Effect of Phase Transitions on Thermal Depoling in Lead-Free 0.94

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Phase Transitions Model on Thermal Depoling in Lead-Free Bi Na TiO – 0.6BaTiO Based Piezoelectrics 0.5

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Amit Mahajan, HangFeng Zhang, Jiyue Wu, E. Venkata Ramana, Mike J. Reece, and Haixue Yan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12501 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017

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The Journal of Physical Chemistry

Effect of Phase Transitions on Thermal Depoling in Lead-Free Bi0.5Na0.5TiO3 – 0.6BaTiO3 Based Piezoelectrics Amit Mahajan1*, Hangfeng Zhang1, Jiyue Wu1, E. Venkata Ramana2, M. J. Reece1 and Haixue Yan1* 1

School of Engineering & Materials Science, Queen Mary University of London, Mile End

Road, E1 4NS, London, United Kingdom 2

I3N-Aveiro, Department of Physics, University of Aveiro, Aveiro-3810 193, Portugal.

* Corresponding authors: [email protected] and [email protected]

Abstract Bi0.5Na0.5TiO3 – 0.6BaTiO3 (BNTBT) is a potential lead-free piezoelectric candidate to replace lead-based PZT ceramics. Thermal depoling temperature sets the upper limit for the high temperature application of piezoelectric materials. Recently, an interface model was proposed to explain the good resistance to thermal depoling in BNTBT-ZnO composite. However, we found that the presence of ZnO was not limited to the interface, but contributed intrinsically to the BNTBT lattice. This played a critical role in the structural changes of BNTBT, confirmed by a unit volume change supported by XRD, which was further proved by Raman, EDS and dielectric characterization at different temperatures. The previous interface model is not correct because BNTBT shows thermally stable piezoelectric properties, even though there is no interface between BNTBT and ZnO. The thermal depoling behaviour of BNTBT-based materials is directly related to the transition temperature from rhombohedral phase to tetragonal phase in our phase transition model, which is consistent with four current peaks in their ferroelectric loops close to the depoling temperature.

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Introduction: The large share of commercially used piezoelectric devices such as actuators, sensors and transducers are made from lead-based compounds.1-2 In 2003, the European parliament passed the RoHS/WEEE regulation on the restrictive use of toxic materials in electronic devices, one of which is lead.3 At present, there is no definite alternative that can fully replace PbZr(1-x)TixO3 (PZT) based materials for commercial electronic devices. The electro ceramic community came up with lead free materials including, lead free compounds: BaTiO3 (BT) – based,

4-5

4-10

the three best alternative

(K1-xNax)NbO3 (KNN) based,

6-7

and

Bi0.5Na0.5TiO3 (BNT) - based8-9 materials. BNT with 6 mol. % of BT (BNTBT) is of great interest due to its higher electromechanical properties (160 pC/N for randomly oriented ceramics, 322 pC/N for textured ceramic and 483 pC/N for Mn - doped single crystal).11-12 A comprehensive structural and domain investigation using in-situ TEM for unpoled and poled BNTBT by Ma et al.13-14 concluded that at room temperature BNT with 6 mol% of BT consisted of both R3c (polar) and P4bm (weak polar) phases, and with the application of an electric field (> 2.7 kV/mm) there was a phase transition from a weak polar to a polar symmetry (P4mm). However, the structural symmetry of BNTBT and BNT base compound is still open to discussion, particularly concerning the existences of a monoclinic phase or coexistence of orthorhombic and rhombohedral symmetries.15-17 Thermal depoling temperature sets the upper limit for the high temperature application of piezoelectric materials. Recently, the disappearance of a thermal depoling temperature was reported for BNTBT with ZnO composites.18 The thermally stable piezoelectric properties and disappearance of the depoling temperature were attributed to the contribution from the interface between BNTBT and ZnO. The claims were based on piezoelectric measurements and on the hypothesis of charge – order model. According to this model, the charges in the ZnO grains become polarized (present at the BNTBT grain


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boundaries) with the application of an externally applied electric field, as did the ferroelectric domains in the BNTBT grains. On the removal of the external field, the polarized charges in the ZnO grains created a local field which acted as a source to stabilise the BNTBT domains in the direction of the applied field. Hence, it eliminated the depolarization effect from the BNTBT system, which produced thermally stable electromechanical properties up to high temperature (> 125 oC). However, the authors did not consider the possibility that the thermally stable properties could have been be related to the effect of a phase transition induced by Zn doping in the BNT lattice, which was apparent in the ferroelectric data presented in their article. 18 In this work, BNTBT and BNTBT-ZnO ceramics were prepared to systematically investigate the possible role of ZnO on the disappearance of the depolarization temperature (Td) and the increased thermal stability of the electromechanical properties of BNTBT. A detailed study was performed of the temperature dependent phase transitions using X – Ray Diffraction (XRD) and ferroelectric properties. The phase analysis was supported by complementary Raman and energy dispersive spectroscopy (EDS). A new phase transition model on thermal depoling was proposed to explain the thermal depoling behaviour of lead free BNT based materials.

Experiment 0.94Bi0.5Na0.5TiO3 (BNT) – 0.06BaTiO3 (named as BNTBT) powders were synthesised using a solid-state reaction method starting from Na2CO3 (Alfa Aesar, 99.5 %), Bi2O3 (Alfa Aesar, purity 99.975 %), TiO2 (Alfa Aesar, purity 99.9 %) and Ba2CO3 (Alfa Aesar, 99.8 % purity). Prior to weighing, the powders were dried in oven > 250 °C in order to remove any moisture content. The powders were weighted according to the required composition and ball milled in teflon jars containing an ethanol media and zirconia balls. The



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ball milling was performed for 4 h at 360 rpm. After drying, the powders were calcined in air at 900 °C for 2 h with a heating rate of 10 °C/min. The calcined powders were ball milled (4 h at 360 rpm) to obtain a fine particle size, which is important to achieve dense ceramics. After obtaining monophasic BNTBT, it was mixed with 30 mol % of ZnO with a particle size of 25 nm (PlasmaChem, Germany). The BNTBT with ZnO was ball milled in a teflon jar using ethanol as the mixing media. After mixing, the powders were dried at 100 oC for 24 h. The dried powders were uniaxially pressed at 60 MPa to obtain green pellets of BNTBT-ZnO. The pellets were sintered at 1100 oC for 2 h to obtain dense pellet with density >95 %, measured using Archimedes Principle. Similar sintering cycles were used as reported by Zhang et al.18 in order to avoid discrepancies. The surface morphology and elemental mapping of the pellets was evaluated by using a Scanning Electron Microscope equipped with Energy dispersive spectroscopy (EDS) (SEM, FEI Inspect-F Oxford, operated at 20 kV). The crystallographic phases at RT and as a function of temperature were studied for BNTBT and BNTBT-ZnO powders using in-situ XRD (Panalytical Xpert Pro diffractometer using CuKα radiation). Raman spectra were obtained from sintered discs at RT using a micro-Raman spectrometer (Horiba Jobin Yvon) with a 532 nm excitation laser, an edge filter for Rayleigh line rejection, and a CCD detector. The laser was focused on the sample to a spot size of ~2 mm using a 50x objective lens. The electrical measurements were conducted using a metal-insulator-metal configuration by painting the pellets with silver paste (Gwent Electronic Materials Ltd. Pontypool, U.K.) as to produce top and bottom electrodes. The relative permittivity (εr) and loss tangent (tan δ) as a function of temperature were evaluated using an LCR meter (Agilent 4284A, 40 Hz - 110 MHz, Hyogo, Japan) equipped with vertical tube furnace (Lenton, LTF). Current – polarization – electric field (I-P-E) hysteresis loops were obtained at 1 Hz using


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hysteresis tester (NPL, UK)

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in a silicon oil bath from RT to 150 oC. The samples were

polled using TREK, HV amplifier (Model 610 E). Direct piezoelectric coefficient (d33) was measured using Berlincourt meter at RT (Model ZJ-3B, PIEZO d33 meter, Institute of Acoustics Academia Sinica). Also, d33 was measured at room temperature after subjecting the samples to different temperatures for a holding time of 10 min.

Results and Discussion: The crystal structures of BNTBT and BNTBT-ZnO (poled and unpoled) were characterized using XRD (Figure S1). The Bragg reflections matched those of BNT with pseudo cubic phase (JCPDS #01-3109). In addition, BNTBT-ZnO showed second phase peaks of ZnO, indexed to JCPDS 36-1451. To quantify the phases and the effect of poling on phase transformation, Rietveld refinement was carried out using the GSAS software (Figure 1). The fitting analysis showed the presence of rhombohedral and tetragonal symmetries for both unpoled and poled BNTBT and BNTBT-ZnO at room temperature (Table 1). After poling, the weight fraction of the rhombohedral BNTBT phase increased from ~56 % to ~60 % (tetragonal phase from 44 % to 40 %) for BNTBT. Whereas the change in the symmetry from tetragonal to rhombohedral phase for BNTBT-ZnO was not obvious due to the presence of the second phase, which overshadowed the weight fraction results. The BNTBT-ZnO unit cell volume (119.203 Å3) with tetragonal symmetry is larger than that of BNTBT (119.022 Å3). The same is true for BNTBT-ZnO with rhombohedral symmetry. The change in volume indicates that Zn+2 cations (ionic radius 0.74 Å) probably diffused into the lattice and replaced Ti+4 ions (ionic radius 0.605 Å) on the B site of perovskite. If this is true, this would have resulted in the formation of oxygen vacancies in order to maintain the overall charge neutrality, represented by the following equation:



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2ZnO

.. ′′ BNTBT 2Zn + 2V + O2 (g)

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Eq. 1

This observation is substantiate by the work of Wang et al.20, where they investigate the effect of Zn occuppancy on the B site of BNT lattice. In order to verify the distribution of Zn ions in BNTBT-ZnO, the BNTBT-ZnO was characterised using SEM equipped with energy dispersive X - Ray (elemental mapping). SEM micrographs showed that BNTBT-ZnO consisted of BNTBT grains and ZnO grains. The average grain size for BNTBT was ~4 µm, whereas for ZnO the size ranged from 50 nm to 1 µm (Figure S2). In order to avoid the effect of polishing on the element distribution, the elemental mapping was carried out on the fractured surface of BNTBT-ZnO (Figure 2a). The elemental mapping shows regions of high and low concentrations of Zn ions, indicated by red pixels. The highly concentrated regions of red pixels are from the ZnO grains, whereas the lower concentrated regions in the BNTBT grains indicates the presence of Zn ions. A point elemental analysis along the straight line in a BNTBT-ZnO was carried out (Figure 2b). The point profile shows that the BNTBT grains consisted of ~ 2 weight % of Zn ions (Table S1); the Zn ions observed in the elemental analysis and EDS point profile probably diffused in to the BNTBT lattice during the sintering step. The elemental mapping results indicate the presence of Zn ions inside the BNTBT lattice and corroborate the finding from the analysis of the XRD data, an increase in the unit cell volume of BNTBT produced by the diffusion of Zn ions into the lattice. However, the elemental mapping results are supportive but not very conclusive due to the close proximity of Kα energy level of Na and Zn ions. Therefore the role of ZnO was further investigated by investigating structural changes using RAMAN and XRD as discussed below. BNTBT crystal shows rhombohedral symmetry with R3c space group and tetragonal symmetry with P4bm. Based on group theory, the R3c symmetry has 13 Raman active modes, 7A1 + 6E.21-22 In Figure 3, for BNTBT and BNTBT-ZnO unpoled samples, the Raman active modes were observed at ~136 cm-1 assigned to A - O vibration 6 ACS Paragon Plus Environment

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(where A is cation ), whereas the peaks at 260, 316, 535, and 619 cm-1 are assigned to O – Ti - O vibrations, consistent with earlier reports.

15, 22

The peak positions were calculated by

fitting the spectra using Gaussian and Lorentz equation shown in Figure 3.The peak around 316 cm-1 belongs to B1/E (To + LO) mode, and is a fingerprint for the tetragonal phase.23 Electrical poling resulted in a change in the coherence length of polar nano regions by transition of part of the symmetry from tetragonal to rhombohedral, indicated by the disappearing of the peak at 316 cm-1 for poled BNTBT (Figure 3b); a similar observation has been reported for BNTBT elsewhere.22, 24 In the case of BNTBT-ZnO, the peak at 316 cm-1 remained even after poling with a small Raman shift to lower wavenumber by 1 cm-1. The presence of a tetragonal peak even after poling of BNTBT-ZnO reflects the presence of the second phase over shadowing the effect of poling by suppressing the transition from tetragonal to rhombohedral symmetry. This might be due to the changes in the BNTBT lattice due to diffusion of Zn ions, as discussed above. To further clarify the role of Zn in BNTBT, its structure was studied using in–situ XRD as a function of temperature. The effect of electric poling on structural changes in BNTBT with and without ZnO were characterized using XRD as a function of temperature. Extended XRD patterns of pseudocubic (pc) planes (110)pc (111)pc and (200)pc for powders prepared from unpoled and poled BNTBT and BNTBT-ZnO ceramics were analyzed. BNTBT and BNTBT-ZnO at room temperature consisted of both rhombohedral and tetragonal symmetry. For BNTBT unpoled (Figure 4a) the weekly polar symmetry (tetragonal) transformed to long-range ferroelectric symmetry (rhombohedral) with the application of an external field (~40 kV/cm). The increased rhombohedral symmetry was indicated by the formation of shoulders and asymmetric broadening of the (110)pc reflection for poled BNTBT (Figure 4b, red arrows indicate rhombohedral phase ((012)R and (110)R planes). Similar changes were noticed for the (111)pc reflection (not shown here); this observation is consistent with the literature.25



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Whereas, for BNTBT-ZnO the changes in the XRD diffraction peaks after poling were very minute, which indicates that the weekly polar phase does not significantly transform to R3c symmetry on poling (Figure 4c - d). This might have been due to extrinsic and/or intrinsic contributions from ZnO, where ZnO acts as a second phase and suppressed the poling effect and /or the diffusion of Zn ions into the BNTBT lattice, stabilizing both tetragonal and rhombohedral phases, respectively. These observations are consistent with XRD fitting results and RAMAN observations. Furthermore, the transformation of R3c + P4bm phases to a dominant tetragonal phase above Td was monitored by analyzing the changes in Bragg diffraction peaks as a function of temperature for BNTBT and BNTBT-ZnO (Figure 4). By comparing the diffraction peak profile from (110)pc reflections for unpoled and poled BNTBT, it was observed that the tetragonal phase dominates at > 80 oC for unpoled BNTBT. Whereas, poled BNTBT shows the tetragonal domination at slightly higher temperature (>100 oC). This demonstrates a shift in depolarization temperature with poling for BNTBT. Whereas, for unpoled and poled BNTBT-ZnO, the tetragonal phase dominated above 140 oC, as evidenced by the (110)pc reflections. The diffraction peaks at 160 oC for BNTBT-ZnO (unpoled and poled) indicates the maximum transition for the FE (rhombohedral + tetragonal) to tetragonal symmetry. Likewise, the (111)pc reflections also showed that the tetragonal symmetry dominated at 160 oC (not shown here). The XRD results are consistent with the RAMAN observations, and clearly suggest that the shift in phase transition from FE to dominate tetragonal phase to a higher temperature for BNTBT-ZnO, compared to BNTBT (at 100 oC). To further strengthen the argument, the volume change for tetragonal phase for BNTBT and BNTBT-ZnO (poled and unpoled) were plotted as a function of temperature (Figure 5). There is an anomaly in the tetragonal volume change marked as a transition from R3c + P4bm phases to dominate tetragonal phase, at 120 oC for BNTBT and at 160 oC for BNTBT-ZnO.


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The existence of rhombohedral phase below the transition temperature exhibits distortion (due to the difference in the lattice parameters of both phases) in the volume of tetragonal phase. This results in the uneven (not linear) volume change as a function of temperature. However, above the transition temperature, where the tetragonal symmetry dominates, there is a linear volume change due to the expansion of the lattices with temperature. These results unambiguously suggest a shift in depolarization temperature to high temperature with the presence of Zn, instead of disappearing as claimed by Zhang et al.18. The dielectric permittivity (εr) and losses (tan δ) as a function of temperature (from 25 o

C to 550 oC) at various frequencies for BNTBT and BNTBT-ZnO (unpoled and poled) are

shown in Figure 6 and 7. εr as a function of temperature for BNTBT shows two/three anomalies for unpoled/poled samples. The first anomaly in εr at 97 oC for poled BNTBT (Figure 6b), above which there is significant frequency dispersion of εr, is referred to as the depolarization temperature (Td). This behaviour occurs due to a FE to weakly polar phase transition. Correspondingly, an anomaly was observed in the tan δ data at 97 oC. However, due to a diffuse distribution of polar nano regions in unpoled BNTBT, the frequency dispersion of εr presents at RT. The first/second anomaly is present at 141 oC and 146 oC for unpoled/poled BNTBT respectively, from here on the frequency dispersion of εr starts to disappear. This temperature is known as the shoulder temperature (Ts). In the work of Viola et al.

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, they used impedance data to study the Ts temperature and concluded that the two

different sets of relaxation processes with different energy barriers occurred below and above Ts, which results in an anomaly in the εr profile. However, the occurrence of shoulder temperature is unique to the BNT system and its origin is still open for discussion.27 The second/third anomaly present at ~ 252 oC for unpoled/poled BNTBT, referred as Tm, is the maximum temperature above which the dielectric permittivity starts to decrease exponentially.



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εr and tan δ as a function of temperature is shown in Figure 7 for poled and unpoled BNTBT-ZnO. Unlike for BNTBT, the dielectric permittivity for unpoled BNTBT-ZnO does not show significant frequency dispersion at RT. The dispersion in dielectric permittivity further reduced for the poled BNTBT-ZnO due to a small phase transition from weakly polar to polar, induced by electric field, as apparent in the XRD data (Figure 4). The poled BNTBT-ZnO shows Td at 130 oC, indicated by a small peak above which the dispersion in εr and tan δ increased (Figure 7b). The εr and tan δ shows an increase in frequency dispersion at >150 oC due to point defects such as oxygen vacancies28 described by equation 1. The effect of these defects is frequency dependent, therefore the Tm temperature was identified from εr profile at high frequency (> 10 kHz) as function of temperature, marked at ~ 278 oC. The increase in Td for BNTBT-ZnO compared to BNTBT points to the structural changes at lattice level as reflected by XRD. The structural changes are the plausible reason for the difference in the dielectric data of BNTBT and BNTBT-ZnO confirmed by the XRD fitting and RAMAN analysis. Even, in the work of Wang et al.20, they formed a solid solution between BNT and Bi(Zn0.5Ti0.5)O3, where Zn ions partially occupy the B sites. This lead to an increase in Td and Tm, and a decrease in the electromechanical response. The εr and tan δ as function of temperature presented by Zhang et al.18 is obscured because the data is presented up to 350 oC with improper scaling. Our observation and interpretation from Wang et al.20 work clearly shows the shift in depolarization temperature to high temperature related to the intrinsic contribution of Zn ions in the BNTBT lattice. Nonlinear polarization (P) - current (I) - electric field (E) hysteresis loops as a function of temperature for BNTBT are shown in Figure 8. At RT the P (E) loops obtained with the maximum applied field of 40 kV/cm exhibit a typical ferroelectric loop with remanent polarization (Pr) of 35 µC/cm2 and coercive field (Ec) 28 kV/cm. The coercive field


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values correspond to the peak in the current (0.75 mA), which is produced by ferroelectric switching. The P-E loops above 100 oC show a decrease in Ec and Pr, suggesting the incipient presence of weakly polar ordering. This is corroborated by the dielectric data for poled BNTBT, where Td is close to 100 oC (Figure 6). At 120 oC the I-E loops for BNTBT (Figure 8d) has four current peaks, two peaks each in the positive (P1 and P2) and negative (P’1 and P’2) parts of a cycle; similar I – E plots for BNTBT have been reported elsewhere.29-31 The occurrence of four current peaks can be explained based on the TEM investigation by Ma et al.

14

, where they demonstrated the electrical field induced transition from weakly polar

(P4nm) to strongly polar (R3c + P4mm) ferroelectric phase symmetry at room temperature. The origin of the four current peaks during electrical loading can be described as follow: at the maximum applied field during the initial cycle of the electrical loading the BNTBT has polar structure corresponding to current peak P2 (Figure 8d), upon unloading the polar structure remains. With the application of a relatively small negative applied bias the polar phase transforms back to the weakly polar phase corresponding to current peak P’1. With the maximum negative applied field the weakly polar phase changes again to polar structure corresponding to current peak P’2. Similarly, on reversal of the applied field the polar structure changes back to the non-polar phase corresponding to the current peak P1. This means that the near zero polarization condition exists in the valley of two current peaks P1 and P2 / P’1 and P’2. At much higher temperatures (150 oC), the polar phase transforms to the weakly polar phase during the unloading cycle of the electric field (P’1) (Figure 8e). This implies that the field induced polar structure is unstable at high temperature. The ferroelectric and current loops for BNTBT-ZnO were obtained with an applied field of 50 kV/cm at different temperatures (Figure 9). At RT the P-E loop exhibits as typical ferroelectric signature, with Pr of 23.8 µC/cm2 and Ec of 31 kV/cm. However, the significant decrease in the Pr and increase in Ec for BNTBT-ZnO compared to BNTBT (Pr 35 µC/cm2,



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Ec 28 kV/cm) is due to the presence of the ZnO as second phase along grain boundaries of BNTBT. Unlike for BNTBT, the BNTBT-ZnO does not show four current peaks up to 125 o

C. However, the P-E loop obtained at 150 oC shows one sharp and one broad current peak

during the loading (P2 and P’2) and reversible loading (P1 and P’1) of electric field (marked in Figure 9e). This indicates the presence of a weakly polar phase (corresponding to current peaks P1 and P’1) along with a small amount of polar phase (corresponding to current peaks P2 and P’2), all of which is corroborated by the XRD and dielectric permittivity results. A big tilt in the I-E peak (at 150 oC) at high field indicates increasing conductivity with temperature due to increasing mobility of the oxygen vacancies. This is consistent with the dispersion of permittivity as a function of temperature at a higher temperatures (> 140 oC) for BNTBTZnO (Figure 7b). In the work of Patterson et al.

32

, they investigated BNT and

Bi(Zn0.5Ti0.5)O3 (BZT) solid solutions and found that with 2 mol % of BZT the double loops appeared at around 150 oC accompanied by a decrease in the remanent polarization. This observation supports our observation of a shift of the depolarization temperature to higher temperature instead of vanishing of depolarization temperature with ZnO as reported by Zhang et al..18 The electromechanical response (d33) of BNTBT and BNTBT-ZnO were measured at RT after holding the sample at different temperatures, and normalized d33 as a function of temperature is presented in Figure 10. The piezoelectric coefficient (d33) for BNTBT and BNTBNT-ZnO at room temperature are 130 and 78 pC/N respectively. The lower d33 for BNTBT-ZnO is due to the presence of ZnO as a second phase and / or Zn ions in the BNTBT lattice. A similar decrease was reported for BNT - Bi(Zn0.5Ti0.5)O3, where Zn occupied B sites .20 The stable d33 below 110 oC for poled BNTBT and 125 oC BNTBT-ZnO is consistent with the stable remanent polarization presented in the P – E loops (Figure 8 and Figure 9). The sudden drop in d33 above 110 oC for BNTBT is due to a diffuse transition from FE to


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weakly polar phase, which is consistent with the structural changes observed by XRD (Figure 4). Similarly, the sudden drop in d33 for BNTBT-ZnO observed above 130 oC is associated with the phase transition from FE to weakly polar phase. Zhang et al.18, reported a linear decrease in d33 as a function of temperature for BNTBT, and with the incorporation of ZnO in BNTBT the d33 was stabilized up to 125 oC. Based on this, they highlight that the incorporation of ZnO stabilizes the d33 to higher temperatures (125 oC). However, the I – E curve for BNTBT reported by Zhang et al.18 shows the appearance of four current peaks at much lower temperature (50 oC) compared to what was observed in the present study (>100 oC) and in the literature.29-30 The appearance of four current peaks indicates the phase transition from FE to weakly polar phase, which results in the decrease in d33 at much lower temperatures for BNTBT in the work of Zhang et al.18. The earlier phase transition and decrease in d33 observed for BNTBT might be due to processing (results in offstoichiometry due to hydroscopic and volatile precursors) and has nothing to do with ZnO and BNTBT interface as suggested by Zhang et al.18. The comprehensive investigation on the BNTBT-ZnO system and comparison with the counterpart BNTBT clearly suggests that the presence of Zn was not only limited to the grain boundaries but diffused into the BNTBT lattice. The intrinsic contribution of Zn ions results in a shifted the depolarization temperature to higher temperatures rather than vanishing of the depolarization effect, which was proposed as a new phase transition model on thermal depoling in BNTBT based ferroelectrics.

Conclusions: The FE to weak polar phase transition is related with the transition from polar phase (R3c +P4mm) to weakly polar phase (P4bm) as a function of the temperature for BNTBT, and is related to the depolarization temperature. The present results suggest that the Zn ions from



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ZnO diffuse into the BNTBT lattice during sintering, results in a stabilizing of the ferroelectic phase up to high temperature (140 oC) and, resulting in an increase in the phase transition for BNTBT-ZnO composites by ~40 oC. Further, the Zn ions in BNTBT lattice produces oxygen vacancies, which is apparent in the dielectric permittivity as a function of temperature data. Moreover, the stability in d33 for BNTBT-ZnO has nothing to do with BNTBT-ZnO interface. According to our phase transition model on thermal depoling, the shift in depolarization temperature is due to the intrinsic contribution of Zn ion in the BNTBT lattice.

Acknowledgement: Author’s thanks for financial support by EPSRC, MASSIVE project no. EP/L505183/1.

Supporting Information:

Effect of Phase Transitions on Thermal Depoling in Lead-Free Bi0.5Na0.5TiO3 – 0.6BaTiO3 Based Piezoelectrics


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Tarakina, N. V.; Abrahams, I.; Yan, H., High energy density in silver niobate ceramics. J. Mater. Chem. A 2016, 4, 17279 -17287. 11.

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Figures:

(a)

(b)



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(c)

(d) Figure 1. Rieveld refinement by GSAS of BNTBT (a) unpoled, (b) poled and BNTBT-ZnO (c) unpoled, (d) poled.


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(a)

(b)

Figure 2 Elemental mapping of BNTBT-ZnO, (a) red pixels indicate the distribution of ZnO and (b) line scan on the fracture surface of BNTBT-ZnO and element analysis data presented in Table S1.



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(a)

Page 22 of 31

(b)

(c)

(d)

Figure 3. Fitting of RAMAN data of BNTBT (a) unpoled, (b) poled and BNTBT-ZnO (c) unpoled, (d) poled. The presence of a peak at 316 cm-1 for poled BNTBT-ZnO suggest the Zn stabilize the tetragonal symmetry in BNTBT-ZnO.


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(a)

(b)

(c)

(d)

Figure 4. XRD pattern from (100)pc reflection plans for BNT-BT unpoled (a), BNTBT poled (b), BNTZnO unpoled (c) and BNTBT-ZnO poled (d), marked phases relates to room temperature. The diffraction peak from (100)pc plans for poled BNTBT-ZnO suggests shift in the FE to weakly polar transition temperature >140 oC in comparison to BNTBT (120 oC).



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Figure 5. Change in tetragonal volume (Å3) for BNTBT (poled, unpoled) and BNTBT-ZnO (poled, unpoled) as a function of temperature. The anomaly in the volume change at 120 oC for BNTBT and 160 oC for BNTBT-ZnO followed by linear change in volume indicates the transition temperature from polar to weak polar symmetry.


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(a)

(b)

Figure 6. Dielectric permittivity and loss tangent as a function of temperature at various frequency for (a) unpoled and (b) poled BNTBT. Poled BNTBT shows depolarization temperature at 97 oC.



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(a)

Page 26 of 31

(b)

Figure 7. Dielectric permittivity and loss tangent as a function of temperature at various frequency for (a) unpoled and (b) poled BNTBT-ZnO. The poled BNTBT-ZnO shows appearance of frequency dispersion at 130 oC referred as depolarization temperature.


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(a)

(b)

(d)

(c)

(e)

Figure 8. P-I-E loops of BNTBT obtained at various temperatures. At 120 oC the four current peaks indicates the existence of polar and weakly polar phase.



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(a)

(b)

(d)

Page 28 of 31

(c)

(e)

Figure 9. P-E-I loops of BNTBT-ZnO obtained at various temperatures. BNTBT-ZnO does not shows show four current peaks corresponding to rhombohedral and tetragonal phase up to 120 oC, but tilt in a current peak at high field indicates high conductivity and it increase with temperature. At 150 oC the four peaks were observed, where one sharp peak and another broad peak.


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Figure 10. Normalized d33 as a function of temperature for BNTBT-ZnO and BNTBT. Indicates the low electromechanical response for BNTBT-ZnO compared to BNTBT.



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Table 1. Rietveld refinement structural parameters for tetragonal (P4bm) and rhombohedral (R3c) phases for BNTBT and BNTBT-ZnO. In addition, the second phase parameters for ZnO are also presented for BNTBT-ZnO. Sample

Unit cell parameters (Å) (Phase 1, P4bm)

Unit cell parameters (Å) (Phase 2, R3c)

BNTBT unpoled

a = b = 5.503054 c = 3.93023 c/a = 0.71419 α = β = γ = 90o Volume = 119.022 (Å3)

a = b = 5.506892 c = 13.563714 c/a =2.46304 α = β = 90o, γ = 120 Volume = 356.222992 (Å3)


a = b = 5.502608 c = 13.59871 c/a =2.47132 α = β = 90o, γ = 120o Volume = 356.587 (Å3)

N.A


a=b= 5.509579 c = 13.586673 c/a = 2.46600 α = β = 90o, γ = 120o

a=b= 3.248873 c = 5.204162 c/a = 1.60184 α = β =90o γ = 120 Volume = 47.572 (Å3)

P4bm = 51.5

a=b= 3.249619 c = 5.205120 c/a = 1.601763 α = β =90o γ = 120o Volume = 47.602 (Å3)

P4bm = 44.9

BNTBT poled

BNTBTZnO unpoled

Volume = 357.175 (Å3)

BNTBTZnO poled


a=b= 5.507014 c = 13.598321 c/a = 2.46927 α = β = 90o, γ = 120o Volume = 357.148 (Å3)

Unit cell parameters (Å) (Phase 3, ZnO) N.A

Weight fraction in %

Fitting parameter

P4bm = 44.0

χ2 = 1.388 wRp = 0.1232 Rp = 0.0976

R3c = 55.983

P4bm = 40.4 R3c = 59.6


R3c = 48.5

R3c = 55.1

χ2 = 1.387 wRp = 0.1200 Rp = 0.0942

χ2 = 1.3644 wRp = 0.1265 Rp = 0.0982

χ2 = 1.457 wRp = 0.1452 Rp = 0.1164

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TOC


Effect of Phase Transitions on Thermal Depoling in Lead-Free 0.94

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