Lead zirconate titanate (Pb(Zr,Ti)O3) (PZT), a non-centrosymmetric material obtained via solid-solution of ferroelectric PbTiO3 (Tc = 490°C) and antiferroelectric PbZrO3 (Tc = 230°C) compounds, has been widely used in piezoelectric and ferroelectric devices such as transducers, sensors and ferroelectric random access memory. In modified form, this material is revolutionizing applications ranging from medical ultrasound and sonar to energy harvesting (Cohen 2006; Ahart et al. 2008; Cowley et al. 2011). Recent investigations have focused on the development of PZT-based high-performance relaxor modified compositions as exemplified by the pseudo-ternary composition Pb(Zr,Ti)O3–Pb(Zn1/3Nb2/3)O3 (PZT–PZN) system (Uchino 1997; Yin, Zhu, and Zeng 2009). Both PZN and PZT exhibit perovskite (ABO3) lattice structure in which the A-site is occupied by Pb2+ ions and the B-site by Zn2+, Zr4+, Ti4+ and Nb5+ ions (Nonnenmann, Gallo, and Spanier 2010; Ryu et al. 2011). The solid solution of PZN–PZT has a rhombohedral/tetragonal morphotropic phase boundary (MPB) at or above ≥ 70 mol% PbZrxTi1−xO3 (0.52 ≤ x ≤ 0.54) (He et al. 2009; Fan and Kim 2002; Wang et al. 2012). Monoclinic phases such as Pm and Cm have been reported to exist around the MPB, acting as an intermediate bridging phase that allows polarization to rotate from  to  (Ahart et al. 2008; La-Orauttapong et al. 2002; Rahman, Metselaar, and Karmaker 2014; Pandey and Singh 2014; Li and Chou 2003, 2006).
PZN ceramics exhibit good dielectric and electrostrictive properties; however, it is difficult to synthesize single phase polycrystalline perovskite PZN ceramic because steric and electrostatic interaction between high polarization of the Pb2+ and the Zn2+ cations makes it thermodynamically unstable over a wide range of temperatures (600–1,400°C). This leads to the formation of a pyrochlore phase and PbO given by the decomposition reaction [3Pb(Zn1/3Nb2/3)O3 → Pb2Nb2O7 + ZnO + PbO] (Fan and Kim 2002). The columbite method has been found to be effective in eliminating the formation of a pyrochlore phase in some relaxor ceramics, e.g. Pb(Mg1/3Nb2/3)O3 (Haertling 1999; Wu et al. 2008). However, this method is still challenging for PZN-based ceramic systems; and PZN ceramics synthesized using these processes are opaque. Although pure PZT is opaque, lanthanum doping on the A-site has been shown to impart transparency (Zeng et al. 2005a, 2005b, 2006, 2007; Shannon, 1976; Etin et al. 2007). Hence, La incorporation in PZN–PZT ceramics has been an effective method to induce transparency and at the same time improve the dielectric, piezoelectric and opto-electric properties of compositions near the MPB.
Furthermore, La-doped Pb(ZrTi)O3 (PLZT) has been used to harvest thermal energy using pyroelectric effect (Chin et al. 2012). During pyroelectric energy harvesting, system generates power from temperature fluctuations in contrast to the case of thermoelectric energy harvesting, which utilizes temperature gradient (Bowen et al. 2014). The PLZT-based epitaxial thin films on Nb-doped SrTiO3 substrate, with Au/PLZT/Pt structure, has been reported to exhibit large photovoltaic output (Qin, Yao, and Liang 2010). Moreover, researchers investigated the effect of UV light on the output of the radial mode piezoelectric transformer based on PLZT (Kozielski, Adamczyk, and Erhart 2011). They reported substantial effect of UV light on efficiency and voltage step-up ratio of these piezoelectric transformers due to coupling between piezoelectricity and photostriction. Therefore, the PLZT-based materials are photoactive and can be used not only to harvest vibration energy (like other piezoelectric materials) but also can be used to harvest light and thermal energy. Other wide band materials such as zinc oxide have also been used for thermal energy harvesting (Zhao et al. 2014; Hussain et al. 2014; Feldhoff and Geppert 2014).
In this study, solid solutions of La-doped PZN–PZT ceramics with composition of 0.3Pb(Zn1/3Nb2/3)O3 –0.7Pb0.97La0.03(ZrxTi1−x)O3 (0.52 ≤ x ≤ 0.54) have been synthesized and investigated for their electro-mechanical, ferroelectric, dielectric and opto-electrical properties. Illumination wavelength-dependent current–voltage behavior was quantified. Raman studies revealed the presence of polar nanoregions (PNRs) persisting much above the Curie temperature. The interaction of these PNRs with optical radiation in the given temperature regime could present intriguing opto-electric interactions.
Transparent ceramics with a composition of 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (0.52 ≤ x ≤0.54) were synthesized through the solid state reaction using PbO, ZnO, Nb2O5, La2O5, ZrO2 and TiO2 powders. The precursor ZnNb2O6 (JCPDS # 37–1371) used for the synthesis was obtained by the solid state reaction of ZnO and Nb2O5. Structural analysis was conducted by x-ray diffraction with CuKα radiation. Microstructures were observed using a scanning electron microscope (Zeiss Model Leo 1550). P–E hysteresis measurements were performed using a ferroelectric tester (Radiant Technologies Inc. Model Precision Premier II). Electromechanical coupling factor (kp) was calculated using resonance and anti-resonance frequencies (Hewlett Packard Model 4194A) and a UV-Vis-NIR spectrophotometer (Hitachi U-4100) was used for transmittance measurements. Raman spectra were recorded in the range of 100–1,000 cm−1 using a Raman spectrometer (JY Horiba LabRam HR 800, Horiba Ltd., Japan) equipped with a CCD detector and a 514.5 nm argon laser of 50 mW power at the source. All samples had a Zr/Ti ratio of 0.54/0.46 and were ball milled for 48 h if not mentioned otherwise.
Results and discussion
X-ray diffraction (XRD) patterns of 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (0.52 ≤ x ≤ 0.54) transparent ceramics are shown in Figure 1(a). These exhibit formation of a perovskite phase with or without a pyrochlore (Pb2Nb2O7, JCPDS # 40–0828) phase, depending upon the processing duration, initial precursors and stoichiometric excess of elements such as Pb, Nb, La or Zn and/or Zr/Ti ratio. Figure 1 (a-u2) and (b-1) and (a-u16) and (b-5) shows XRD patterns of 5% stoichiometric excess of Pb at Zr/Ti ratios of 54/46 (x = 0.54) and 52/48 (x = 0.52), respectively. The excess lead revealed selective elimination of the Pb2Nb2O7 phase, i.e. samples with a Zr/Ti ratio of 52/48 exhibited complete removal of Pb2Nb2O7 phase. The ceramics synthesized with a Zr/Ti ratio of 54/46 exhibited reduced formation of the Pb2Nb2O7 phase when the ball milling duration was increased to 144 h. Use of the ZnNb2O6 precursor in place of ZnO and Nb2O5 at the Zr/Ti ratio of 54/46 also showed reduced formation of the Pb2Nb2O7 phase. These ceramics were further investigated for the stoichiometric excess of elements such as Nb, La and Zn. Stoichiometric excess of La and Nb showed none or negligible effect on the formation of the Pb2Nb2O7 phase (Figure 1(a)–(b)). However, at 5% stoichiometric excess of Zn, the complete leaching of a pyrochlore phase was obtained (Figure 1(b-4)). Scanning electron micrographs of the cross-section of 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 ceramics (figures not shown due to space limitations) showed high uniformity and density.
The formation of secondary phase has been attributed to the Pb deficiency, and therefore, excess Pb could lead to crystallization of pure perovskite phase. However, the complete extinction of the Pb2Nb2O7 phase (at 5% excess stoichiometric Pb) was observed only at a Zr/Ti ratio of 52/48 (but not at Zr/Ti ratio of 54/46) and can be interpreted as follows. Zirconium (Zr) is a tetravalent ion, while titanium (Ti) can occur in the +4, +3, and +2 valence states. However, in this study samples were synthesized in oxygen ambient and, therefore, Ti will be mainly in Ti4+ state. The ionic radii of Zr4+ and Ti4+ in sixfold coordination are 0.72 and 0.605 Å, respectively (Miller, Vandome, and McBrewster 2010). Replacing Zr with Ti results in lattice contraction because of the smaller radius of Ti4+ compared to Zr4+ ions. This, in turn, may reduce the B–O bond length of ABO3 type structure and provide stability to the perovskite structure (Zhang et al. 2006). Further, using the columbite method and increasing ball milling duration both results in reduced formation of Pb2Nb2O7. Addition of excess ZnO has been found to stabilize the perovskite phase. Also, 5% stoichiometric excess of zinc resulted in the complete removal of the Pb2Nb2O7 phase (Zeng et al. 2005a). Figure 1(c) shows the variation in the electro-mechanical coupling factor (kp) as a function of stoichiometric excess of various elements and processing conditions. Increasing Pb content reduces kp in all samples. Samples synthesized using individual oxide powders as precursors and using the columbite method both show similar behavior of kp with Pb excess. However, the columbite method resulted in a lower value of kp ~53%. In lead-based piezoelectric materials, Pb vacancies are inevitable during processing at higher temperature. However, these Pb vacancies could generate oxygen vacancies according to Schottky equilibrium (null → VPb + V˚˚O) to maintain local charge neutrality (Huang, Chen, and Wu 2004; Wu and Cao 1999). Niobium and lanthanum act as soft doping ions on B- and A-sites, respectively. The substitutions of Nb5+ on B4+ sites and La3+ on A2+ sites, results in extra positive charges. This extra charge could reduce the formation of oxygen vacancies due to Schottky equilibrium leaving only Pb vacancies in the system (Kubinski and Holloway 1995). These Pb vacancies have been considered to facilitate the domain wall motion resulting in high electro-mechanical response in soft piezoelectric materials (Flygare and Gierke 1974). Therefore, the increased value of kp, in the case of excess niobium and lanthanum content could be attributed to these Pb vacancies. Moreover, excess zinc addition resulted in a continuous increase in kp, due to reduced pyrochlore phase (Zeng et al. 2005a). The specimen processed with extended ball milling duration (144 h) resulted in higher kp values, independent of the processing method or excess stoichiometric content of various elements. The highest values of kp ~62.3% were observed in samples with 5% stoichiometric excess of Zn and 5% Pb (Zr/Ti ratio of 0.52/0.48).
Figure 1(d) shows the variation in dielectric permittivity with temperature in the frequency range 100 Hz–15 MHz for each sample. The dielectric permittivity variation with frequency is not significant, and, therefore, the various curves taken at different frequencies seem to overlap (Figure 1 (d)). The Curie temperature (Tc) for the stoichiometric sample synthesized at a Zr/Ti ratio of 54/46 is about 222°C. However, the sample containing 5% excess Pb exhibited Tc of 245°C at a Zr/Ti ratio of 52/48. As noted above, the ionic radii of Zr4+ and Ti4+ in sixfold coordination are 0.72 and 0.605 Å, respectively (Miller, Vandome, and McBrewster 2010). The smaller size of the Ti4+ ion in the octahedral site increases the rattling tendency, which, in turn, elevates the Curie temperature (Flygare and Gierke 1974). The substitution of La3+ and Nb5+ upto certain extent of concentration results in similar effect on lattice relaxation and could lead to the relaxation of oxygen framework to the tilted structure (Seinfeld and Pandis 2006). Also, there is none or negligible formation of the Pb2Nb2O7 phase, indicating that most of the Nb5+ ions occupy the “B” lattice sites. The Curie temperature of 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (x = 0.54) at 0% stoichiometric excess lead content and synthesized using single oxides or ZnNb2O6 precursor processed for 48 h, is 222°C. The Curie temperature is marginally decreased (~220°C) at increased ball milling time (144 h). The sample with 5% excess stoichiometric Zn exhibited the lowest Curie temperature (217°C). Excess Zn content restricted the formation of the Pb2Nb2O7 phase completely and, therefore, retained all the Nb5+ in the ABO3 type lattice. Inserting zinc as +2 and Nb as +5 into “B” sites in an ABO3 type structure requires charge compensation and, in turn, may reduce the formation of oxygen vacancies. Also, the larger ionic radius of Zn2+ (0.74 Å) in sixfold coordination in comparison to Nb5+ (0.64 Å) and Ti4+ (0.605 Å) reduces the size of rattling space and, in turn, will reduce the tendency of Zn2+ ions to rattle, leading to a reduced Curie temperature.
Figure 1(e) shows the polarization versus electric field (P–E) hysteresis curves for the 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 samples. Samples with a stoichiometric composition and Zr/Ti ratio of 54/46 exhibit a saturation polarization (Ps) value of ~39.4 µC/cm2. This value is slightly increased (~40.0 µC/cm2) if the samples were synthesized using ZnNb2O6 precursor. Increased ball milling duration of precursors (~144 h) resulted in a further increase in the Ps value (~41.6 µC/cm2) due to reduced formation of secondary phase. An excess stoichiometric content of 5% Zn increased Ps values further to 42.5 µC/cm2. The samples with 5% excess stoichiometric lead content at x = 0.52 exhibited the highest Ps values of 43.6 µC/cm2. Furthermore, the stoichiometric 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 sample was found to exhibit coercive field (Ec) ~9.49 kV/cm. This value is slightly decreased in the samples prepared using the columbite method (9.1 kV/cm). Ball milling of the samples for longer durations, i.e. up to 144 h, further reduced the coercive field to 8.76 kV/cm. The values of Ec for 5% excess Pb (Zr/Ti ratio 52/48) and 5% Zn were found to be 9.7 and 9.9 kV/cm, respectively.
Figure 2(a) shows the optical transmittance of the 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 samples. The samples with 5% excess stoichiometric zinc content at x = 0.54 exhibited the highest transmittance in the visible region followed by the transmittance of the samples containing 5% excess lead at x = 0.52 (the transmittance values of the samples with 5% excess lead at x = 0.52 were highest above a wavelength of 800 nm). The transmittance (T) values for other samples increased according to the relation: samples synthesized using individual oxides < samples synthesized using the columbite method < samples synthesized with extended ball milling time of 144 h. The T values for these samples were smaller than those of the samples synthesized using 5% excess Zn content and/or 5% excess Pb (at Zr/Ti ratio of 52/48). Insets in Figure 2(a) show the transmittance image and a photomicrograph of a typical sample. Microstructural variables such as density, grains size, microscopic pores, point defects and secondary phase modify the transmittance behavior (Hu et al. 2010). The poor transmittance value in pyrochlore containing sample can be attributed to the increased scattering through its segregation at grain boundaries. Also, the progressive increase in transmittance value with increase in wavelengths results due to grain size and wavelength-dependent scattering effects (Wang 2010).
The variation of optical absorbance as a function of wavelength, where h denotes Planck constant and ν is the frequency of radiation, is shown in Figure 2(b). The absorbance data were obtained in transmission mode for the sample thickness of 0.35 mm (for each sample). The variation of (αhν)2 as a function of energy (hν), where α is the absorption coefficient, is displayed in the inset of Figure 3(b). The band gap energy Eg is obtained by extrapolating the linear portion of the graph to energy axis at α = 0. The optical band gap (Eg) estimated using the relation, (αhυ) = C(hυ−Eg)n, where C is the constant and n = ½ for a direct allowed transition, were 2.88, 2.90, 2.90, 2.92, and 2.92 eV for the samples with excess (1) 0% Pb, (2) 0% Pb-columbite method, (3) 0% Pb ball milling for 144 h, (4) 5% Zn and (5) 5% Pb, Zr/Ti ratio 52/48, respectively. Chemistry at A- and B- sites, more specifically, the ionic sizes modify the band gap (Wang 2010). However, the B–O bonding characteristics predominates the optical band-gap over A-site chemistry. In the present study, the higher band gap values in the case of 5% excess Zn (Zr/Ti ratio 54/46) and 5% excess Pb (Zr/Ti ratio of 52/48) can be attributed to the increased content of larger size ions, i.e. Zn2+ (0.74 Å) and Zr4+ (0.72 Å) at B sites. Since, 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (x = 0.54) ceramics at the stoichiometric excess of 5% Zn exhibit the highest optical band gap and transmittance (in the visible range), these ceramics were further investigated to examine their current–voltage behavior to identify their possible applications in electro-optical-devices.
The low stoichiometric excess of all elements, e.g. Nb, La and Zn in 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (x = 0.54), revealed the improved characteristics. To understand its relation with domain shape, size and orientation, we investigated two samples; one of stoichiometric composition (sample code S1) and other with 1.5% stoichiometric excess of Nb (sample code S2) using Piezoresponse force microscopy (PFM, Dimension Icon, Bruker). Figure 3 (a)–(d) shows the PFM phase and amplitude mapping of samples S1 and S2 respectively (of approximately 100 μm thickness). It was noticed that all the piezoreponse images exhibited a strong domain contrast: deep bright and dark areas indicating significant out-of-plane component of polarization. We further observed that along with the decrease in size, domains themselves became more oriented for the specimen with stoichiometric excess of 1.5% Nb. Nano-sized strip domains with size ~100–200 nm were found in the sample S2. The piezoelectric response has been considerably found to depend on the domain sizes as the systems with easily switchable nano-sized domains could exhibit a large improvement in functional response. The field-induced polarization transition in S2 sample is explored through the polarization switching experiments (Figure 3 (e)–(h)). Prior to PFM characterization, square polarization pattern (2.5 μm × 2.5 μm) was generated in the sample by scanning the surface with an applied dc bias voltage of 0 V, +4 V, +6 V and +10 V, respectively. The enlarged bright contrast of the phase image before and after the application of electric field confirms the strong polarization switching in the sample. In the absence of external electric field, the strip bright/dark contrast is uniform. After the application of external electric field, the corresponding polarization dramatically increased with deep bright area, due to reversal in polarization orientation. Meanwhile, PFM amplitude images taken at each voltage step exhibit a uniform contrast, with the polarized-square pattern amplitude steadily increasing with each incremental step. The depolarization field, due to presence of these ferroelectric domains having different polarization, could facilitate the separation of photo-generated charge carriers modulating the electrical output of the system.
Figure 4(a) shows the spectral dependence of the current–voltage (I–V) curves of a typical 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (x = 0.54) ceramic at the stoichiometric excess of 1.5% Nb. The current varies as a function of illumination wavelength; and the illumination of the sample with the shorter wavelength radiation exhibits higher current density. Obviously, the value of current density at 300 nm radiation is higher than of 700 nm, though the lamp profile shows comparative higher power at 700 nm. This is mainly because of high absorbance of the sample at 300 nm than at 700 nm (Figure 2(b)). The value of current density at 500 nm is higher than of 300 nm due to strong power of lamp at 500 nm. In Figure 4(a) these samples were illuminated with xenon lamp, which, in turn, has lower irradiance in comparison to laser light (Figure 4(b)). Figure 4(b) shows the I–V curves obtained in dark (before light interaction), by illumination of the sample with 530 nm laser light, and in dark (after light interaction), respectively. Obviously, the laser (530 nm) light exhibits larger change in the value of current in comparison to the lamp light. The dark current measurements performed after illumination study matches well with the curve collected before measurements (inset Figure 4(a)). The mechanism of light interaction may be interpreted as follows. For piezoelectric materials, recently, it has been proposed that the stress can lead to change in local electronic band structure and cause its tilt (Seinfeld and Pandis 2006; Hu et al. 2010). The 10% tensile stress in zigzag boron nitride nano-ribbons was found to reduce the band gap energy value from 3.5 eV to 1.0 eV (Qi et al. 2012). Thus, the applied voltage during the I–V measurement may lower the band-gap energy slightly at the +ve potential (tensile stress) and increased at –ve (compressive stress), which, subsequently, will lead to light interacting charge recombination device similar to P–N junction (Qi et al. 2012). The other possible contributions of current may be due to heating through absorbed light, and the interaction of radiation with crystal lattice and generation of electric dipoles (though transmitted light).
To examine the effect of temperature and suitability of such devices, if operated in extreme conditions, we collected laser Raman spectra of the sample 0.3Pb(Zn1/3Nb2/3)O3–0.7Pb0.97La0.03(ZrxTi1−x)O3 (x = 0.54) having stoichiometric excess of 5% Zn. In Figure 5, the Raman modes Rh, R1 and E(2TO2) correspond to the R3m structure (rhombohedral), modes A1(1TO), A1(3LO), E(4LO), A1(3TO), E(4TO) and E(2TO1) appear due to P4mm (tetragonal) structure, and modes E(2LO), A1(2TO) and Silent (E + B1) are caused by both R3m and P4mm structures. Clearly, the sample consists of a MPB phase with rhombohedral and tetragonal structure and a total of 12 Raman modes (He et al. 2009; Fan and Kim 2002; Wang et al. 2012; La-Orauttapong et al. 2002; Rahman, Metselaar, and Karmaker 2014; Pandey and Singh 2014). These samples were further investigated as a function of temperature. Interestingly, the soft transverse modes A1(1TO) were present at higher temperature than the macroscopic transition temperature and exhibited a redshift in frequency with increase in temperature. Appearance of these phonon modes at higher temperature than the macroscopic transition temperature indicates order–disorder type transition due to the presence of PNRs with rhombohedral–tetragonal phases (similar features can also be observed in E phonon modes) (Zhang et al. 2011). The localized PNRs observed at higher temperatures may have different nature of interaction with optical radiation (than observed at room temperature Figure 3) and, in turn, could lead to intriguing opto-electric-interactions.
In conclusion, stoichiometric excess Pb, Nb, La and Zn revealed variable effects on properties. Improved properties were noticed in stoichiometric excess of (i) 5% Zn at Zr/Ti ratio of 54/46 and (ii) 5% Pb at Zr/Ti ratio of 52/48. At lower excess stoichiometric content of various elements, the reduced domain size and their ordering contributed toward improved properties. The results of this study will lead to synthesis of materials with superior electro-mechanical, ferroelectric, dielectric, optical, opto-electrical properties for applications in wireless opto-electric-devices working over a wide range of temperatures.
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About the article
Published Online: 2014-12-02
Published in Print: 2014-12-01
Research funding: This work was supported by the Office of Basic Energy Sciences, Department of Energy through the grant number DE-FG02-06ER 46290.