Rhombohedral-tetragonal Phase Transition Research Articles

Enhanced electrocaloric effect in Sr-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 lead-free thin films near room temperature

Ferroelectric thin films with giant room-temperature electrocaloric effect (ECE) could be very useful in design of solid-state refrigeration devices. Here, lead-free Ba0.85Ca0.15-xSrxZr0.1Ti0.9O3 (BCZT-xSr, 0.01 ≤ x ≤ 0.10) thin films are successfully prepared on the substrates (Pt/Ti/SiO2/Si) via sol-gel route, and the effect of the Sr substitution on the structure, electric properties and ECE of the films were investigated. The dielectric analyses show that the rhombohedral-tetragonal phase transition temperature (TR-T) move to a higher temperature and the tetragonal-cubic phase transition temperature (Tm) shift to a lower temperature with increasing Sr content, and finally the TR-T peaks merge into the Tm peaks at room temperature. The BCZT-0.05Sr film presents a giant ECE with ΔT = 27.1 K and ΔS = 30.6 J K−1 kg−1 at 800 kV cm-1 near room temperature (301 K). The giant ΔT benefits from a multiphase coexisting region with successive R-T and T-C phase transition character near room temperature. These results indicate that the lead-free Sr-doped BCZT thin film is an attractive material for application in ECE refrigeration devices.

Research on optimized piezoelectric properties of PNN-PMN-PT ceramics and corresponding underlying factors based on component regulation

To meet the increasing demands for developing miniaturized high-precision modern piezoelectric devices, researchers are focusing their attention on PbTiO3 (PT) relaxor ferroelectrics having excellent comprehensive performance. Although the partial substitution of Pb(Mg1/3Nb2/3)O3 (PMN)-PT with Pb(Ni1/3Nb2/3)O3 (PNN)-PT has been reported to improve dielectric and piezoelectric properties, the factors underlying the performance improvement of the PNN-PMN-PT ternary system remain unclear. Therefore, based on PT content regulation, the 0.128PNN-(0.872-x)PMN-xPT (x = 0.316, 0.326, 0.336, 0.346, and 0.356) ternary ceramics were fabricated, and the crystal structure and electrical properties were systematically investigated during the rhombohedral-tetragonal phase transition. The 0.128PNN-0.536PMN-0.336PT composition exhibited optimum performance with the piezoelectric charge coefficient, effective piezoelectric strain coefficient, dielectric constant were 691 pC/N, 1060 pm/V and 4545 respectively at a maximum permittivity temperature of 136 ℃, signifying its potential for use in transducers and actuators. The outstanding properties of this system were mainly derived from the lattice distortion and reversible domain wall motion. The results indicated that the multiphase coexistence and enhanced B-site disorder in the perovskite structure contribute to the improvement in the piezoelectric properties, which would facilitate the design and development of novel piezoelectric materials.

Water Quenched and Acceptor-Doped Textured Piezoelectric Ceramics for Off-Resonance and On-Resonance Devices.

Piezoelectric materials should simultaneously possess the soft properties (high piezoelectric coefficient, d33 ; high voltage coefficient, g33 ; high electromechanical coupling factor, k) and hard properties (high mechanical quality factor, Qm ; low dielectric loss, tan δ) along with wide operation temperature (e.g., high rhombohedral-tetragonal phase transition temperature Tr-t ) for covering off-resonance (figure of merit (FOM), d33 × g33 ) and on-resonance (FOM, Qm × k2 ) applications. However, achieving hard and soft piezoelectric properties simultaneously along with high transition temperature is quite challenging since these properties are inversely related to each other. Here, through a synergistic design strategy of combining composition/phase selection, crystallographic texturing, defect engineering, and water quenching technique, <001> textured 2mol% MnO2 doped 0.19PIN-0.445PSN-0.365PT ceramics exhibiting giant FOM values of Qm × (227-261) along with high d33 × g33 (28-35 × 10-12 m2 N-1 ), low tan δ (0.3-0.39%) and high Tr-t of 140-190°C, which is far beyond the performance of the state-of-the-art piezoelectric materials, are fabricated. Further, a novel water quenching (WQ) room temperature poling technique, which results in enhanced piezoelectricity of textured MnO2 doped PIN-PSN-PT ceramics, is reported. Based upon the experiments and phase-field modeling, the enhanced piezoelectricity is explained in terms of the quenching-induced rhombohedral phase formation. These findings will have tremendous impact on development of high performance off-resonance and on-resonance piezoelectric devices with high stability.

Natural day–night temperature variations driving pyrocatalytic wastewater treatment and CO2 reduction based on room-temperature rhombohedral–tetragonal phase transition of lead-free ferroelectric BZT-0.5BCT fibers

Environmental remediation approaches in terms of wastewater treatment and CO2 reduction using environmental energy are highly desirable for an industrialized society. Herein, the pyrocatalytic performance of electrospun lead-free ferroelectric Ba(Ti0.8Zr0.2)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT, x = 0.2, 0.3, 0.4, 0.5, 0.6) micro-nano fibers is investigated, and BZT-0.5BCT is identified as an efficient pyrocatalyst for water pollutants decomposition driven by rhombohedral–tetragonal (R-T) phase transition. Phase changes can induce high pyroelectric coefficients. In response to 0–60 °C thermal cycles, the decomposition ratio of Rhodamine B (RhB) by BZT-0.5BCT fibers after nine thermal cycles is approximately 70%. Moreover, under outdoor day-night temperature fluctuations, about 76% of RhB and 95% of methylene blue (MB) can be degraded after 10 and 7 days respectively because BZT-0.5BCT has an R-T phase change point of around 30 °C. For Cr(VI) (potassium dichromate) and tetracycline (TE), the fibers also show good pyrocatalytic degradation ability. Due to destruction of bacterial cell membranes by superoxide groups, high levels of antimicrobial activity could also be achieved against Escherichia coli (99.99%) and Staphylococcus aureus (92.83%). Furthermore, the pyrocatalytic reduction of CO2 to acetaldehyde by BZT-0.5BCT can reach approximately 382.47 μmol g−1 after 20 thermal cycles of 0–60 °C. Therefore, such outstanding catalytic performance of pyroelectric BZT-0.5BCT fibers enables pyroelectric catalysis as a potential approach for both all-round wastewater treatment and CO2 reduction.

Natural Day–Night Temperature Variations Driving Pyrocatalytic Wastewater Treatment and Co2 Reduction Based on Room-Temperature Rhombohedral–Tetragonal Phase Transition of Lead-Free Ferroelectric Bzt-0.5bct Fibers

Natural Day–Night Temperature Variations Driving Pyrocatalytic Wastewater Treatment and Co2 Reduction Based on Room-Temperature Rhombohedral–Tetragonal Phase Transition of Lead-Free Ferroelectric Bzt-0.5bct Fibers

Textured ferroelectric ceramics with high electromechanical coupling factors over a broad temperature range

The figure-of-merits of ferroelectrics for transducer applications are their electromechanical coupling factor and the operable temperature range. Relaxor-PbTiO3 ferroelectric crystals show a much improved electromechanical coupling factor k33 (88~93%) compared to their ceramic counterparts (65~78%) by taking advantage of the strong anisotropy of crystals. However, only a few relaxor-PbTiO3 systems, for example Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3, can be grown into single crystals, whose operable temperature range is limited by their rhombohedral-tetragonal phase transition temperatures (Trt: 60~120 °C). Here, we develop a templated grain-growth approach to fabricate <001>-textured Pb(In1/2Nb1/2)O3-Pb(Sc1/2Nb1/2)O3-PbTiO3 (PIN-PSN-PT) ceramics that contain a large amount of the refractory component Sc2O3, which has the ability to increase the Trt of the system. The high k33 of 85~89% and the greatly increased Trt of 160~200 °C are simultaneously achieved in the textured PIN-PSN-PT ceramics. The above merits will make textured PIN-PSN-PT ceramics an alternative to single crystals, benefiting the development of numerous advanced piezoelectric devices.

Domain patterns and super-elasticity of freestanding BiFeO3 membranes via phase-field simulations

Super-elasticity of functional ferroelectric oxides offers promises for integrating ferroelectric films into flexible electronics. However, super-elastic deformation is a complex phenomenon related to possibly multiple concurrent mechanisms. Fundamentally understanding how multiple mechanisms contribute to the super-elasticity of ferroelectric oxides is crucial to realizing their potential flexible electronic applications. Here, we employ phase-field simulations to model the dynamics of ferroelectric domain patterns of freestanding BiFeO3 membranes to understand the origin of their super-elasticity under substantial bending deformation (5% strain). It is demonstrated that both a reversible Rhombohedral-Tetragonal (R-T) phase transition and a nearly reversible domain evolution of BiFeO3 membranes contribute to accommodating the large deformation and thus their super-elasticity. The dynamics of domain evolution also reveal the formation of an exotic ferroelectric vortex and polarization rotation before the phase transition. We constructed a diagram of phases and domain patterns as a function of the membrane thickness and bending angle, which allows one to readily predict the emergence of T phase and ferroelectric vortex in bent BFO membranes. These results not only provide fundamental understanding of mesoscale super-elastic mechanisms but also reveal exotic domain states of ferroelectric membranes.

Effects of Nano-Sized In2O3 on Sintering Behavior and Electrical Properties of 0.28Pb(In1/2Nb1/2)O3-0.40Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 Ceramics

0.28Pb(In1/2Nb1/2)O3-0.40Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 ceramics were synthesized by conventional solid-phase method using nano-sized In2O3. All the sintered 0.28PIN-0.40PMN-0.32PT ceramics have pure perovskite structure with composition locating at rhombohedral side around the morphotropic phase boundary (MPB) region. The 0.28PIN-0.40PMN-0.32PT ceramics belong to displacive driving ferroelectrics with apparent diffused phase transition characteristic. Although the rhombohedral-tetragonal ferroelectric phase transition temperature (TR-T) increases only to ∼120 °C due to the solid solution of Pb(In1/2Nb1/2) (PIN), the 0.28PIN-0.40PMN-0.32PT ceramics present high piezoelectric thermal stability till around the Curie temperature (Tm), which extremely widens the working temperature range and makes the 0.28PIN-0.40PMN-0.32PT ceramics suitable for high-temperature piezoelectric applications.

Electrical and magnetodielectric coupling properties of Co-doped PMN-32PT ferroelectric single crystal

Co-doped Pb(Mg1/3Nb2/3)O3–32PbTiO3 (Co-doped PMN–32PT) single crystals were grown by a flux method. The phase structure, dielectric, piezoelectric, ferroelectric and magnetodielectric coupling properties of the crystals were characterized. The result of X-ray diffraction showed Co-doped PMN-32PT single crystal was perovskite phase. The dielectric properties of the Co-doped PMN-32PT single crystal along [001]-orientation has a higher rhombohedral-tetragonal phase transition temperature (Trt~107 °C) and larger coercive field (EC~7.91 kV/cm) than the [001] oriented PMN-32PT single crystal. The piezoelectric constant d33 is 1200 pC/N. Meanwhile, Under the external magnetic field, Co-doped PMN-32PT single crystal has negative magnetodielectric coupling effect.

New strategy to solve temperature sensitivity in (K, Na)NbO3: Modified slope of R-T phase boundary through phase content regulation

The temperature stability of potassium sodium niobate based materials has become a huge challenge due to its polymorphic phase boundary. How to thoroughly solve temperature sensitivity is one of the urgent aims in practical applications. Here, a new strategy of modifying R-T slope through phase content regulation has been achieved in lead-free ceramics of 0.96K0.5Na0.5Nb0.955Sb0.045O3–0.04[(1-x)Bi0.5Na0.5ZrO3-xBi0.5Na0.5SnO3] (0 ≤ x ≤ 0.20) aiming to verify the relationship between phase content and temperature insensitivity. The temperature dependent on Suni, d33, and domain has proved that decreasing of tetragonal (T) phase content as rising rhombohedral-tetragonal phase transition temperature (TR-T) results in poor temperature stability. Moreover, an ideal R-T phase boundary similar to MPB model based on the results is proposed to possibly solve the fundamental problem of temperature sensitivity via increasing T phase content to modify the slope of R-T phase boundary. Therefore, it can be inferred that a more favorable temperature insensitivity can be obtained if one can construct a more slope R-T phase boundary, providing a new thought to consider how to thoroughly solve temperature sensitivity in KNN-based ceramics.

High-field electrostatic control of ferromagnetic resonance in NiFe/PIN-PMN-PT multiferroic heterostructures

We report electric-field control of ferromagnetic resonance in NiFe film, which was deposited on (011)-cut 0.3Pb(In1/2Nb1/2)O3-0.4Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PIN-PMN-PT) single crystals. An upward frequency shift from 11.69 to 12.53 GHz is achieved in our heterostructures for electric field from 0 to 15 kV/cm as an external magnetic field of 2090 Oe was applied along in-plane [100] direction, and a downward shift from 13.61 to 13.25 GHz is investigated for electric field from 0 to 7 kV/cm, while an external magnetic field of 2723 Oe was applied along [01$$\stackrel{-}{1}$$] direction. The large operational electric field and the high rhombohedral-tetragonal phase transition temperature TR−T (~ 127 °C) of the crystal make NiFe/PIN-PMN-PT composites great candidates for electrically tunable microwave magnetic device applications with broad electric field and temperature range.

Electronic switching by metastable polarization states in BiFeO3 thin films

We present an approach to control resistive switching in metal-ferroelectric contacts using a radially symmetric electric field. In ferroelectrics with significant polarization along the corresponding field lines, the field above a critical threshold will induce polarization discontinuity, a corresponding nanoscale volume of space charge, and a conducting junction. We demonstrate this principle using nanoscale polarization switching of a conventional (001)-oriented thin film of $\mathrm{BiFe}{\mathrm{O}}_{3}$. Without any optimization, the conducting state created in this regime of resistive switching exhibits local currents of $\ensuremath{\sim}1\text{--}10\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$, approaching the $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{nA}$ threshold required for device implementation [J. Jiang et al., Nat. Mater. 17, 49 (2018)]. The corresponding electronic function is that of a volatile resistive switch, which is directly compatible with neuristor functionality that encodes the functioning basis of an axon [M. D. Pickett et al., Nat. Mater. 12, 114 (2013)]. Phase-field modeling further reveals that in the strongly charged local configuration, $\mathrm{BiFe}{\mathrm{O}}_{3}$ locally undergoes a rhombohedral-tetragonal (R-T) phase transition, in part due to substantial piezoelectric expansion of the lattice. The estimated local charge density can be as high as $\ensuremath{\sim}{10}^{21}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$, which would be extremely difficult to achieve by conventional doping approaches without altering other material properties. Therefore, this method for creating stable and reproducible strongly charged ferroelectric junctions enables more systematic studies of their physical properties, such as the possibility of structural and electronic phase transitions, and it can lead to new ferroelectric devices for advanced information functions.

Thickness-dependent structural and electromechanical properties of (Na0.85K0.15)0.5Bi0.5TiO3 multilayer thin film-based heterostructures

(Na0.85K0.15)0.5Bi0.5TiO3 (NKBT) multilayer thin films with different thicknesses of 100–700nm, corresponding to 2–14 layers with each layer of ~50nm thickness, are synthesized on Pt(111)/Ti/SiO2/Si substrates to form Pt/NKBT/Pt/Ti/SiO2/Si heterostructures using different spin-coating and annealing conditions in a modified aqueous sol-gel process. The multilayer thin films spin-coated by two steps (step 1/2) at 600/4000rpm for 6/30s and annealed at 700°C for 5min with a heating rate of 30°C/s show a dense, uniform, and continuous morphology as well as a pure perovskite structure with a rhombohedral–tetragonal phase transition at ~140°C and no preferential orientation in the heterostructures. Their structural and electromechanical properties exhibit consistent improvement trends with increasing thickness from 100 to 550nm (i.e., 2–11 layers). The 550nm-thick, 11-layer films demonstrate the best ferroelectric, dielectric, piezoelectric, and electric performance in terms of the highest remnant polarization, saturation polarization, dielectric constant, and effective piezoelectric constant of 18.3μC/cm2, 53.6μC/cm2, 463, and 64pm/V, as well as the lowest coercive field, dielectric loss tangent, and leakage current density of 116kV/cm, 0.057, and 27μA/cm2, respectively. The observed thickness-dependent improvement is explained by an interfacial passive layer effect where the motion of both 180° and non-180° domain walls is enhanced in the thicker multilayer thin films by weakening the influence of domain pinning in the interfacial passive layers between the multilayer thin films and the substrates.

Damping constant, dielectric susceptibility, inverse relaxation time and the activation energy calculated as a function of temperature from the Raman frequency for the rhombohedral-tetragonal phase transition in BaCeO3

Temperature dependences of the damping constant (half width at half maximum), dielectric susceptibility, inverse relaxation time and the activation energy of the 110 cm -1 and 125 cm -1 Raman modes are calculated in the ferroelectric phase (T<T C ) of BaCeO 3 . Observed frequencies of the Raman modes studied are related with the order parameter (spontaneous polarization) to calculate the damping constant from the pseudospin-phonon coupled (PS) model and the energy fluctuation (EF) model for the orthorhombic-tetragonal transition in BaCeO 3 (T C = 427 K). Our calculated values for the damping constant from both models are in good agreement with the observed data. The inverse relaxation time of those Raman modes are predicted using the calculated values of the damping constant from both models (PS and EF) and the values of the order parameter (squared). Dielectric susceptibility is also predicted through the observed Raman frequencies of the modes studied by using the Landau phenomenological theory. Our calculated values of the inverse relaxation time and the dielectric susceptibility of the 110 cm -1 and 125 cm -1 Raman modes of BaCeO 3 can be compared with the experimental data when they are available in the literature. Finally, we extract the values of the activation energy from the damping constant calculated from both models for the Raman modes studied in the ferroelectric phase of BaCeO 3 .

Surface acoustic wave properties in 0.955Pb(Zn1/3Nb2/3)O3-0.045PbTiO3 multidomain single crystal poled along [001

ABSTRACTSurface acoustic wave propagation in a 0.955Pb (Zn1/3Nb2/3)O3-0.045PbTiO3 multidomain single crystal poled along [001]c has been analyzed theoretically using Christoffel wave equations. From the measured experimental data, the orientational dependence of surface acoustic wave phase velocities, electromechanical coupling coefficients and power flow angles was calculated. The results showed that surface acoustic wave propagations of 0.955Pb (Zn1/3Nb2/3)O3-0.045PbTiO3 are poor than 0.93Pb(Zn1/3Nb2/3)O3-0.07PbTiO3 crystal slightly. However, the 0.955Pb (Zn1/3Nb2/3)O3-0.045PbTiO3 single crystals have higher rhombohedral-tetragonal phase transition temperature and much stabler electromechanical properties. This may expand applications range of SAW devices.

Investigation of morphotropic phase boundaries in PIN–PSN–PT relaxor ferroelectric ternary systems with high Tr-t and Tc phase transition temperatures

New morphotropic phase boundary (MPB) compositions with relatively high Tcs were projected in Pb(In1/2Nb1/2)O3–Pb(Sc1/2Nb1/2)O3–PbTiO3 (PIN–PSN–PT) solid solution based on the perovskite tolerance factor, and were experimentally confirmed. The phase structure, dielectric, pyroelectric, piezoelectric and ferroelectric properties of PIN–PSN–PT ceramics were investigated. The rhombohedral–tetragonal phase transition temperatures Tr-t on the order of 189–210°C, Curie temperatures Tc on the order of 274–285°C and piezoelectric coefficients d33 in the range of 310–360pC/N, were achieved in xPIN– (1−x)PSN–0.37PT (x=0.15–0.23) ceramics, demonstrating a promising relaxor–PbTiO3 system with high phase transition temperatures. In addition, the maps of Tc, Tr-t, d33 and εr with respect to composition were established, based on which, a clear direction for composition screening in future crystal growth of the PIN–PSN–PT system was given.

Study of temperature-induced structural evolution in (Na0.5Bi0.5)TiO3-(K0.5Bi0.5)TiO3-(K0.5Na0.5)NbO3 lead-free ceramics

In this work, the temperature-induced structural evolution in 0.79(Na0.5Bi0.5)TiO3-0.2(K0.5Bi0.5)TiO3-0.01(K0.5Na0.5)NbO3 (NKBNT) lead-free ceramics was investigated by Raman microscopic spectroscopy combined with electrical macroscopic measurements. The NKBNT ceramics possess the local structure with the coexisted rhombohedral R3c and tetragonal P4bm polar-nano-regions (PNRs). The R3c and P4bm PNRs coexist in a wide temperature range, then the local structure transforms to the P4bm PNRs around the temperature of dielectric maximum (Tm) evidenced by the doublet splitting of Ti-O modes (peak B) and oxygen octahedral vibrational modes (peak C). The discontinuous changes of wavenumber and line-width of peak B2 and peak C3 as well as the dielectric local maxima around the rhombohedral-tetragonal phase transition temperature (TRT) are considered to result from the thermal evolution of R3c and P4bm PNRs. The macroscopic changes of non-polar phase with electric field and temperature were investigated by the temperature-dependent polarization-electric field (P-E) loops, current-electric field (I-E) loops and bipolar strain (S-E) curves. The electric-field level necessary to form the long-range ferroelectric order from non-polar phase associated with the stability of the induced ferroelectric phase depends strongly on the temperature.

High temperature limitation due to onset of depoling in BiScO3–PbTiO3

Thermal depoling is investigated with impedance spectroscopy as a function of temperature and frequency for unmodified and ZnSc′ modified compositions around the morphotropic phase boundary (MPB) in the high temperature material (1−x)BiScO3–xPbTiO3 (BSPT). By tracking the shape and width of the fundamental resonance peaks and phase angle during impedance analysis, it was shown that the depoling temperature (Td), usually considered a single temperature, can actually be characterized as occurring over a range with an onset temperature (Tdonset) and a final temperature (Tdfinal). When this technique is applied to the rhombohedral compositions near the MPB of BSPT, it is seen that the peak narrowing in phase angle closely correlates to the rhombohedral-tetragonal phase transition decreasing from 350 °C to 300 °C when increasing from 58% to 62% PT. The Tdonset and the subsequent correlation to the phase transition are investigated further through aliovalent substitution of 1%, 2%, and 5% ZnSc in BS-PT. The Tdonset for the rhombohedral compositions displays enhancement over the unmodified compositions at ∼12 °C/1% ZnSc.

Electric-field-induced tetragonal phase in [Pb(Mg1/3Nb2/3)O3]0.68–[PbTiO3]0.32 by Raman scattering

ABSTRACTThe paper describes investigation of the electric-field-induced phase transition in PMN-xPT single crystals with x = 0.32 by means of detailed polarized Raman scattering measurements. Field-induced tetragonal state was reached by applying an electric field up to 30 kV/cm along pseudo-cubic direction at a temperature slightly below that of the zero-field rhombohedral–tetragonal phase transition (TRT). In this field-induced tetragonal state, the angular dependencies of the polarized Raman spectra were recorded and compared with the similar data available for the rhombohedral single-domain and multidomain states.

Giant electric-field-induced strain in lead-free piezoelectric materials.

First-principles calculations are performed to investigate the structures, electrical, and magnetic properties of compressive BiFeO3 films under electric-field and pressure perpendicular to the films. A reversible electric-field-induced strain up 10% is achieved in the compressive BiFeO3 films. The giant strain originates from rhombohedral-tetragonal (R-T) phase transition under electric-filed, and is recoverable from tetragonal-rhombohedral (T-R) phase transition by compressive stress. Additionally, the weak ferromagnetism in BiFeO3 films is largely changed in R-T phase transition under electric-filed and T-R phase transition under pressure – reminiscent of magnetoelectric effect and magnetoelastic effect. These results suggest exciting device opportunities arising from the giant filed-induced strain, large magnetoelectric effect and magnetoelastic effect.