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We have investigated the effect of sintering temperature on the microstructure, crystal structure, dielectric, ferroelectric, and piezoelectric properties of the morphotropic phase boundary (MPB) composition lead-free piezoelectric 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (NBT-6BT). We have found that the specimen sintered at an optimized temperature (1150 °C) exhibits the maximum piezoelectric response (d33 ~ 198 pC/N). The maximum piezoresponse is associated with the presence of optimized structural disorder amidst long-range ferroelectric order in the poled state of the specimen. The structural disorder in the present case is caused by the propensity for stabilizing in-phase octahedral tilt.

A-site off-stoichiometry has been an efficient method to enhance the electromechanical properties of lead-free Na0.5Bi0.5TiO3(NBT)-based piezoceramics. In this work, we have reported the effect of Na/Bi off-stoichiometry on the microstructural, structural, and electromechanical properties of the tetragonal 0.80Na0.5Bi0.5TiO3–0.20BaTiO3 (NBT–20BT) ceramic. The maximum piezoresponse ~ 115 pC/N is obtained for 4 mol% Na-deficient (20BT–Na46) and 2 mol% Bi-excess (20BT–Bi52) compositions. It is a 25% increment over the piezoresponse of the stoichiometric composition (20BT–Na50 ~ 90 pC/N). The enhancement in piezoresponse has been attributed to an optimized presence of polar-structural heterogeneity/disorder in the poled ceramic matrix and an optimized distortion of the long-range tetragonal ferroelectric phase. It is established that A-site off-stoichiometry is influencing the polar-structural heterogeneity/disorder and tetragonal lattice distortion via the grain size.

We investigated the phase boundaries of three ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}$-(NBT) based piezoelectric systems NBT-${\mathrm{BaTiO}}_{3}$ (NBT-BT), NBT-${\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}$ (NBT-KBT) and NBT-${\mathrm{SrTiO}}_{3}$ (NBT-ST), all exhibiting similar piezoelectric responses (${d}_{33}\ensuremath{\sim}180\phantom{\rule{0.16em}{0ex}}pC/N$). We found that, apart from a qualitative difference (NBT-KBT/NBT-BT exhibits R3c-P4mm interferroelectric instability and NBT-ST does not), the critical compositions of all the three systems exhibit onset of a disordered P4bm phase. Our study suggests that the preponderant effect of this distortion overrides the interferroelectric instability, limiting the piezoelectric response of NBT-BT/NBT-KBT like that in NBT-ST.

One of the important requirements for piezoelectric materials for use as high strain actuators is that they exhibit large unipolar electrostrain with minimum hysteresis. While large unipolar electrostrain >1% is generally achievable in good quality single crystals, most polycrystalline piezoelectric show low values 0.5Bi0.5TiO3-based compositions at the non-ergodic ergodic boundary. Not amenable to poling, such materials exhibit almost nearly zero direct piezoelectric coefficient ( d33 ∼ 0 pC/N) and cannot be simultaneously used as a sensor. In this paper, we report a combination of large unipolar electrostrain of ∼0.6% with small strain hysteresis of 25% in a Sn-modified relaxor ferroelectric system PbTiO3–Bi(Ni1/2Zr1/2)O3. It exhibits d33 ∼ 340 pC/N, which is stable up to 130 °C, and large signal converse piezoelectric coefficient d33* ∼ 1200 pm/V. A combination of large d33 and d33* in the same material makes it an important candidate for simultaneous use as a sensor and high strain actuators. X-ray diffraction study in situ with the electric field suggests that large electrostrain with low strain hysteresis in this system is because of the increased reversible switching of the field stabilized tetragonal ferroelastic domains.

Driven by environmental concerns and governmental directives, a sustained research effort in the last decade and half has led to the development of lead-free alternatives which can potentially replace the commercial lead-based piezoceramics in niche applications. Na0.5Bi0.5TiO3 (NBT)-based lead-free piezoceramics have found acceptance as promising lead-free transducers in high power ultrasonic devices. An issue of concern however is the low depolarization temperature which limits the device's tolerance for temperature rise during operation. While several strategies have been reported to improve thermal depolarization in NBT-based piezoceramics, there is a lack of consensus regarding the most fundamental factor/mechanism which enhances the depolarization temperature. In this paper we unravel a coupled microstructural-structural mechanism which controls the thermal depolarization in NBT-based piezoceramics. First, we demonstrate the phenomenon of a considerable increase in the depolarization temperature, without significantly losing the piezoelectric property in unmodified NBT by increasing the grain size. We then establish a grain size controlled structural mechanism and demonstrate that the rise in depolarization temperature is primarily associated with the bigger grains allowing relatively large lattice distortion to develop in the poling stabilized long range ferroelectric phase. We reconfirmed the validity of this mechanism in the model morphotropic phase boundary (MPB) composition 0.94Na0.5Bi0.5TiO3-0.06BaTiO3. For the sake of generalization, we demonstrate that the same mechanism is operative in another lead-based relaxor-ferroelectric system 0.62PbTiO3-0.38Bi(Ni0.5Hf0.5)O3. Our study provides the fundamental structural basis for understanding thermal depolarization delay in relaxor ferroelectric based piezoceramics.

Polycrystalline samples of Bi0.9Eu0.1FeO3, Bi0.9Gd0.1FeO3, and Bi0.9Eu0.05Gd0.05FeO3 ceramics were synthesized by a solid-state reaction method, followed by rapid quenching to room temperature. Bio(0.9)Eu(0.1)FeO(3) shows higher magnetization than Bi0.9Gd0.1FeO3 and Bi5.9Eu0.05Gd0.05FeO3. This can be explained by lattice distortion, and the change in the statistical distribution of Fe3+ /Fe2+. Similarly, ferroelectric polarization of Bi0.9Eu0.1FeO3 was found to be higher than that of Bi0.9Gd0.1FeO3 and Bi0.9Eu0.05Gd0.05FeO3. However, the absolute value of the magnetocapacitance of Bi0.9Eu0.05Gd0.05FeO3 (similar to 1.6%) was found to be higher than that of Bi0.9Eu0.1FeO3 (similar to 1.3%) and Bi0.9Gd0.1FeO3 (similar to 1.1%). In addition, the quadratic component of magnetoelectric coupling is higher for Bi0.9Eu0.05Gd0.05FeO3, which makes Bi0.9Eu0.05Gd0.05FeO3 attractive for further study.

Among the different piezoceramics, ${\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$ (NBT)-based systems have the unique distinction of exhibiting very large electrostrain $(g0.5%)$ when pushed at the threshold of ergodic--nonergodic relaxor transition. However, this strategy severely compromises the system's weak-signal piezoelectric response (${d}_{33}$). Here, we show that a similar level of electrostrain is feasible together with high ${d}_{33}$ by a different approach. In our approach, the system is kept well within the nonergodic relaxor regime close to an interferroelectric instability, and the inherent structural (tilt) disorder is exploited for large reverse switching of the ferroelastic domains in the field-stabilized ferroelectric phase. A synergy of this combination is demonstrated in $(1\ensuremath{-}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3\ensuremath{-}}(x){\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}$; $x=0.25$ which exhibit electrostrain of \ensuremath{\sim}0.6% and ${d}_{33}$ (\ensuremath{\sim}210 pC/N). With the added advantage of high depolarization temperature $({T}_{d}\ensuremath{\sim}185{\phantom{\rule{4pt}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ this piezoceramic stands unique for showing the best combination of the three most sought after properties.

The structural complexities of the lead-free piezoelectric system $(1\text{\ensuremath{-}}x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}{\mathrm{TiO}}_{3}\text{\ensuremath{-}}x{\mathrm{BaTiO}}_{3}$ (NBT-BT) continues to pose challenge regarding understanding the mechanisms underlying several interesting phenomena. Issues like (i) whether thermal depoling across compositions is triggered by a structural transformation event or not, (ii) what causes the average Cc structure to partially transform to $R3c$ at $x\ensuremath{\sim}0.03$ in unpoled specimens, (iii) what makes complete depoling of the compositions $0.03\ensuremath{\le}x\ensuremath{\le}0.05$ occur in a considerably small temperature interval as compared to those for $xl0.03$, (iv) what makes the $R3c$-$P4bm$ transition temperature $({T}_{2})$ abruptly become smaller than the depolarization temperature $({T}_{d})$ at $x=0.06$, etc., have remain unresolved. Here, we offer structural insights on these issues by carrying out a detailed investigation using a set of complementary tools involving temperature-dependent x-ray powder diffraction, neutron powder diffraction, dielectric, ferroelectric, piezoelectric, and thermally induced depoling current measurements. We show that onset of thermal depoling in NBT $(x=0)$ well below its depolarization temperature is caused by abrupt reduction of intrinsic polarization in the ferroelectric $R3c$ phase, triggered by the appearance of the $P4bm$ phase. Our study suggests that partial conversion of the Cc average structure to $R3c$ in unpoled NBT-BT at $x\ensuremath{\sim}0.03$ (more precisely in the range $0.03\ensuremath{\le}x\ensuremath{\le}0.05$) is catalyzed by the appearance of $P4bm$ phase. The overlap of ${T}_{d}$ and ${T}_{2}$ for this composition range is correlated with the collapse of the tetragonality of the $P4bm$ phase and significantly reduced kinetic barrier associated with the $R3c\ensuremath{\rightarrow}P4bm$ transformation. We show that the abrupt crossover between ${T}_{d}$ and ${T}_{2}$ at $x=0.06$ is due to takeover of the thermal depoling process by an emergent tetragonal $(P4mm)$-like ferroelectric distortion. We present updated phase diagrams of poled and unpoled specimens which highlight all the subtle details needed to explain the temperature-dependent properties of this complex piezoelectric alloy system.

The complex interplay of local disorder with the structure and dynamics and their role in enhancing the electromechanical response makes relaxor ferroelectric materials fascinating both from the scientific and technological standpoints. It is generally believed that in chemically disordered ferroelectric solid solutions, the normal ferroelectric state gradually yields to a relaxor ground state on increasing the concentration of localized (point-defect-like) random-field centers. That a different kind of process can spontaneously stabilize a relaxor ground state is less known, especially in the family of ferroelectric perovskites. We demonstrate the occurrence of this less-known phenomenon in $(1\ensuremath{-}x){\mathrm{K}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}\text{\ensuremath{-}}(x){\mathrm{Na}}_{0.5}{\mathrm{Bi}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}[(1\ensuremath{-}x)\mathrm{KBT}\ensuremath{-}(x)\mathrm{NBT}]$. Unlike the gradual evolution common to most ferroelectric solid solutions, KBT-xNBT exhibits an abrupt crossover from a normal ferroelectric ground state to a full-blown relaxor ground state for $xg0.58$. We show that this abrupt crossover to the relaxor state is caused by stabilization of an incommensurate-like/highly disordered $M$-point ferroelastic distortion in the ergodic/paraelectric temperature regime. The abrupt crossover manifests as composition driven anomalous changes in several important properties such as dielectric response, electromechanical properties, tetragonality, and coercive field, electrostrain, and mimics a scenario often encountered in composition driven interferroelectric transformations.

Lead free (0.94-x)Na0.5Bi0.5TiO3–0.06BaTiO3-xK0.5Na0.5NbO3 ceramics are under the spotlight owing to their multifunctional nature. We have systematically investigated the microstructural, structural, dielectric, piezoelectric and energy storage properties of the system in the range 0 ≤ x ≤ 0.08. There is a systematic reduction in grain size with increasing K0.5Na0.5NbO3(KNN) concentration. The enhancement in disorder with K0.5Na0.5NbO3(KNN) doping is evident from the increase in slope of the dielectric dispersion plots. The unpoled compositions have a cubic like structure in the whole range i.e. from x = 0 to x = 0.08. However, the application of electric field(poling) results in a structural transformation to lower symmetry upto x = 0.04. Beyond that the disorder dominates over the applied electric field and the structure is cubic like even in the poled state. The energy storage density increases from 0.08 J/cm3 for x = 0 to 1.19 J/cm3 for x = 0.05 and saturates thereafter. Discharge efficiency also follows a similar trend. The piezoresponse decreases with KNN doping from 151 pC/N for x = 0 to 4 pC/N for x = 0.08. It is observed that energy storage density and piezoresponse are complimentary properties which increase at the expense of each other.

Artificial magnetoelectric material with enrichment of magnetoelectric (ME) coupling coefficient was achieved via mixed-oxide route in cobalt substituted BaTiO3 multiferroics. The observed large ME coupling coefficient induced by spin–orbit interaction through the symmetry breaking due to the off-stoichiometry oxygen. Structural details extracted from the Rietveld refinement indicate the rapid arrival of a single hexagonal phase (P63/mmc) at the expense of the tetragonal phase (P4mm) above x = 0.03. SAED patterns evidence to the structural coexistence, morphotropic phase boundary (MPB), (P4mm + P63/mmc) in x = 0.01 and 0.03 while hexagonal phase alone in x ≥ 0.05. Core-level XPS spectrum of Ba(3d), Ti (2p), Co (2p), O (1s) evidences the occurrence of the single oxidation state of Ba ion (Ba2+) and aliovalent of Ti (Ti4+ and Ti3+), Co (Co3+ and Co2+) and O (O2− and O1−) ions respectively. The weakening of ferroelectric loops arises from the formation of the non-ferroelectric hexagonal phase by the non-stoichiometric oxygen. For the first time, a high value of energy efficiency of 54.7% was achieved in the BaTi0.99Co0.01O3 sample. The ferromagnetism originated from the contributions of the super-exchange interaction of Co3+(octahedral)-O2−-Co3+(pentahedral) and double-exchange interaction (Co2+-O2−-Co3+) Among all the samples, an excellent magnetoelectric coupling coefficient (αME) value of 29.6 mV/cm Oe is attained in the hexagonal BaTi0.93Co0.07O3 sample.

Morphotropic phase boundary (MPB) and polymorphic phase boundary ferroelectric solid solutions show enhanced electromechanical response and are sought after as actuator and transducer materials. Compared to others, the lead-free piezoelectric solid solution ${\mathrm{N}}_{0.5}\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{Ti}{\mathrm{O}}_{3}\ensuremath{-}\mathrm{BaTi}{\mathrm{O}}_{3}$ stands out in two important respects: (i) low piezoelectric coefficients $(\ensuremath{\sim}200\phantom{\rule{0.16em}{0ex}}\mathrm{pC}/\mathrm{N})$ and (ii) anomalous decrease in the depolarization temperature at the MPB, the reason for which is still unclear. Here we show that both features are related; this is caused by an increasing intervention of a nonferroelectric structural distortion which compromises the gains the system acquires at the MPB via the interferroelectric instability. The propensity of the intervening nonferroelectric distortion, identified as in-phase octahedral tilt, grows considerably as the MPB is approached causing considerable structural-polar disorder, weakening the overall strength of ferroelectric interaction.

We demonstrate that the lead-free piezoelectric compound Na0.5Bi0.5TiO3 (NBT) exhibits random lattice strain in the ferroelectric phase, and that this feature primarily dictates the way the system evolves towards the morphotropic phase boundary in the unpoled state of NBT-based piezoelectrics. Investigations on two different morphotropic phase boundary (MPB) systems, namely Na0.5Bi0.5TiO3 - K0.5Bi0.5TiO3 (NBT-KBT) and Na0.5Bi0.5TiO3 - BaTiO3 (NBT-BT), revealed that the coupled structural-polar evolution towards the MPB is primarily driven by the necessity to minimize this strain. Our study suggests that the random lattice strain originates in the random stacking of the in-phase tilt and antiphase octahedral tilted regions, and that the system is able to minimize it by adopting a sequential stacking of the two tilt types, leading to a long-period modulation in the octahedral tilt configuration over large parts of the sample volume. This hinders the development of long-range ferroelectric order as the MPB is approached. We also demonstrate that the composition showing the maximum piezoelectric coefficient corresponds to a structural state wherein considerable polar-structural disorder coexists with the field-stabilized long-range rhombohedral ferroelectric order after poling, and not coexistence of two ferroelectric phases (tetragonal and rhombohedral), generally believed.

The inherent structural disorder has a profound effect on the dielectric, ferroelectric and the electromechanical response of the Na0.5Bi0.5TiO3 (NBT) based lead-free piezoelectrics. While analogous to the lead-based classical morphotropic phase boundary (MPB) systems the existence of MPB has been recognized in some derivatives of NBT displaying enhanced electromechanical response, there is a lack of clarity on the structural state of the MPB compositions on NBT-based systems on the global length scale. We have examined this issue on the well known MPB system (1-x)Na0.5Bi0.5TiO3-(x)K0.5Bi0.5TiO3(NBT-KBT) by carrying out structural investigations on local and global length scales using Eu+3 photoluminiscence and high-resolution neutron powder diffraction techniques, respectively. Our study reveals that the MPB of this system is characterized by the onset of a long-period modulated structure with a periodicity of similar to 40 angstrom on the global scale. Temperature depedent neutron diffraction study revealed that the intermediate temperature P4bm phase which appears in NBT is suppressed for the MPB composition. The MPB composition rather develops a long-period modulated phase on cooling from the cubic phase. The ergodic-nonergodic relaxor ferroelectric transition occurs within this long-period modulated phase. In the non-ergodic regime, however, strong electric field irreversibly transforms the long-period modulated phase to the rhombohedral ferroelectric (R3c). We demonstrate that thermal depolarization of this system is a distinct structural event characterized by the system losing its field-induced long range rhombohedral (R3c) coherence and transforming back to the long-period modulated phase. Our study suggests that the long-period modulated phase is the primary structural feature of the MPB compositions in NBT-based piezoelectrics. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We show that the lead-free piezoelectric system (x)Na1/2Bi1/2TiO3-(1 − x)K1/2Bi1/2TiO3 [xNBT-(1 − x)KBT] exhibits a boundary within its tetragonal phase regime at 0.58 0.58. We demonstrate that the abrupt changes, mimicking a composition-driven phase transition like scenario, are due to a sudden increase in the structural disorder for x > 0.58.

Morphotropic phase boundary (MPB) composition 0.94Na0.5Bi0.5TiO3–0.06BaTiO3 (NBT–6BT) has received industrial acceptance in certain aspects such as ultrasonic cleaners to replace those of lead-based systems. While detailed structure–property analysis on NBT–6BT is mostly confined to room temperature and above, less attention is paid with regard to their structure and property understanding below room temperature. In this work, utilizing the complementarity of Raman spectroscopy, x-ray diffraction, and neutron powder diffraction, we unravel low temperature polar-structural behavior of the MPB composition. While x-ray diffraction shows the persistence of a cubic like global structure in the temperature interval from 300 to 100 K, neutron diffraction reveals a structural crossover from long period modulated to non-modulated P4bm + R3c phase coexistence at ∼150 K. We show that the tendency of growing ferroelectric ordering with reducing temperature is responsible for the structural crossover. Concurrence of weak anomaly in dielectric loss (tan δ) at the same temperature seems to be correlated to the structural crossover.

We have systematically investigated the effect of rare earth erbium (Er3+) substitution on the structural, ferroelectric, piezoelectric, and dielectric behaviour of the morphotropic phase boundary lead-free piezoelectric 0.94Na(0.5)Bi(0.5)TiO(3)-BaTiO3(NBT-6BT). We found that the system shows very good thermal stability of energy density, discharge efficiency, and dielectric permittivity. Detailed structural studies confirmed that Er modification induces a great deal of structural frustration which causes the system to thermally evolve towards the paraelectric state in a very sluggish manner. Our results seem to suggest that, among other factors, this sluggish temperature variation of the structural distortion is an important factor which imparts thermal stability to the energy storage and dielectric properties. Published by AIP Publishing.