High-temperature Paraelectric Phase Research Articles

Structural changes in Rochelle salt on phase transitions revisited in a multi-temperature single-crystal X-ray diffraction study.

Phase transitions in Rochelle salt [sodium potassium L(+)-tartrate tetrahydrate] are revisited in a single-crystal X-ray diffraction multi-temperature study on cooling from 308 to 100 K across the high-temperature paraelectric (PE) ↔ ferroelectric ↔ low-temperature PE phase transition points. The results of structure refinement using three different models (a harmonic with and without disorder, and an anharmonic) were compared. The temperature dependencies of anisotropic displacement parameters (ADPs) and Ueq, which can be calculated directly from ADPs, for the low-temperature PE phase indicate clearly the dynamic nature of disorder of the K1 atoms. The structures of the low-temperature and the high-temperature PE phases are compared for the first time at multiple temperatures for each phase based on diffraction data collected from the same single crystal. The data indicate that the high-temperature and the low-temperature paraelectric phases are probably not two different phases, as was assumed in earlier works, but are structurally the same phase at different temperatures.

Polar topological superdomain arrays in PMN-PT crystals engineered via a voltage-free method

Domain engineering is an active field of research that aims to enhance the functional properties of ferroelectric materials for various applications. This work presents a voltage-free method for fabricating topological superdomain arrays in 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-28PT) crystals. During the cooling process from high-temperature paraelectric phase to room temperature ferroelectric phase, the out-of-plane polarization of PMN-28PT crystals in contact with aluminum can be fully regulated downward, attributed to the difference in the work function of aluminum and PMN-28PT. In conjunction further with photolithography and ultraviolet light illumination, it is easy to obtain periodic strip domain structures with alternating up and down polarization. Moreover, this method can also control in-plane polarization, resulting in a large-scale central divergent/convergent topological superdomain array. These findings provide insights into ferroelectric domain engineering and have implications for the development of electromechanical, piezoelectronic, and photonic devices.

Stochastic theory of ferroelectric domain structure formation dominated by quenched disorder

A self-consistent stochastic model of domain structure formation in a uniaxial ferroelectric, quenched from a high-temperature paraelectric phase to a low-temperature ferroelectric phase, is developed with an account of the applied electric field and the feedback effect via local depolarization fields. Both polarization and field components are considered as Gauss random variables. A system of integro-differential equations for correlation functions of all involved variables is derived and solved analytically and numerically. Phase diagram in terms of the average value and dispersion of polarization reveals different possible equilibrium states and available final single-domain and multi-domain states. The time-dependent evolution of the average polarization and dispersion discloses a bifurcation behavior and the temperature-dependent value of the electric field, deciding between the single-domain and multi-domain final states, which can be interpreted as the coercive field. Analytical and numerical results for the time-dependent correlation length and correlation functions exhibit plausible agreement with available experimental data.

In search of Pca21 phase ferroelectrics

In recent years, hafnia-based ferroelectrics have attracted enormous attention due to their capability of maintaining ferroelectricity below 10 nm thickness and excellent compatibility with microelectronics flow lines. However, the physical origin of their ferroelectricity is still not fully clear, although it is commonly attributed to a polar Pca21 orthorhombic phase. The high-temperature paraelectric phases (the tetragonal phase or the cubic phase) do not possess a soft mode at the Brillouin zone center, thus the ferroelectric distortion has to be explained in terms of trilinear coupling among three phonon modes in the tetragonal phase. It is necessary to explore new materials with possible ferroelectricity due to the polar Pca21 phase, which in turn should be very helpful in evaluating the microscopic theory for ferroelectric hafnia. In this work, based on the idea of the Materials Genome Engineering, a series of hafnia-like ferroelectrics have been found, exemplified by LaSeCl, LaSeBr, LuOF and YOF, which possess adequate spontaneous polarization values and also relatively favorable free energies for the polar phase. Their common features and individual differences are discussed in detail. In particular, a promising potential ferroelectric material, Pca21 phase LuOF, is predicted and recommended for further experimental synthesis and investigation.

Phase transitions study of the liquid crystal DIO with a ferroelectric nematic, a nematic, and an intermediate phase and of mixtures with the ferroelectric nematic compound RM734 by adiabatic scanning calorimetry.

High-resolution calorimetry has played a significant role in providing detailed information on phase transitions in liquid crystals. In particular, adiabatic scanning calorimetry (ASC), capable of providing simultaneous information on the temperature dependence of the specific enthalpy h(T) and on the specific heat capacity c_{p}(T), has proven to be an important tool to determine the order of transitions and render high-resolution information on pretransitional thermal behavior. Here we report on ASC results on the compound 2,3',4',5'-tetrafluoro[1,1'-biphenyl]-4-yl 2,6-difluoro-4-(5-propyl-1,3-dioxan-2-yl) benzoate (DIO) and on mixtures with 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). Both compounds exhibit a low-temperature ferroelectric nematic phase (N_{F}) and a high-temperature paraelectric nematic phase (N). However, in DIO these two phases are separated by an intermediate phase (N_{x}). From the detailed data of h(T) and c_{p}(T), we found that the intermediate phase was present in all the mixtures over the complete composition range, albeit with strongly decreasing temperature width for that phase with decreasing mole fraction of DIO (x_{DIO}). The x_{DIO} dependence on the transition temperatures for both transitions could be well described by a quadratic function. Both these transitions were weakly first order. The true latent heat of the N_{x}-N transition of DIO was as low as L=0.0075±0.0005J/g and L=0.23±0.03J/g for the N_{F}-N_{x} transition, which is about twice the previously reported value of 0.115 J/g for the N_{F}-N transition in RM734. In the mixtures both transition latent heats decrease gradually with decreasing x_{DIO}. At all the N_{x}-N transitions pretransition fluctuation effects are absent and these transitions are purely but very weakly first order. As in RM734 the transition from the N_{F} to the higher-temperature phase exhibits substantial pretransitional behavior, in particular, in the high-temperature phase. Power-law analysis of c_{p}(T) resulted in an effective critical exponent α=0.88±0.1 for DIO and this value decreased in the mixtures with decreasing x_{DIO} toward α=0.50±0.05 reported for RM734. Ideal mixture analysis of the phase diagram was consistent with ideal mixture behavior provided the total transition enthalpy change was used in the analysis.

Phase Competition in High-Quality Epitaxial Antiferroelectric PbZrO3 Thin Films.

Antiferroelectric PbZrO3 has attracted renewed interest in recent years because of its unique properties and wide range of potential applications. However, the nature of antiferroelectricity and its evolution with the electric field and temperature remain controversial, mostly due to the difficulty of obtaining high-quality single-crystal samples. The lack of consensus regarding the phase transition in PbZrO3 is not only important on a fundamental side but also greatly hinders further applications. Herein, high-quality PbZrO3 epitaxial thin films are successfully fabricated by pulsed laser deposition. The structural and physical properties of the films are systematically studied via a combination of electric property measurements, X-ray diffraction, scanning transmission electron microscopy imaging, and second-harmonic generation studies. Our studies unveil the noncentrosymmetric nature of PbZrO3 films at room temperature. Moreover, the Curie temperature increased to 270°, ∼40° higher than that in the bulk, and no intermediate ferroelectric phase was observed. Besides, an incipient ferroelectric with relaxor-like behavior above the Curie temperature due to the existence of a local polar cluster in the high-temperature paraelectric phase is experimentally observed for the first time. Our studies provide a better understanding of PbZrO3 thin films and pave the way for practical applications of antiferroelectric material in modern electronic devices.

Role of A and B site doping on antiferrodistortive phase transition in sodium bismuth titanate

We report the results of temperature-dependent powder X-ray diffraction studies on lead-free piezoelectric sodium bismuth titanate (NBT) modified by partially substituting Ba at the A site or Nb at the B site. We observed the disappearance of certain reflections and a change in the main perovskite peaks of the rhombohedral phase suggesting a structural phase transition. Detailed Rietveld refinements revealed that both the ferroelectric and the antiferrodistortive phase transitions occur together at high temperature leading to a change in crystal structure from the low-temperature ferroelectric rhombohedral phase (space group: R3c) to the high-temperature paraelectric cubic phase (space group:Pm3¯m). This suggests the suppression of the intermediate tetragonal phase of pure NBT. We found that substituting Ba onto the A site enhances the lattice strain and lowers the antiferrodistortive phase transition temperature.

Morphotropic phase boundaries of (1−x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 probed by Raman spectroscopy at high temperature

Morphotropic phase boundaries (MPBs) in relaxor Pb(Zn1/3Nb2/3)O3–ferroelectric PbTiO3 systems were investigated by means of angle-resolved polarized Raman spectroscopy at 800 K in the paraelectric phase. The features of the spectra represented by contour maps were extracted through the multivariate curve resolution (MCR) method, rendering the comparison among the data with six compositions more accurate and convenient. The result indicates that the structure becomes unstable at x = 0.07, which is slightly smaller than the MPB occurring at x = 0.09. The additional analysis using the MCR method to investigate the x dependence of the extracted Raman spectra shows anomalies at the MPB. The information related to the MPB near room temperature could be extracted through Raman spectroscopy even in a simple high-temperature paraelectric phase.

Quantum dynamics of Mn2+ in dimethylammonium magnesium formate.

Dimethylammonium magnesium formate, [(CH3)2NH2][Mg(HCOO)3] or DMAMgF, is a model used to study high temperature hybrid perovskite-like dielectrics. This compound displays an order-disorder phase transition at about 260 K. Using multifrequency electron spin resonance in continuous wave and pulsed modes, we herein present the quantum dynamics of the Mn2+ ion probe in DMAMgF. In the high temperature paraelectric phase, we observe a large distribution of the zero field splitting that is attributed to the high local disorder and further supported by density functional theory computations. In the low temperature ferroelastic phase, a single structure phase is detected and shown to contain two magnetic structures. The complex electron paramagnetic resonance signals were identified by means of the Rabi oscillation method combined with the crystal field kernel density estimation.

Soft mode theory of ferroelectric phase transitions in the low-temperature phase

Historically, the soft mode theory of ferroelectric phase transitions has been developed for the high-temperature (paraelectric) phase, where the phonon mode softens upon decreasing the temperature. In the low-temperature ferroelectric phase, a similar phonon softening occurs, also leading to a bosonic condensation of the frozen-in mode at the transition, but in this case the phonon softening occurs upon increasing the temperature. Here we present a soft mode theory of ferroelectric and displacive phase transitions by describing what happens in the low-temperature phase in terms of phonon softening and instability. A new derivation of the generalized Lyddane–Sachs–Teller (LST) relation for materials with strong anharmonic phonon damping is also presented which leads to the expression ɛ 0/ɛ ∞ = |ω LO|2/|ω TO|2. The theory provides a microscopic expression for T c as a function of physical parameters, including the mode specific Grüneisen parameter. The theory also shows that , and again specifies the prefactors in terms of Grüneisen parameter and fundamental physical constants. Using the generalized LST relation, the softening of the TO mode leads to the divergence of ϵ 0 and to a polarization catastrophe at T c. A quantitative microscopic form of the Curie–Weiss law is derived with prefactors that depend on microscopic physical parameters.

A combinatory ferroelectric compound bridging simple ABO3 and A-site-ordered quadruple perovskite

The simple ABO3 and A-site-ordered AA′3B4O12 perovskites represent two types of classical perovskite functional materials. There are well-known simple perovskites with ferroelectric properties, while there is still no report of ferroelectricity due to symmetry breaking transition in A-site-ordered quadruple perovskites. Here we report the high pressure synthesis of an A-site-ordered perovskite PbHg3Ti4O12, the only known quadruple perovskite that transforms from high-temperature centrosymmetric paraelectric phase to low-temperature non-centrosymmetric ferroelectric phase. The coordination chemistry of Hg2+ is changed from square planar as in typical A-site-ordered quadruple perovskite to a rare stereo type with 8 ligands in PbHg3Ti4O12. Thus PbHg3Ti4O12 appears to be a combinatory link from simple ABO3 perovskites to A-site-ordered AA′3Ti4O12 perovskites, sharing both displacive ferroelectricity with former and structure coordination with latter. This is the only example so far showing ferroelectricity due to symmetry breaking phase transition in AA′3B4O12-type A-site-ordered perovskites, and opens a direction to search for ferroelectric materials.

Local properties and phase transitions in Sn doped antiferroelectric PbHfO3 single crystal

Pb(Hf1−xSnx)O3 single crystals with x = 0.23 were characterized using single-crystal x-ray diffraction in the wide temperature range. The information on the structure of two intermediate phases, situated between low temperature antiferroelectric and high temperature paraelectric phases, has been obtained. The lower-temperature intermediate AFE2 phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate IM phase is characterized by rotations of oxygen octahedra, primarily in the form of anti-phase tilts, which are also present in the lower-temperature AFE2 phase. For the same crystal, 119Sn Mossbauer effect in the temperature range from 300 K to 600 K has been used to study phase transitions mechanism. Two kinds of quadruple splitting have been found. It implies that two different environments of the central Sn ion exist. The observed two kinds of quadruple splitting do not disappear in the whole investigated temperature range which confirm that even far above TC the structure of paraelectric phase is locally non-centrosymmetric.

Lead-free molecular ferroelectric [N,N-dimethylimidazole]3Bi2I9 with narrow bandgap

Lead-based organic-inorganic hybrid perovskite materials are widely applied in solar cells due to their excellent optoelectronic properties. These materials usually appear ferroelectricity with the built-in electric field, which can be used to improve the power conversion efficiency (PCE) of solar cells. However, the toxicity of lead has severely hampered their applications. Hence, it is of utmost significance to explore novel organic-inorganic hybrid perovskite materials with non-toxicity, ferroelectricity, and narrow bandgap at room temperature to enhance the PCE for broad practical applications. Herein, we reported a lead-free hybrid molecular ferroelectric material, i.e. (C5N2H9)3Bi2I9, with zero-dimensional perovskite-like structure. (C5N2H9)3Bi2I9 undergoes first-order ferroelectric phase transition with a Curie temperature of 327 K. Moreover, the dielectric constants of (C5N2H9)3Bi2I9 exhibit a step-like anomaly varied between the high-temperature paraelectric phase and the low-temperature ferroelectric phase. Furthermore, (C5N2H9)3Bi2I9 demonstrates a narrow bandgap of 2.1 eV. Hence, the as-reported novel member of the organic-inorganic hybrid family renders great promise for optoelectronic devices.

Temperature dependence of polar modes in hybrid improper ferroelectrics

An atomistic effective Hamiltonian, along with a presently developed analytical model, are employed to investigate and analyze low-frequency polar, antipolar and antiferrodistortive phonons at finite temperature in a prototypical hybrid improper ferroelectric, that is, the $({\mathrm{BiFeO}}_{3})/({\mathrm{NdFeO}}_{3})$ [abbreviated as ${(\mathrm{BFO})}_{1}/{(\mathrm{NFO})}_{1}$] 1:1 superlattice. In the high-temperature paraelectric phase, phonons having both polar and antipolar characters are found to exist, as a result of a bilinear coupling between different cation motions, with these phonons having frequencies that are basically independent of temperature. In contrast, phonons having fluctuations of either in-phase or antiphase octahedral tiltings are soft in this high-temperature phase (with these two fluctuations being uncoupled), which results in their condensation below some critical temperature and the emergence of a low-temperature phase. In this latter low-temperature phase, trilinear energetic couplings between these two types of octahedral tiltings and Bi and Nd cations' motions lead to the appearance of a spontaneous polarization, consistent with the nature of hybrid improper ferroelectricity. These trilinear energetic couplings also yield an increase in the number of phonons possessing both polar and antipolar characters in the low-temperature phase, with most of these phonons softening when approaching the ferroelectric-to-paraelectric transition from below, as a result of the fact that they also exhibit antiferrodistortive features. The different temperature behaviors of polar modes at high versus low temperatures emphasize the uniqueness of hybrid improper ferroelectrics.

Raman interrogation of the ferroelectric phase transition in polar metal LiOsO3

Ferroelectric (FE) distortions in a metallic material were believed to be experimentally inaccessible because itinerant electrons would screen the long-range Coulomb interactions that favor a polar structure. It has been suggested by Anderson and Blount [P. W. Anderson, E. I. Blount, Phys. Rev. Lett. 14, 217-219 (1965)] that a transition from paraelectric phase to FE phase is possible for a metal if, in the paraelectric phase, the electrons at the Fermi level are decoupled from the soft transverse optical phonons, which lead to ferroelectricity. Here, using Raman spectroscopy combined with magnetotransport measurements on a recently discovered FE metal LiOsO3, we demonstrate active interplay of itinerant electrons and the FE order: Itinerant electrons cause strong renormalization of the FE order parameter, leading to a more gradual transition in LiOsO3 than typical insulating FEs. In return, the FE order enhances the anisotropy of charge transport between parallel and perpendicular to the polarization direction. The temperature-dependent evolution of Raman active in-plane 3Eg phonon, which strongly couples to the polar-active out-of-the-plane A2u phonon mode in the high-temperature paraelectric state, exhibits a deviation in Raman shift from the expectation of the pseudospin-phonon model that is widely used to model many insulating FEs. The Curie-Weiss temperature (θ ≈ 97 K) obtained from the optical susceptibility is substantially lower than T s, suggesting a strong suppression of FE fluctuations. Both line width and Fano line shape of 3Eg Raman mode exhibit a strong electron-phonon coupling in the high-temperature paraelectric phase, which disappears in the FE phase, challenging Anderson/Blount's proposal for the formation of FE metals.

Influence of native structural defects activated by illumination and under the memory effect conditions on ultrasonic wave propagation in TlInS2 ferroelectric–semiconductor with incommensurate phase

This paper presents data about experimental observation and theoretical validation effect of native defect states activated by illumination and under the memory effect conditions on the velocity of longitudinal and shear elastic waves propagating in layered TlInS2 ferroelectric-semiconductor in different directions. These longitudinal and transverse ultrasonic velocities were worked out by using the pulse echo overlap technique in the range of temperature ∼90–300 K and correspond to and elastic constants of TlInS2. The investigations reveal the illumination-induced spike-like peaks in the temperature dependence of longitudinal wave velocity centered at different temperatures in the range of temperature 225–300 K of high-temperature paraelectric phase. The temperature dependence of ultrasonic velocity of shear wave corresponding to mode exhibits a steep decrease in the slope under illumination at ∼190 K. Influence of the memory effects on the velocities of ultrasonic waves, which is associated with annealing times of about five hours inside the incommensurate (INC) phase, show similar features in the same temperature ranges: a similar well-defined spike-like anomalies in the temperature dependence of longitudinal ultrasonic wave velocity at and pronounced shear acoustic-mode softening below ∼190 K. The experimental results have been discussed in the frame of phenomenological the Landau phase transition theory. It has demonstrated that native deep level defects and crystal imperfections charged under light excitation and the memory effect condition and may essentially distort a crystal lattice of TlInS2 in the high-temperature paraelectric phase.

Effects of heat treatment and in situ high-temperature X-ray diffraction study on the formation of ferroelectric epitaxial Y-doped HfO2 film

The process of forming the ferroelectric orthorhombic phase was investigated for epitaxial 7% Y-doped (YHO7) films using in situ high-temperature X-ray diffraction. Epitaxial YHO7 films were grown on (111) ITO-coated (111)YSZ substrates by pulsed laser deposition at room temperature and a subsequent heat treatment process. Films deposited at room temperature were crystallized as paraelectric monoclinic phase. The monoclinic phase partially changes to tetragonal phase above 600 °C and perfectly transformed around 950 °C during heating. The change from tetragonal phase to orthorhombic phase was detected at 300 °C, corresponding to the Curie temperature under the cooling process. These results clearly suggest that the tetragonal phase was more stable at 1000 °C for YHO7 films on heating than the other phases, and the formation of this tetragonal phase—the high-temperature paraelectric phase of the ferroelectric orthorhombic phase—is key to the formation of the ferroelectric orthorhombic phase.

Chemically engineered multiferroic morphotropic phase boundary in BiFeO3-based single phase multiferroics

As the reach points of different phases with complex structural features, a morphotropic phase boundary (MPB) in ferroelectric and ferromagnetic solid solutions can significantly enhance the piezoelectric performance and magnetostrictive response, respectively. Recently, the phase-change functional responses related to the multiferroic MPB are proposed to be a promising way to enhance the magnetoelectric coupling in BiFeO3-based single phase multiferroics. In this work, we verify the tunable magnetic ordering and the construction of the multiferroic MPB by engineering the chemical concentrations of the ferroelectric/non-magnetic PbTiO3 end in the (1 – x)Bi0.9Dy0.1FeO3-xPbTiO3 binary solid solution ceramic system. Based on the results obtained in this work and reported in the literature, the structure-ferroic properties phase diagram of the BiFeO3-DyFeO3-PbTiO3 ternary system is established, where a compositional region with coexisting ferroelectric polarization and ferromagnetic moment is found. More importantly, a multiferroic MPB line separating two chemical regions with distinct crystal structures and ferroic orderings is discovered in the phase diagram. The phase changing nature of MPB compositions with temperature and compositions is investigated from room temperature to high temperature paraelectric phase. This work could provide a promising system to explore the highly desired colossal effects on magnetoelectric coupling in single phase multiferroics by phase-change functional responses.

Stabilization of the NOP ferroelectric phase in potassium nitrate–barium titanate ferroelectric composites

The study of temperature evolution of KNO3 (NOP) structure in ferroelectric (1−x)KNO3 + (x)BaTiO3 composites with BaTiO3 concentrations х = 0.25, 0.50 and 0.53 has been carried out on cooling with the use of the neutron diffraction technique. It was shown that, on cooling, the phase transition temperature (Tc) from the high-temperature paraelectric phase into the ferroelectric one did not depend on barium titanate concentration and practically coincided with Tc for pure NOP. Moreover, it was found that the admixture of BaTiO3 essentially enlarged the temperature range of NOP ferroelectric phase stability in the composites with BaTiO3 concentrations x = 0.25 and 0.50. The suppression of the ferroelectric phase was observed for the composite with x = 0.53.

Characterization of mechanosynthesized Bi 1−x Sm x FeO 3 samples unencumbered by secondary phases or compositional inhomogeneity

High-quality samples, in terms of phase purity and dielectric properties, of composition Bi1−xSmxFeO3 (0.05 ≤ x ≤ 0.20) have for the first time been prepared by mechanosynthesis. Close inspection of the powder diffraction data, analysis via Rietveld refinement and TEM microscopy demonstrates that the Bi1−xSmxFeO3 samples contain only perovskite phases. Additionally, by a combination of Rietveld analysis, TEM, DSC, temperature-dependent XRD and permittivity data a tentative phase diagram has been proposed where the high temperature paraelectric phase Pnma has been confirmed for samarium substituted BiFeO3. Regarding the physical properties, the samples resulted to be electrically homogenous and highly insulating at room temperature, suggesting that other sources of conductivity, such as mixed valence of Fe associated with possible oxygen non-stoichiometry, have been avoided during the samples synthesis. In spite of the high quality of the samples, the dielectric and magnetic behaviour of the Bi1-xSmxFeO3 samples change only modestly on Sm substitution, with neither a great change in the resistivity or remnant magnetisation of Sm substituted samples in comparison with BiFeO3.