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Publicly Available Published by De Gruyter December 7, 2021

Investigation of the BaO-CaO-CuO Ternary System at 850 °C

  • J.Q. Li , C.C. Lam , J. Feng , K.C. Hung and Eric C.L. Fu

Abstract

The phase relationships of the BaO–CaO–CuO ternary system at 850 °C in air was investigated by X-ray powder diffraction, scanning electron microscopy and electron probe microanalysis techniques. The samples were prepared by solid state reaction method. The existence of three binary oxide compounds, Ba2CuO3, BaCuO2 and Ca2CuO3 was confirmed and no other intermediate phases were found in this system at 850 °C. The isothermal section consists of six single-phase regions, nine two-phase regions and four three-phase regions. No carbonate was detected. However, barium dioxide, BaO2, was found in BaO-rich corner of this system.

At 850 °C, Ba2CuO3 keeps its high temperature structure, being tetragonal with a = 1.2978 nm and c = 0.3992 nm. BaCuO2 has a cubic crystal structure with a = 1.82855 nm, and Ca2CuO3 is orthorhombic with a = 0.32573 nm, b = 0.37769 nm and c = 1.2234 nm.

1 Introduction

Since the newly discovered high-temperature superconductor of mercury-copper oxide, many scientists all over the world were stimulated with a great interest to investigate the properties of these high-temperature superconductors, see [1, 2]. It is interesting to note that the synthesis of these materials has met some difficulties due to the decomposition behaviour of mercury oxide, HgO, into its constituents, namely Hg and O2. As reported by many researchers, the Hg-based high-Tc superconductors of the homologous series HgBa2Can–1CunO2n+2 (abbreviated as Hg – 12 (n – 1)n) were achieved by a solid state reaction either in vacuum or under high pressure. For the successful preparation of samples of the homologous series Hg – 12(n – l)n, one crucial factor is the quality of the Ba–Ca–Cu–O precursor [3, 4]. Ref. [5] studied the influence of the precursor on the formation of the Hg-1223 phase, namely HgBa2Ca2Cu3O8+δ, under high pressure. Generally, the starting materials, BaO, CaO and CuO, used for preparing the precursor were of analytical purity grade. Relationships between phases existing in the BaO–CaO–CuO ternary system have not yet been reported. To provide the necessary information for the synthesis of good quality Hg-based superconductors, a phase diagram is required for illustrating the relationship between phases possibly existing during the synthesis. In this work, the phase relations in the ternary system BaO– CaO–CuO at 850 °C under air were investigated. The crystal structures of phases in sintered sample were studied by means of X-ray diffraction (XRD) with CuKα radiation.

The phase relations of the BaO–CuO binary system are taken from Ref. [6]. The compounds appearing in this system are Ba2CuO3 and BaCuO2. Note that the Ba2CuO3 compound changes its crystal structure from orthorhombic to tetragonal at 810 °C. Similarly, the phase relations of the CaO–CuO binary system are adopted from Ref. [7]. Two intermediate compounds, Ca2CuO3 and CaCuO2, exist in this system. The compound Ca2CuO3 decomposes incongruently at 1030 °C into CaO and liquid. CaCuO2 is formed by a peritectoid reaction between Ca2CuO3 and CuO at 740 °C. The phase diagram of the BaO–CaO binary system is adopted from Ref. [8]. No intermediate compound exists in this system. To verify the binary phase relations, we have prepared several samples in each of the three binary systems. The results are incorporated in the three boundaries of the ternary phase diagram.

2 Experimental Detail

Twenty eight samples were prepared by the conventional solid state reaction method using BaO, CaO and CuO of analytical purity grade. The powders of these reagents were weighed according to their molar fraction in the corresponding sample, and then they were mixed and ground carefully in an agate mortar. Our experiments showed that barium oxide, BaO, turn into BaO2 and became liquid above 450 °C by absorbing O2 from air during its heating process. Therefore, mixtures of raw powders in the samples which contain more than 50 mol.% BaO were calcined at 420 °C in an alumina crucible in air for 48 h to avoid the formation of liquid BaO2. The other samples were calcined in an alumina crucible in air at 900 °C for 30 h. After calcination the powders were ground, pressed into small pellets, and sintered at 850 °C for 30 h in air. The sintering steps were performed twice after intermediate grinding and pressing during the calcination to assure complete homogeneity. After sintering, the sample were taken from the furnace and normalised in still air. Since the samples were small, the normalization process, in fact, was equivalent to a quenching heat treatment. Thus, the phase structures of the samples were retained at 850 °C. The crystal structures were substantially verified by XRD patterns taken at room temperature.

Phase identification was performed by using XRD with CuKα (Siemens D500) and JCPDS standard data. Judging from the XRD patterns of various samples, we found that the solid state reactions having occurred in the samples were fully complete. Scanning electron microscopy and electron probe microanalysis (JSM-820, 20 kV) have been performed on selected samples. The results are combined with the X-ray diffraction analysis to achieve reliable interpretations.

3 Results

3.1 Phase Analysis

The existence of the three intermediate compounds, Ba2CuO3, BaCuO2 and Ca2CuO3 in the ternary system at 850 °C was confirmed by XRD (Fig. 1). Single-phase samples were obtained at the respective nominal stoichiometry, i. e. the solid state reactions are sufficiently complete under the present experimental conditions.

Figs. 1a to c. X-ray diffraction patterns for specimens with compositions of (a) Ba2CuO3, (b) BaCuO2 and (c) Ca2CuO3.
Figs. 1a to c.

X-ray diffraction patterns for specimens with compositions of (a) Ba2CuO3, (b) BaCuO2 and (c) Ca2CuO3.

Comparing the X-ray diffraction data of BaCuO2 and Ca2CuO3 with the JCPDS cards for BaCuO2 (38-1402) and Ca2CuO3 (34-282) leads to the conclusion that they correspond to individual single phases. The XRD analysis shows that BaCuO2 is cubic with a = 1.82855 nm instead of orthorhombic as reported in Ref. [9], while Ca2CuO3 has an orthorhombic crystal structure with a = 0.32573 nm, b = 0.37769 nm and c = 1.2234 nm. Ba2CuO3, formed at high temperatures, crystallizes tetragonally. Despite quenching it remains in this structure with lattice constants a = 1.2975 nm and c = 0.3992 nm being in good agreement with [10].

3.2 Phase Relations at 850 °C

By comparison and analysis of the X-ray diffraction patterns of 28 samples, and combination with the results obtained from the scanning electron microscopy and electron energy-dispersive spectroscopy (EDS), we can accurately identify the phases in each sample. Thus, we can determine the phase relations in the BaO–CaO–CuO ternary system at 850 °C in air, as shown in Fig. 2. The isothermal section displays six single-phase regions, nine two-phase regions and four three-phase regions. The six single-phase regions are BaO, CaO, CuO, Ba2CuO3, BaCuO2 and Ca2CuO3. No carbonate was detected, while barium dioxide, BaO2, was found in the BaO-rich corner of this system. This is due to the reaction between BaO and the oxygen in air. Since the appearance of the BaO2 phase is referred to temperatures below 450 °C, it can be excluded from the 850 °C section.

Fig. 2. Phase relationship of the BaO–CaO–CuO ternary system at 850 °C. ● Single-phase region; ◒ Two-phase region; ○ Three- phase region.
Fig. 2.

Phase relationship of the BaO–CaO–CuO ternary system at 850 °C. ● Single-phase region; ◒ Two-phase region; ○ Three- phase region.

The X-ray diffraction patterns of samples No. 21, 25 and the precursor of the Hg-1223 superconductor are shown in Fig. 3. The pattern of sample No. 21 (BaO-30 mol% CaO-40 mol% CuO) indicates three phases: BaCuO2, Ca2CuO3 and CaO (Fig. 3a). Thus, this sample lies in the corresponding three-phase region (see microstructure in Fig. 4). For sample No. 25 (BaO-30 mol% CaO-30 mol% CuO), the XRD pattern is an overlay of the patterns of Ba2CuO3, BaCuO2 and CaO (Fig. 3b), hence, its composition lies in the three- phase region Ba2CuO3 + BaCuO2 + CaO. The Hg-1223- precursor has a composition of BaO-28.57 mol.% CaO-42.86 mol.% CuO, thus it just lies in the (BaCuO2 + Ca2CuO3) two-phase region, as shown in Fig. 2. The correspondent XRD pattern is shown in Fig. 3c. The results obtained from scanning electron microscopy and the electron probe microanalysis support the XRD results.

Figs. 3a to c. X-ray diffraction patterns for specimens with compositions of (a) sample No. 21: BaO-30 mol.% CaO-40 mol.% CuO, (b) sample No. 25: BaO-30 mol.% CaO-30 mol.% CuO and (c) precursor for Hg-1223 superconductor: BaO-28.7 mol.% CaO-42.86 mol.% CuO.
Figs. 3a to c.

X-ray diffraction patterns for specimens with compositions of (a) sample No. 21: BaO-30 mol.% CaO-40 mol.% CuO, (b) sample No. 25: BaO-30 mol.% CaO-30 mol.% CuO and (c) precursor for Hg-1223 superconductor: BaO-28.7 mol.% CaO-42.86 mol.% CuO.

Fig. 4. Microstructure of sample No. 21 (BaO-30 mol.% CaO-40 mol.% CuO) lying in the BaCuO2 + Ca2CuO3 + CaO region (SEM).
Fig. 4.

Microstructure of sample No. 21 (BaO-30 mol.% CaO-40 mol.% CuO) lying in the BaCuO2 + Ca2CuO3 + CaO region (SEM).

The range of solid solubility for each single-phase at 850 °C has been determined by XRD, viz. the appearance of reflections of secondary phases as well as the variation of lattice parameters with composition. By the latter method, the lattice constants need to be accurately determined. As a consequence of our XRD experiments, no shift of diffraction peaks of the different phases in the ternary system was observed with respect to the variation of composition. Consequently, no mutual solubility of the compounds appears in the phase diagram (Fig. 2).

4 Discussion

There is no evidence for the existence of CaCuO2 in the CaO–CuO binary system at 850 °C being in good agreement with [7], probably because of the instability of CaCuO2 at temperatures above 740 °C. Tsang [11] pointed out that the intermediate phase Ca3Cu7O10 was formed and became stable at 977 °C. However, in our experiment, the diffraction pattern of the sample corresponding to this composition indicates a mixture of CuO and Ca2CuO3. Therefore this sample lies in the respective two-phase region rather than in the single phase field of Ca3Cu7O10.

Wu et al. [12] regarded the Hg-1223-precursor, namely Ba2Ca2Cu3O7, as a single-phase with tetragonal structure. However, composition of the respective diffraction pattern with that of our precursor sample (Fig. 3c) revealed complete correspondence. Hence, the precursor is not a single phase but rather those of Ca2CuO3 and BaCuO2, obviously we can conclude that the precursor consists of Ca2CuO3 and BaCuO2. Similarly, we found that the Hg-1212 precursor consists of BaCuO2 and CaO, thus lying in the two phase region BaCuO2 + CaO.

Lin et al. [5] found that the ratio of Ca and Cu ions strongly affects the formation of the Hg-1223 superconductor. Furthermore, they also found that the presence of some large CaO grains in the precursor increases the synthesis temperature or time. We prepared the Hg-1223 superconductor from a mixture of HgO and the “Ba2Ca2Cu3O7" precursor by sintering at 860 °C for 6 h in a sealed silica tube. If the precursor has been prepared only by calcinating a powder mixture of BaO, CaO and CuO at 900 C for 30 h, we received the Hg-1212 superconductor. If the sintered precursor has been ground again, pressed into pellets and then calcinated at 900 °C for 30 h, we received the Hg- 1223 superconductor. This might be due to nonreacted CaO in the first precursor, whereas after the retreatment the precursor might be free of nonreacted CaO.


J.Q. Li, C.C. Lam, J. Feng, K.C. Hung, E.C.L. Fu Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue, Kowloon, Hong Kong

  1. The authors would like to express their gratitude to the Research Committee of the City University of Hong Kong for their financial support to this research topic.

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Received: 1996-12-18
Published Online: 2021-12-07

© 1997 Carl Hanser Verlag, München

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