal Cu-O planes. Figure 20E is a Fourier transform of HREM image of Figure 20B, and filtered inverse Fourier transform of Figure 20E is shown in Figure 20F, in which linear bright stripes with the distance of 2a are observed along the a-axis. The twin boundary can be clearly seen at a glancing view parallel to the a- or b-axis in Figure 20F.

(A) TEM image and (B) lattice image of YBa2Cu3O7-x taken with the incident beam parallel to the c-axis. Twin boundaries (TB) are indicated by arrows. Electron diffraction patterns of YBa2Cu3O6.68 taken with the incident beam parallel to the (C) c-axis and (D) b-axis. (E) Fourier transform of (B). (F) Filtered inverse Fourier transform of (E).
Figure 20

(A) TEM image and (B) lattice image of YBa2Cu3O7-x taken with the incident beam parallel to the c-axis. Twin boundaries (TB) are indicated by arrows. Electron diffraction patterns of YBa2Cu3O6.68 taken with the incident beam parallel to the (C) c-axis and (D) b-axis. (E) Fourier transform of (B). (F) Filtered inverse Fourier transform of (E).

Electron diffraction patterns of oxygen-deficient YBa2Cu3O6.80, YBa2Cu3O6.47, YBa2Cu3O6.23, and YBa2Cu3O6.29 taken along the c-axis are shown in Figure 21A–D, respectively. In Figure 21A, weak diffuse streaks are observed along the a-axis, which would indicate short-range ordering of oxygen atoms. In Figure 21B and C, superstructures with a modulation wave vector q=<1/3 0 0> and <0 1/3 0>, which indicates the superstructure are formed along the a-axis and b-axis of the orthorhombic cell, respectively. In addition, a superstructure with a modulation wave vector q=<1/4 0 0> is observed along the a-axis, as shown in Figure 21D. These modulated structures are summarized as listed in Table 9.

Electron diffraction patterns of (A) YBa2Cu3O6.80, (B) YBa2Cu3O6.47, (C) YBa2Cu3O6.23, and (D) YBa2Cu3O6.29 taken along the c-axis.  (E) HREM lattice image and (F) electron diffraction pattern of YBa2Cu3O6.47 taken along the c-axis.
Figure 21

Electron diffraction patterns of (A) YBa2Cu3O6.80, (B) YBa2Cu3O6.47, (C) YBa2Cu3O6.23, and (D) YBa2Cu3O6.29 taken along the c-axis. (E) HREM lattice image and (F) electron diffraction pattern of YBa2Cu3O6.47 taken along the c-axis.

Table 9

Summary of modulated structures of YBa2Cu3Oy.

Figure 21E and F are a lattice image and an electron diffraction pattern of YBa2Cu3O6.47 taken with the [001] incidence. Satellite peaks with a modulation wave vector q=<1/3 0 0> are observed together with q=<1/2 0 0> in the diffraction pattern. Linear bright stripes with the distance of 3a are observed along the two principal lattice directions in the HREM image of Figure 21E.

From these observations, a model for the ordered arrangement of oxygen vacancies is proposed, as shown in Figure 22A. The fundamental unit cell is orthorhombic, with a dimension of 2a×b×c. Other oxygen-ordering models of basal planes (Cu-O) of the YBa2Cu3O7-x are also proposed as shown in Figure 22B, which depends on the oxygen content. These phases would correspond to the ortho-II phase and ortho-III phases [90–96], which results in the changes of Tc, and the control of oxygen atoms in the oxide crystals is important [97–99].

Models for ordered arrangement of oxygen vacancies in YBa2Cu3O7-x. (A) A 2-times model along the a-axis. (B) Oxygen-ordering models of basal planes (Cu-O) of the YBa2Cu3O7-x.
Figure 22

Models for ordered arrangement of oxygen vacancies in YBa2Cu3O7-x. (A) A 2-times model along the a-axis. (B) Oxygen-ordering models of basal planes (Cu-O) of the YBa2Cu3O7-x.

9 Structures of Bi-based copper oxides

Bi-based copper oxides with Ag are expected for wire application [100–102]. A spray-dried aqueous solution of nitrates with atomic ratio Bi1.5Pb0.5Sr2Ca2Cu3 was calcined at 650°C for 15 h, resulting in a mixture of oxides [103–105]. The resulting precursor powder with a grain size of 3 μm was mixed with 30 vol% Ag whiskers with a diameter of 20–50 μm and a length of a few hundred micrometers. The Ag whiskers were synthesized via an electrochemical reduction of a Ag nitrate solution by a copper wire at pH 2. The mixture was pressed into bars and sintered at 853°C for 170 h in air to obtain the Bi-2223/Ag composites.

Figure 23A is a TEM image of the Bi-2223/Ag whisker interface in a sintered composite. A thin layer with a different contrast is observed at the Bi-2223/Ag interface. Electron diffractions of Ag and Bi-2223 phase are also shown in Figures 23B and C, respectively. Figure 23B is observed along the [11̅0] of the face-centered cubic Ag crystal. The diffraction pattern of Bi-2223 phase in Figure 23C was taken along the a-axis, which indicates a modulated structure with a modulation wave vector of q∼<0 1/4 0>. The origin of the modulated superstructure would be metal atom displacements in the crystal [106–114].

(A) TEM image of (Bi,Pb)2Sr2Ca2Cu3Ox/Ag whisker interface in a sintered composite. Electron diffraction of (B) Ag and (C) (Bi,Pb)2Sr2Ca2Cu3Ox.
Figure 23

(A) TEM image of (Bi,Pb)2Sr2Ca2Cu3Ox/Ag whisker interface in a sintered composite. Electron diffraction of (B) Ag and (C) (Bi,Pb)2Sr2Ca2Cu3Ox.

Figure 24A is a TEM image of the Bi-2223/Ag whisker composite with Ag-rich phase. Lattice fringes of c-planes of Bi-2223 phase are observed, and an amorphous/nanocrystalline (AM-NC) structure is observed at the Ag/Bi-2223 interface. The white areas are the result of preferential ion milling, probably of amorphous phases, which are more easily removed compared to Bi-2223. The superconducting phase is oriented with the c axis perpendicular to the interface. An EDX spectrum of the AM-NC phase in Figure 24A is shown in Figure 24B, which indicates a composition of Ag2Bi2.4Pb0.6Sr2Ca0.8Cu4.3 and an increase of Ag, Bi, Pb and Cu concentration in the amorphous phase. An enlarged HREM image and an electron diffraction pattern at the interface are shown in Figure 24C and D, respectively. The Bi-2223/AM-NC interface exhibits small steps of half or one unit cell of Bi-2223, and such intermediary phase was also observed [115]. The diffraction pattern exhibits [110] incident of the Bi-2223 crystal, and the observed streak along the c-axis indicates that there is a small amount of Bi-2212 or Bi-2234 phase to form an intergrowth structure. A diffuse ring is also observed as indicated by arrows, which exhibits the amorphous-like structure of AM-NC phase. The influence of Ag on the yield in Bi-2223 synthesis can be explained in terms of the shift in the incongruent melting point.

(A) TEM image of the Bi-2223/Ag whisker composite with Ag-rich phase. (B) EDX spectrum of the AM-NC phase in (A). (C) HREM and (D) electron diffraction pattern at the interface.
Figure 24

(A) TEM image of the Bi-2223/Ag whisker composite with Ag-rich phase. (B) EDX spectrum of the AM-NC phase in (A). (C) HREM and (D) electron diffraction pattern at the interface.

In order to observe the atomic arrangements of the Bi-2223 phase more clearly, crystallographic image processing was carried out for the Fourier transform of the HREM image. Figure 25A is a HREM image after crystallographic image processing of Bi2223, together with a projected structure model and a simulated image enclosed by a white square. The HREM image clearly shows the metal atom arrangements in the crystal, and the lines indicate the unit cell. The shading and size of the black spots corresponding to (Bi, Pb), Sr, Cu, and Ca positions can be identified as being nearly proportional to their atomic numbers. Although oxygen atoms are not represented as dark spots in Figure 25A, information on the oxygen positions should be included in the image [14, 116]. In the Ca layers, oxygen positions indicated by Ov show a brighter contrast than the other white regions, which indicates the existence of an ordering of oxygen vacancies in the (Bi, Pb) layers. A structure model of Bi(Pb)-2223 determined by X-ray diffraction [117] is inserted into Figure 25A and agrees well with the arrangements of the black dots. To investigate the observed HREM image in detail, HREM images were calculated based on the structure model for Bi-2223. Figure 25B shows calculated HREM images for Bi-2223 along the [11̅0] direction. Image calculations were carried out for various defocus values (under defocus) and crystal thickness to determine the imaging conditions of the observed images. The contrast changes of the images are very sensitive to both defocus value and crystal thickness, and (Bi, Pb) double layers are observed as dark dots around the defocus value of 30 nm. The simulation image calculated at the defocus value of -30 nm and crystal thickness of 2.7 nm is inserted into Figure 25A and agrees well with the observed HREM image.

(A) HREM image after crystallographic image processing of Bi2223, together with a projected structure model and a simulated image enclosed by a white square. Oxygen vacancies are indicated by Ov. (B) Calculated image of Bi-2223 as a function of crystal thickness and defocus values of the TEM objective lens.
Figure 25

(A) HREM image after crystallographic image processing of Bi2223, together with a projected structure model and a simulated image enclosed by a white square. Oxygen vacancies are indicated by Ov. (B) Calculated image of Bi-2223 as a function of crystal thickness and defocus values of the TEM objective lens.

10 Defects, interfaces, and surface structures

As mentioned in the previous sections, the perovskite-type superconducting copper oxides have many types of layer structures with slightly different compositions. Thus, their crystals always include a high density of intergrowth with various types of structures, in addition to well-ordered regions. A typical example of such disordered regions is shown in Figure 26A, which is a one-dimensional lattice image of TlBa2Ca3Cu4O11 taken with the incident beam perpendicular to the c-axis. Although the image does not represent each atom, stacking sequence of the layered structure can be distinguished; that is, thick black lines correspond to Tl and Ba layers, and thin white lines correspond to Ca layers with oxygen vacancy. In Figure 26A, two-, three-, five-, and six-fold Cu sequences (white lines) between Tl and Ba layers (thick black lines) are observed in addition to the usual fourfold Cu sequences. This intergrowth gives rise to steps decreasing of electron conductivity near Tc, showing multiple transition temperatures [15] because the structures with different numbers of Cu-O layers show different transition temperatures. This defect is very sensitive to annealing condition of the sample. The lattice images such as Figure 26A can be easily observed and have enough information on the intergrowth of various layer structures. Figure 26B is also a HREM image of TlBa2Ca3Cu4O11, which has a higher resolution than that of Figure 26A. Although the HREM image is a two-dimensional image, it is not a structure image. It is interesting that the periodic intergrowth forms a new ordered structure with a long period. It would be difficult to synthesize this type of crystal with a single phase.

High-resolution lattice images of TlBa2Ca3Cu4O11 taken with the incident beam (A) perpendicular to the c-axis and (B) parallel to the a-axis.
Figure 26

High-resolution lattice images of TlBa2Ca3Cu4O11 taken with the incident beam (A) perpendicular to the c-axis and (B) parallel to the a-axis.

It is well known that Tl atoms in the Tl-based superconductors are easily vapored at high temperatures, and Tl content deceases by annealing at high temperatures. Figures 27 are one-dimensional lattice images of Tl-vaporized regions. Starting composition of the sample was Tl:Ba:Ca:Cu=2:2:2:3 and was sintered at 890°C for 10 h. In the lattice image in Figure 27A, black strain contrasts are observed, which resulted from lattice deformation around the terminations of Tl-O layers, being about to disappear by the vaporization of Tl, as indicated by small arrows. In Figure 27B, the vaporization of Tl progressed furthermore. The regions of Cu-O layers were expanded, and the strain field contrasts become stronger. After Tl-O layers disappear, the remaining Ba-O layers are observed, as indicated by arrows in Figure 27B.

(A) High-resolution lattice images taken with the incident beam perpendicular to the c-axis. (B) More Tl-vaporized region.
Figure 27

(A) High-resolution lattice images taken with the incident beam perpendicular to the c-axis. (B) More Tl-vaporized region.

Similar structures were often observed in the Pb-based superconductors. An intergrowth of (Pb, Cu) layers is observed in PbBaSrYCu3O7 (Pb-2212), as shown in Figure 28A. In the image, the intergrowth of two types of units, (a) and (b), which have double (Pb, Cu) layers and stacks of two (Pb, Cu) and one Cu layers is observed. A high density intergrowth in PbBa0.7Sr1.3YCe3Cu3O13 is also shown in Figure 28B. In this image, the intergrowth of various numbers of (Y, Ce) layers was observed, and disappearance of (Pb, Cu) layers is also observed as indicated by arrows, and extended regions of the (Y, Ce) layers can be seen. Observations of the intergrowth give us variable information on the possibility of appearance of new unknown structures and on the transitional structures to the equilibrium state.

HREM images of (A) PbBaSrYCu3O7 and (B) PbBa0.7Sr1.3YCe3Cu3O13 taken with the incident beam parallel to the a-axis. Number of (Y, Ce) layers are shown in (B).
Figure 28

HREM images of (A) PbBaSrYCu3O7 and (B) PbBa0.7Sr1.3YCe3Cu3O13 taken with the incident beam parallel to the a-axis. Number of (Y, Ce) layers are shown in (B).

Lattice defects, particularly dislocations in the superconductors having complex structures, are an interesting subject to study mechanical and electrical properties of these materials. Figure 29A is an end-on view of a dislocation observed in the Tl2Ba2CuO6. The dislocation is laid along the [11̅0] direction parallel to the incident beam direction. An extra plane can be seen by obliquely viewing Figure 29A along the vertical direction. Figure 29B is a Fourier transform of Figure 29A, and Figure 29C is an image reconstructed by using 110 and 1̅1̅0 reflection in the Fourier diffractogram of Figure 29B. Figure 29D is an enlarged image of Figure 29C. In the image of Figure 29D, a long range strain field over 4 nm in diameter is observed around the dislocation core. Although a Burgers vector of this dislocation cannot be determined from only this image, it would be [100] of the perovskite unit. The smallest Burgers vector in the perovskite unit is 1/2 [111] position, but the dislocation with the [100] Burgers vector is possibly stable in this structure because a different type of atom is placed at the 1/2 [111] position. This dislocation would be considered to be formed during crystal growth, and it would be placed in the most stable state.

(A) High-resolution image of an end-on view of a dislocation in Tl2Ba2CuO6, taken with the incident beam parallel to the [11̅0] direction. (B) Fourier transform of (A). (C) Image reconstructed by using 110 and 1̅1̅0 reflection in the Fourier diffractogram of (A).  (D) Enlarged image of (C).
Figure 29

(A) High-resolution image of an end-on view of a dislocation in Tl2Ba2CuO6, taken with the incident beam parallel to the [11̅0] direction. (B) Fourier transform of (A). (C) Image reconstructed by using 110 and 1̅1̅0 reflection in the Fourier diffractogram of (A). (D) Enlarged image of (C).

Understanding of interface and surface structures is important for application of the superconductors to electronic devices such as Josephson junction and superconducting transistor [118–122], as illustrated in Figure 30A and B, respectively. Recently, the surface structure has been widely studied with scanning tunneling microscopy in the atomic scale. High-resolution electron microscopy has also made possible to observe atomic structures of interfaces and surfaces in the superconductors. Figure 30C is a one-dimensional lattice image of an anti-phase boundary in Tl2Ba2CuO6, which was observed perpendicular to the c-axis. Tl layers are indicated by arrows, and the phase displacement is 0.2c along the c-axis at the interface. This indicates that the displacement corresponds to one block of the perovskite cell, and the Tl layers are connected with Cu layer at the interface. Figure 30D is a structure image of an anti-phase boundary in TlBa2CaCu2O7 taken along the a-axis. The interface is {103} of the TlBa2CaCu2O7 structure, and the phase displacement is 0.21c+0.5a. This indicates that the displacement corresponds to half block of the perovskite cell, and the Tl layers are connected with the Ba layers at the interface. Small disordering of atoms is observed at the interface within ∼2 nm, which would work as insulator layers between two crystals. These interfacial structures with distances of a few nanometers are suitable for the Josephson junction devices as shown in Figure 30A.

Schematic illustration of (A) Josephson junction and (B) superconducting transistor. HREM images at the interfaces of (C) Tl2Ba2CuO6 and (D) TlBa2CaCu2O7.
Figure 30

Schematic illustration of (A) Josephson junction and (B) superconducting transistor. HREM images at the interfaces of (C) Tl2Ba2CuO6 and (D) TlBa2CaCu2O7.

Surface structures are important information on the superconducting transistors as illustrated in Figure 30B. Figure 31A is a HREM structure image of TlBa2Ca3Cu4O11 taken with the incident beam parallel to the a-axis. The image is observed without any contamination layers at the sample edge, and an atomic arrangement can be directly observed at the crystal surface. High-resolution images were obtained from thin samples, which were selected from crushed materials dispersed in a solution of n-butanol and dropped on holely carbon films. A mixing of Tl, Ba, Ca, and Cu atoms near the surface is observed. Figure 31B is a HREM structure image of Tl0.5Pb0.5Sr2CuO5 taken along the a-axis together with a projected structure model, which indicates no preferential surface structure. Figure 31C is a HREM structure image of Pb2Sr2Y0.5Cu3O8, and a characteristic surface structure with valleys at Pb layers and hills at between the Pb layers is observed. In the region of the hills, mixing of atoms appears to occur. A HREM structure image of PbBa0.7Sr1.3EuCeCu3O9 taken along the a-axis is shown in Figure 31D, which indicates preferential atomic arrangements on the c-plane of the crystal. A HREM structure image of TlBa2CaCu2O7 is also shown in Figure 31E. At the crystal edge, preferential fracture occurs between Tl and Ba layers, as indicated by arrowheads, and the fracture surface has steps with Tl layers. This result shows that the fracture surface parallel to the c-plane in the superconductor is stable and chemically unreactive to the solution and the atmosphere. On the other hand, the surface structures of the fracture nearly perpendicular to the c-axis show complex structures formed by rearrangement of atoms near the edge. However, Hg-based copper oxides showed stable surface [123, 124]. The characteristics of various surface structures should be taken into consideration, when the properties are sensitive to surface structures.

HREM images of (A) TlBa2Ca3Cu4O11, (B) Tl0.5Pb0.5Sr2CuO5, (C) Pb2Sr2Y0.5Ca0.5Cu3O8, (D) PbBa0.7Sr1.3EuCeCu3O9, and (E) TlBa2CaCu2O7, taken along the a-axis together with projected structure models.
Figure 31

HREM images of (A) TlBa2Ca3Cu4O11, (B) Tl0.5Pb0.5Sr2CuO5, (C) Pb2Sr2Y0.5Ca0.5Cu3O8, (D) PbBa0.7Sr1.3EuCeCu3O9, and (E) TlBa2CaCu2O7, taken along the a-axis together with projected structure models.

11 Summary

The microstructural information on the perovskite-type copper oxide superconductors is useful for the materials’ development for superconducting devices such as high-performance superconducting wires, nuclear electromagnetic resonance analytical systems, linear motor car and the international thermonuclear experimental reactor [125–127], and for the theoretical analysis of superconducting mechanism [128–130]. High-resolution electron microscopy and electron diffraction are quite useful for characterization of structures of high-Tc superconducting oxides on the atomic scale. Valuable information on crystal structures from the observed high-resolution images, severe conditions, such as thinner crystals than 2 nm, and definite defocus values of -35 to -45 nm are required. The structure images taken under the severe conditions include information not only on arrangements and accurate coordinates of cations but also on ordered arrangements of oxygen atoms and oxygen vacancies. Many examples of crystal structure analysis of the perovskite-type superconductors were presented from the observed structure images and electron diffractions. Defects, surface, interface, intergowth, and dislocations, which cannot be investigated by diffraction methods, were observed and examined. Modulated superstructures were also observed in various copper oxides, which showed satellite reflections with various modulation wave vectors. The appearance of superstructures is sensitive to the oxygen content, and the superlattices would be due to the ordering of oxygen atoms in the Cu-O planes. Thin films and bulk materials with high Jc were also investigated, and many precipitates showing Moiré fringes were observed in the image, which could act as the flux-pinning centers resulting in the high Jc. Such nanostructural analysis will have an important role for the future development of perovskite-type superconducting copper oxide materials.

Acknowledgments

The author would like to acknowledge K. Hiraga, S. Nakajima, A. Tokiwa, M. Kikuchi, D. Shindo, M. Hirabayashi, Y. Syono, T. Kajitani, H. Yamane, K. Takagi, T. Hirai, Y. Tokura, E. Bruneel, S. Hoste, K. Osamura, T. Kizu, K. Kosuge, N. Kobayashi, and S. Hosoya for excellent collaborative works, providing samples and fruitful discussion.

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About the article

Takeo Oku

Takeo Oku is a Professor of Department of Materials Science at The University of Shiga Prefecture (2007–). He received his PhD from Department of Nuclear Engineering at Tohoku University. He joined the Kyoto University as a Research Associate (1992–1996), National Center for HREM at Lund University as a Postdoc (1996–1997), Institute of Scientific and Industrial Research at Osaka University as an Associate Professor (1997–2007), and Department of Physics, Cavendish Laboratory at University of Cambridge as a visiting scientist (2007–2008). His general research areas include structure analysis by high-resolution electron microscopy, fabrication and characterization of next-generation solar cells and other energy materials. He is the author and co-author of more than 210 papers in ISI journals.


Corresponding author: Takeo Oku, Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan, e-mail:


Received: 2014-02-11

Accepted: 2014-03-31

Published Online: 2014-06-19

Published in Print: 2014-10-01


Citation Information: Nanotechnology Reviews, Volume 3, Issue 5, Pages 413–444, ISSN (Online) 2191-9097, ISSN (Print) 2191-9089, DOI: https://doi.org/10.1515/ntrev-2014-0003.

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