Rhombohedral Ferroelectric Phase Research Articles

Phase coexistence and broad depolarization response in (Pb,La)(Zr,Sn,Ti)O3 single crystals

Lead lanthanum zirconate stannate titanate (PLZST) antiferroelectric materials have been intensively studied in the last few decades due to their adjustable phase transition and outstanding performance. Despite the strong interest it has drawn, there is a lack of study of domain structure evolution in antiferroelectric materials with morphotropic phase boundary (MPB) composition, which is of great importance for understanding the phase transition between ferroelectric and antiferroelectric phase. PLZST single crystals with the composition in the MPB region were grown by the flux method in this work. We presented the direct evidence that rhombohedral ferroelectric and tetragonal antiferroelectric phase coexist in the as-grown single crystals. The evolution of phase structure and domain morphology during the temperature-induced phase transition was observed by in situ optical visualization, accompanied by a broadening depolarization response. An enhanced broadening pyroelectric response with a wide temperature window was detected. A modified Ginzburg-Landau-Devonshire theory was employed to elucidate the structural and physical origin of the broadening thermal depolarization. It is anticipated that a further understanding of the phase transition behavior in PLZST single crystals may be helpful in developing novel ferroelectrics and antiferroelectrics with MPB compositions.

Outstanding Piezoelectric Performance in Lead‐Free 0.95(K,Na)(Sb,Nb)O3‐0.05(Bi,Na,K)ZrO3 Thick Films with Oriented Nanophase Coexistence

AbstractLead‐free 0.95(K0.48Na0.52)(Nb0.95Sb0.05)O3‐0.05Bi0.5(Na0.82K0.18)0.5ZrO3 (KNSN‐BNKZ0.05) piezoelectric films with preferred crystal orientation and enhanced thickness are fabricated on silicon substrates from a chemical solution approach. Adequate K excess is introduced to obtain a single perovskite phase in the resulting thicker films. The effects of thickness, crystal orientation, and structure of the films on the performance are investigated. Outstandingly large effective piezoelectric strain coefficient up to 250 pm V−1 is demonstrated over a macroscopic scale using a laser scanning vibrometer in the [100]‐KNSN‐BNKZ0.05 film with an enhanced thickness of 2.7 µm, competitive to the benchmark oriented lead zirconate titanate films on silicon. Atomically resolved electron microscopy reveals the coexistence of oriented ferroelectric rhombohedral (R) and tetragonal (T) phases at the nanometer scale with gradual polarization rotation, which can lower the domain wall energy and facilitate the large piezoelectric response. The increased film thickness reduces the in‐plane mechanical clamping to enable more free deformation in the thickness direction and improve domain wall mobility, both further contributing to enhanced piezoelectric response.

Composition-dependent xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 bulk ceramics for high energy storage applications

This work reports the composition dependent microstructure, dielectric, ferroelectric and energy storage properties, and the phase transitions sequence of lead free xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 [xBZT-(1-x)BCT] ceramics, with x = 0.4, 0.5 and 0.6, prepared by solid state reaction method. The XRD and Raman scattering results confirm the coexistence of rhombohedral and tetragonal phases at room temperature (RT). The temperature dependence of Raman scattering spectra, dielectric permittivity and polarization points a first phase transition from ferroelectric rhombohedral phase to ferroelectric tetragonal phase at a temperature (TR-T) of 40 °C and a second phase transition from ferroelectric tetragonal phase - paraelectric pseudocubic phase at a temperature (TT-C) of 110 °C. The dielectric analysis suggests that the phase transition at TT-C is of diffusive type and the BZT-BCT ceramics are a relaxor type ferroelectric materials. The composition induced variation in the temperature dependence of dielectric losses was correlated with full width half maxima (FWHM) of A1, E(LO) Raman mode. The saturation polarization (Ps) ≈8.3 μC/cm2 and coercive fields ≈2.9 kV/cm were found to be optimum at composition x = 0.6 and is attributed to grain size effect. It is also shown that BZT-BCT ceramics exhibit a fatigue free response up to 105 cycles. The effect of a.c. electric field amplitude and temperature on energy storage density and storage efficiency is also discussed. The presence of high TT-C (110 °C), a high dielectric constant (εr ≈ 12,285) with low dielectric loss (0.03), good polarization (Ps ≈ 8.3 μC/cm2) and large recoverable energy density (W = 121 mJ/cm3) with an energy storage efficiency (η) of 70% at an electric field of 25 kV/cm in 0.6BZT-0.4BCT ceramics make them suitable candidates for energy storage capacitor applications.

Improved pyroelectric figures of merit of Mn-doped Zr-rich lead zirconate titanate bulk ceramics near room temperature for energy harvesting applications

Pyroelectric energy harvesting has been attracting increasing attention due to its abundance and potential applications. In this work, we comparably investigated the pyroelectric properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 bulk ceramics without (PNZT95/5) and with Mn doping (PNMZT95/5). Compared to PNZT95/5, PNMZT95/5 demonstrated a lower dielectric constant and low rhombohedral - high rhombohedral ferroelectric phase transition temperature (TLH from 37 °C to 27 °C). Moreover, the poling time displayed significant effects on the dielectric and pyroelectric properties of the ceramics. A giant pyroelectric response was obtained for the PNMZT95/5 ceramics, of which the maximum pyroelectric figure of merit FE was 16017 J/(m3K2) after 180 min of poling, which increased 62% compared with the FE for PNZT95/5 (9890 J/(m3K2), far higher than those of other pyroelectric materials as well. PNMZT95/5 ceramics is suggested to be a promising candidate for the thermal energy harvesting applications.

A rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films.

Hafnia-based thin films are a favoured candidate for the integration of robust ferroelectricity at the nanoscale into next-generation memory and logic devices. This is because their ferroelectric polarization becomes more robust as the size is reduced, exposing a type of ferroelectricity whose mechanism still remains to be understood. Thin films with increased crystal quality are therefore needed. We report the epitaxial growth of Hf0.5Zr0.5O2 thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 substrates. The films, which are under epitaxial compressive strain and predominantly (111)-oriented, display large ferroelectric polarization values up to 34 μC cm-2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This finding, in conjunction with density functional theory calculations, allows us to propose a compelling model for the formation of the ferroelectric phase. In addition, these results point towards thin films of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

Chrome influence on the physical properties of Bi0.90Ba0.10Fe0.90Ti0.10O3 multiferroic ceramic system

Bi0.90Ba0.10Fe0.90Ti0.10O3 and Bi0.90Ba0.10Fe0.88Cr0.02Ti0.10O3 multiferroic ceramics were prepared, in order to evaluate the chrome influence on the physical properties of Ba2+ and Ti4+ co-doped BiFeO3 system. The structural properties revealed, at room temperature, a rhombohedral (R3c) ferroelectric phase for the studied compositions. The dielectric analysis was carried out in a wide temperature and frequency ranges. The dielectric results have shown a clear magneto-electric coupling, which is associated to the presence of a dielectric anomaly around the antiferromagnetic-paramagnetic transition temperature. Additionally, the Cr3+ ions play an important role decreasing the electrical conductivity of the studied compositions. The doping element has also shown an important influence on the magnetic hysteresis results.

Structural and dielectric properties of the (Bi0.500Na0.500)0.920Ba0.065La0.010TiO3 lead-free ceramic system

Structural and dielectric properties of the (Bi0.500Na0.500)0.920Ba0.065La0.010TiO3 ceramic system have been investigated. The structural properties revealed, at room temperature, the coexistence of both rhombohedral (R3c) ferroelectric and tetragonal (P4bm) antiferroelectric phases. The dielectric response has shown the presence of three anomalies in the whole analyzed temperature range, which have been associated to different phase transitions. The frequency dependent dielectric response has been evaluated for temperatures close (below) the first phase-transition peak. Results revealed a strong dielectric dispersion, which has been associated to an intrinsic conduction related to thermally activated polarons.

Structural and electronic properties of Fe monolayer on BaTiO3(0 0 1)

The structural, electronic, and phonon properties of the BaTiO3(0 0 1) surface and the Fe/BaTiO3(0 0 1) interface have been studied within the density functional theory. Attention is paid to the lattice instabilities (soft phonon modes) that induce ferroelectric distortions in the surface and the interface. A phonon-induced monoclinic (Cm) thin-film counterpart of the low-temperature rhombohedral (R3m) ferroelectric bulk BaTiO3 phase is found. The changes in crystal structure, electronic density of states, atomic charges, and magnetic moments associated with the ferroelectric distortions are discussed comparing the results of the standard GGA and the hybrid DFT calculations. The magnetoelectric coupling at the Fe/BaTiO3(0 0 1) interface is investigated by the analysis of changes in magnetic moments on Fe and Ti atoms induced by the atomic displacements perpendicular and parallel to the surface.

Spectroscopic study of phase transitions in ferroelectric Bi0.5Na0.5Ti1−xMnxO3−δ films with enhanced ferroelectricity and energy storage ability

Lead-free ferroelectric Bi0.5Na0.5TiO3 (BNT) has attracted considerable attention taking into account environment issues and applications in micro electro mechanical systems. The effects of manganese (Mn) substitution on microstructure, lattice dynamics, and optical properties of BNT films have been investigated by X-ray diffraction, Raman scattering and ellipsometric spectra. The phase transitions as a function of temperature have been systemically explored by temperature-dependent Raman and ellipsometric spectra. The anomalous temperature-dependent behavior of Raman-active modes suggests that there is an intermediate phase between ferroelectric rhombohedral and paraelectric tetragonal phase, which is confirmed by the temperature-dependent optical band gap (Eg) and extinction coefficient (κ) extracted from ellipsometric spectra analysis. And then, a phase diagram as a function of Mn composition has been proposed. Finally, we demonstrate that Mn-dopant is an effective approach to enhance ferroelectric properties, improve energy storage ability, and increase permittivity as well as suppress leakage current. Electrical dielectric responses indicate that there is a critical frequency and polarization relaxation behavior in the films. The present results will be helpful for the application of BNT-based lead-free multifunctional devices.

Improving densification and electrical properties of KNN-based lead-free piezoceramics via two-step sintering method

ABSTRACTThe 0.95(Li0.03Na0.5K0.47)(Nb0.92Sb0.05Ta0.03)O3-0.05(Ba0.5Ca0.5)ZrO3 (LNKNST-BCZ) lead-free piezoelectric ceramics were prepared by the conventional ceramic processing via the two-step sintering method. The phase structure purity and electrical properties of the LNKNST-BCZ ceramics can be improved further by tailoring the sintering temperature and sintering time. Rather pure perovskite structure with the coexistence of ferroelectric rhombohedral (R) and tetragonal (T) phases can be obtained in the LNKNST-BCZ ceramics via the two-step sintering method confirmed by the X-ray diffraction (XRD) analysis. Sintered at optimized conditions the LNKNST-BCZ ceramics present high densification and rather homogeneous microstructure morphology. The Curie-Weiss law and the quadratic law fitting prove that the dielectric behavior of the LNKNST-BCZ ceramics approaches closer to the normal ferroelectrics with slight diffused phase transition characteristic. The LNKNST-BCZ ceramics sintered at 1180°C for 0.5 h exhibit the optimum dielectric, ferroelectric and piezoelectric properties. The improvement of the densification and piezoelectric properties of the LNKNST-BCZ ceramics can be attributed to the polymorphic R-T phase boundary and densified microstructure morphology induced by (Ba0.5Ca0.5)ZrO3 (BCZ) doping and two-step sintering method.

Dielectric and ferroelectric properties of lanthanum‐modified lead zirconate stannate titanate (42/40/18) ceramics

AbstractThe effect of lanthanum (La) content on the phase transformation of Pb1−3x/2Lax(Zr0.42Sn0.40Ti0.18)O3 (PLZST 100x/42/40/18, 0 ≤ x ≤ 0.06) ceramics was investigated by the dielectric and ferroelectric properties. The base composition PLZST 0/42/40/18 located in the ferroelectric (FE) rhombohedral phase region. As x increased, the compositions showed successively FE and antiferroelectric (AFE) state at room temperature, and their peak temperatures (Tmax) decreased gradually in line as Tmax = 162.21‐1507x. Evidence was presented that there were two dielectric anomalies in PLZST 2/42/40/18, which were corresponding to the FE‐AFE and AFE‐paraelectric (PE) phase transformations, respectively. With increasing the dc bias fields, the two phases merged into one. PLZST 3/42/40/18 showed AFE characteristics with the first loop outside of the second loop and there was only one dielectric inflection. The critical lanthanum content occurred at x = 0.03 from the dielectric temperature spectra and hysteresis loops. Furthermore increase in La above 0.03, these compositions showed typical antiferroelectric behaviors with double hysteresis loops. The stored energy properties of the three compositions (PLZST 4/42/40/18, 5/42/40/18 and 6/42/40/18) displayed different temperature dependencies from room temperature to 140°C (over their respective Tmax). Comparing the above results with previous investigations on PLZSTs, some questions were discussed.

Electric field induced phase transition and accompanying giant poling strain in lead-free NaNbO3-BaZrO3 ceramics

A series of phase transitions in (1-x)NaNbO3-xBaZrO3 ((1-x)NN-xBZ) ceramics was observed from antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase and finally into ferroelectric rhombohedral phase with increasing x. An electric field induced irreversible phase transition was found in different compositions, irrespective of their virgin phase structures. Particularly, an antiferroelectric orthorhombic phase is irreversibly transformed into a ferroelectric monoclinic phase within 0.02 ≤ x ≤ 0.05, leading to a giant poling strain of ∼0.58%. This is much larger than that observed in ferroelectric orthorhombic (0.06 ≤ x ≤ 0.07) and rhombohedral phases (0.08 ≤ x ≤ 0.11) suffering from an irreversible ferroelectric-ferroelectric (monoclinic) phase transition. The synchrotron x-ray diffraction and the measurement of longitudinal and transverse strains suggest that this irreversible phase transition should involve not only a distinct volume expansion, but also an obvious lattice elongation. The present study demonstrates a unique nature of the composition and field dependent phase stability and an underlying mechanism of giant poling strains in NN-BZ ceramics.

Microstructure and Properties of the Ferroelectric-Ferromagnetic PLZT-Ferrite Composites

The paper presents the technology of ferroelectric-ferromagnetic ceramic composites obtained from PLZT powder (the chemical formula Pb0.98La0.02(Zr0.90Ti0.10)0.995O3) and ferrite powder (Ni0.64Zn0.36Fe2O4), as well as the results of X-ray powder-diffraction data (XRD) measurement, microstructure, dielectric, ferroelectric, and magnetic properties of the composite samples. The ferroelectric-ferromagnetic composite (P-F) was obtained by mixing and the synthesis of 90% of PLZT and 10% of ferrite powders. The XRD test of the P-F composite shows a two-phase structure derived from the PLZT component (strong peaks) and the ferrite component (weak peaks). The symmetry of PLZT was identified as a rhombohedral ferroelectric phase, while the ferrite was identified as a spinel structure. Scanning electron microscope (SEM) microstructure analysis of the P-F ceramic composites showed that fine grains of the PLZT component surrounded large ferrite grains. At room temperature P-F composites exhibit both ferroelectric and ferromagnetic properties. The P-F composite samples have lower values of the maximum dielectric permittivity at the Curie temperature and a higher dielectric loss compared to the PLZT ceramics, however, the exhibit overall good multiferroic properties.

Electric field induced irreversible change and asymmetric butterfly strain loops in Pb(Zr,Ti)O3-Pb(Ni1/3Nb2/3)O3-Bi(Zn1/2Ti1/2)O3 quaternary ceramics

The phase transformation and strain characteristics of (0.9-x)Pb(Zr0.54Ti0.46)O3−0.1Pb(Ni1/3Nb2/3)O3-xBi(Zn1/2Ti1/2)O3 (0.1PNN-(0.9-x)PZT-xBZT) solid-solution ceramics were investigated. The substitution of BZT for PZT can effectively decrease the sintering temperature from 1150 °C at x = 0–975 °C at x = 0.24. A typical morphotropic phase boundary (MPB) between ferroelectric rhombohedral (R) phases and relaxor tetragonal (T) phases was identified in the composition range of x = 0.12–0.18, resulting in enhanced dielectric, piezoelectric and electromechanical properties of d33 = 610 pC/N, kp = 58% an ε33T/εo = 2518 at x = 0.16. Most interestingly, exceptionally asymmetric butterfly strain loops were observed in the first-cycle electric loading, and found to obviously change with increasing the BZT content. The X-ray diffraction analysis on virgin and poled samples suggests that the observed asymmetric strain curves should stem from the electric field induced irreversible lattice shrinkage or lattice elongation in the single-R or single-T phase compositions, respectively. For MPB compositions, a combined effect of both opposite strain contributions would result in a maximum value (in absolute value) in the observed irreversible strain Sirr at x = 0.12, together with the effect of the electric field induced irreversible T-R phase transition. These results provide useful insights into the mechanism of forming asymmetric bipolar strains observed in BZT-substituted PZT-PNN piezoelectric ceramics.

Effect of the lanthanum concentration on the physical properties of the (Bi0.5Na0.5)0.92Ba0.08-3x/2LaxTiO3 ceramic system

The influence of lanthanum concentration on the structural, ferroelectric and dielectric properties of BNT–BT ceramic compounds with a composition close to the morphotropic phase boundary (Bi0.5Na0.5)0.92Ba0.08-3x/2LaxTiO3 (x = 0, 1, 2 and 3 at.%) has been studied. The structural properties reveal the coexistence of both ferroelectric (P4mm) and antiferroelectric (P4bm) tetragonal phases for the lanthanum-free system. Nonetheless, for the La3+ modified samples, both antiferroelectric tetragonal (P4bm) and ferroelectric rhombohedral (R3c) phases coexist. On the other hand, slim-like hysteresis loops for the x = 1 at.% La3+ concentration have been observed, suggesting a higher stability of the antiferroelectric phase. However, for the other compositions (including the lanthanum-free sample) the P–E hysteresis loops suggested a higher stability of the ferroelectric phase. The dielectric response has shown the presence of three anomalies in the entire analyzed temperature range, which have been associated to different phase transitions with an important influence of the lanthanum content.

Effect of chemical pressure on competition and cooperation between polar and antiferrodistortive distortions in sodium niobate

We present results obtained from a combination of dielectric and x-ray diffraction measurements for compositional design of $(1\ensuremath{-}x)\phantom{\rule{0.28em}{0ex}}\mathrm{NaNb}{\mathrm{O}}_{3}\ensuremath{-}x\mathrm{BaTi}{\mathrm{O}}_{3}\phantom{\rule{0.28em}{0ex}}(\mathrm{NNBT}x)$, which can induce interferroelectric phase transitions. Anomalies are observed in dielectric measurements performed for various compositions at 300 K, as well as at different temperatures for NNBT03. We observed the appearance(disappearance) of the superlattice reflections along with change in the intensities of the main perovskite peaks in the powder x-ray diffraction data, which provide clear evidences for structural phase transitions with composition and temperature. We found that increasing the concentration of $\mathrm{BaTi}{\mathrm{O}}_{3}$ leads to the suppression of out-of-phase rotation of octahedra and an increment in tetragonality ($c/a$ ratio), which promotes the polar mode at room temperature. The temperature-dependent powder diffraction study shows that the ferroelectric rhombohedral phase of pure sodium niobate gets suppressed for the composition $x=0.03$, and the monoclinic phase $Cc$ gets stabilized at low temperature. The monoclinic phase is believed to provide for a flexible polarization rotation and is considered to be directly linked to the high-performance piezoelectricity in materials due to presence of more easy axes for spontaneous polarizations than the rhombohedral phase.

Effects of nano-sized BCZT on structure and electrical properties of KNN-based lead-free piezoceramics

The (1 − x)(Li0.03Na0.5K0.47)(Nb0.92Sb0.05Ta0.03)O3–xBa0.85Ca0.15Zr0.1Ti0.9O3 (LNKNST–BCZT, x = 0, 0.5, 1.5, 2.5, and 3.5%) lead-free piezoceramics were synthesized by the conventional solid-state reaction method with adding nano-sized Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) powder to investigate their influences on crystal structure and electrical properties. The X-ray diffraction (XRD) analysis reveals that all the sintered LNKNST–BCZT ceramics exhibit pure perovskite structure with the mainly rhombohedral (R) phase or the coexistence of rhombohedral (R) phase and tetragonal (T) phase. The composition of the LNKNST–1.5%BCZT ceramics locates around the critical point, i.e., the ceramics changing from the coexistence of rhombohedral (R) phase and tetragonal (T) phase to purely rhombohedral (R) phase, and presents the most densified microstructure morphology, which can be confirmed further by the field emission scanning electron microscope (FESEM) observation. There are two dielectric anomalies in the temperature dependent dielectric properties curves of the LNKNST–BCZT ceramics, which correlate with the ferroelectric rhombohedral (R) phase changing to the ferroelectric tetragonal (T) phase, and then to the paraelectric cubic (C) phase. Both the Curie–Weiss law and the power law fittings confirm the diffuse phase transition ferroelectrics characteristic of the LNKNST–1.5%BCZT ceramics, which is considered as correlating with the polar nanoregions (PNRs). Due to the densification effect caused by the chosen amount of nano-sized BCZT doping and the construction of R–T polymorphic phase boundary, the LNKNST–1.5%BCZT ceramics exhibit the best ferroelectric and piezoelectric properties. The complex impedance spectroscopy analysis confirms that the extrinsic electrical conduction mechanism at high temperatures is dominated by the oxygen vacancies induced by the evaporation of the alkali metals during sintering.

Tailored phase transition and electric properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric ceramics by ZnO modification

Microstructure, phase transition and electric properties of Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 (PZTN 95/5) ferroelectric ceramics modified with x wt% ZnO additive (where x = 0, 0.1, 0.2, 0.3 and 0.4) were systematically investigated. It was found that ZnO modification brought out an improvement of densification and an increase in grain size of PZTN 95/5 ceramics. Results of dielectric and pyroelectric measurements demonstrated that, the low temperature rhombohedral-high temperature rhombohedral ferroelectric phase transition temperature (TLH) gradually decreased with increasing x to 0.2, while the Curie temperature (Tc) gradually increased. With further increasing ZnO content, however, TLH and TC almost showed no change. Additionally, the excellent ferroelectric, pyroelectric and piezoelectric properties were all achieved by less than 0.2 wt% ZnO modification. It is suggested that, due to superior room–temperature pyroelectric properties, the 0.2 wt% ZnO-modified PZTN 95/5 ceramics may be a promising material for applications in uncooled pyroelectric infrared detectors.

Ferroelectric, elastic, piezoelectric, and dielectric properties of Ba(Ti0.7Zr0.3)O3-x(Ba0.82Ca0.18)TiO3 Pb-free ceramics

We designed and synthesized a pseudo-binary Pb-free system, Ba(Ti0.7Zr0.3)O3-x(Ba0.82Ca0.18)TiO3, by combining a rhombohedral end (with only cubic to rhombohedral ferroelectric phase transition) and a tetragonal end (with only cubic to tetragonal ferroelectric phase transition). The established composition-temperature phase diagram is characterized by a tricritical point type morphotropic phase boundary (MPB), and the MPB composition has better ferroelectric, piezoelectric, and dielectric properties than the compositions deviating from MPB. Moreover, a full set of material constants (including elastic stiffness constants, elastic compliance constants, piezoelectric constants, dielectric constants, and electromechanical coupling factors) of the MPB composition are determined using a resonance method. The good piezoelectric performance of the MPB composition can be ascribed to the high dielectric constants, elastic softening, and large electromechanical coupling factor.

Large electro-strain response of La3+ and Nb5+ co-doped ternary 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 lead-free piezoelectric ceramics

La3+ and Nb5+ co-doped 0.85Bi0.5Na0.5TiO3-0.11Bi0.5K0.5TiO3-0.04BaTiO3 (BNT-BKT-BT) samples were prepared through conventional solid oxide route, and the effects of doping ions on the phase structure, electrical properties, and electro-strain behavior were investigated. The XRD results showed that La3+ and Nb5+ co-dopant induced a phase transition from coexistence of ferroelectric tetragonal and rhombohedral phase to a pseudocubic phase. With increasing dopant content, the normal ferroelectric-to-ergodic relaxor (FE-to-ER) transition temperature (TF-R) decreased down from 120 °C to room temperature (RT), even below RT. A large unipolar strain of 0.34% (d33∗ = Smax/Emax = 680 pm/V) was achieved under 5 kV/mm and the largest electro-strain of 0.46% was achieved under an applied electric field of 7.5 kV/mm at RT for x = 0.0050. Temperature dependence of both polarization loops and bipolar strain curves from room temperature to 160 °C were also studied, and the results suggested that the origin of the large strain is attributed to a reversible nonpolar isotropic ergodic relaxor state to polar anisotropic ferroelectric phase transition.