Displacive-type Phase Transition Research Articles

Phase Instability and Thermal Properties of Multilayered Vanadium Diselenide: DAC-Based High Pressure Studies

Two-dimensional layered trichalcogenides are of current research interest because of their robust opto-electronics, and thermal properties. Mechanical and phonon vibrational properties of these materials are of significant importance in the field of stress and thermal management. Here we report the pressure dependence of structural and phonon behaviors of high quality few-layers vanadium diselenide nano-sheets grown by hydrothermal method employing synchrotron x-rays and micro-Raman scattering techniques. The high crystalline nature of the nanosheets was examined using transmission electron microscopy. Synchrotron x-ray studies at high pressure (up to 22 GPa) using a diamond-anvil cell identify a displacive type phase transition to a low symmetry monoclinic phase (C2/m) around 7 GPa corroborated with the P-dependent Raman scattering results. Raman spectroscopic studies at high pressure (up to 34 GPa) identify two prominent Raman bands with symmetry Eg and A1g. Eg band at 207 cm-1 shows normal hardening and A1g band at 236 cm-1 softens. The first order pressure derivatives of these phonons at ambient phase (P-3m1) are found to be -3.547(2) and 0.388(6) cm-1/ GPa, respectively. Using the experimental mode Grüneisen parameter of Eg Raman mode, the in-plane thermal expansion coefficient of the titular compound at the ambient phase is estimated. The observed results are expected to be useful in calibration and performance of next generation nano-electronics and optical devices under extreme stress conditions. These experimental results corroborated with our ab-initio calculations will be presented in the ECS meeting.

Experimental and Theoretical Investigations on Structural and Vibrational Properties of Melilite-Type Sr2ZnGe2O7 at High Pressure and Delineation of a High-Pressure Monoclinic Phase.

We report a combined experimental and theoretical study of melilite-type germanate, Sr2ZnGe2O7, under compression. In situ high-pressure X-ray diffraction and Raman scattering measurements up to 22 GPa were complemented with first-principles theoretical calculations of structural and lattice dynamics properties. Our experiments show that the tetragonal structure of Sr2ZnGe2O7 at ambient conditions transforms reversibly to a monoclinic phase above 12.2 GPa with ∼1% volume drop at the phase transition pressure. Density functional calculations indicate the transition pressure at ∼13 GPa, which agrees well with the experimental value. The structure of the high-pressure monoclinic phase is closely related to the ambient pressure phase and results from a displacive-type phase transition. Equations of state of both tetragonal and monoclinic phases are reported. Both of the phases show anisotropic compressibility with a larger compressibility in the direction perpendicular to the [ZnGe2O7](2-) sheets than along the sheets. Raman-active phonons of both the tetragonal and monoclinic phases and their pressure dependences were also determined. Tentative assignments of the Raman modes of the tetragonal phase were discussed in the light of lattice dynamics calculations. A possible irreversible second phase transition to a highly disordered or amorphous state is detected in Raman scattering measurements above 21 GPa.

Heat Capacity Study of Relaxors

The thermodynamic properties of various relaxors of PMN, PMT, PZN, PST, PMN–PT, PZN–PT, and BTZ have been reviewed and the excess thermodynamic properties are discussed. In lead-containing complex perovskite, excess heat capacity owing to the formation of PNR and glass-like low-temperature excitations are clearly observed. Studies of solid solutions indicate the difficulty of forming PNR with rhombohedral symmetry in the ferroelectric phase and the possible growth of a polar region with tetragonal symmetry in the compounds. Thus, the lead-free Ba-based relaxor BTZ35 can be considered as the frozen state of a displacive-type phase transition without disorder. Keywords: BTZ, heat capacity, PMN, PMN–PT, PMT, polar nanoregion, PST, PZN, PZN–PT, relaxor.

Conditions of Morphotropic Phase Boundary in Perovskite-Oxides

In order to search for Pb-free relaxor substances, the conditions to derive morphotropic phase boundary (MPB) are studied in perovskite-oxides. Before a microscopic theory for displacive type phase transition, the phenomenological approach is extended by combining with a first-principles computer simulation in several perovskite compounds. The MPB region in mixed perovskite-oxides is described by an anisotropy of the fourth-order term of polarizations, S, as 0 ≤ S ≤ 1.7, which is the region corresponding to individual perovskite-oxides with successive phase transitions. Basing on the present theoretical results, the definition of the MPB is discussed with explaining the pressure effect and the electric-field dependence in relaxor ferroelectrics.

Euler strut: a mechanical analogy for dynamics in the vicinity of a critical point

An anchored elastic filament (Euler strut) under an external point load applied to its free end is a simple model for a second-order phase transition. In the static case, a load greater than the critical load causes a Euler buckling instability, leading to a change in the filament's shape. The analysis of filament dynamics with an external point load at its end shows that when approaching the critical end-load, the period of such an inverted pendulum diverges in a fashion analogous to a ‘soft mode’ critical slowing down in, for example, a ferroelectric phase transition of displacive type. We thus show that an advanced concept of solid state physics, i.e. ‘soft mode’ dynamics and critical slowing down, observable in a variety of second-order phase transitions, can be actualized in this simple mechanical system. The variable loads attached to a vertical spring allow for an experimental implementation and quantitative measurements as an illustration of this analogy.

Theoretical Investigation of the Isotope-Induced Ferroelectric Phase Transition in Quantum Paraelectric Strontium Titanate

The isotope-induced ferroelectric phase transition is theoretically investigated in terms of a compact formula based on the self-consistent phonon approximation (SCA) on the anharmonic lattice vibration system. The T c − T 1 phase diagram, where T c and T 1 are the transition temperature T c and the quantum fluctuation parameter T 1, respectively, is numerically calculated to demonstrate the nonlinear dependence of T c on T 1 sensitive to the increase of the oxygen mass. Experimental results of the small transition entropy and the perfect softening are discussed from the viewpoint of the ideal displacive type phase transition.

Heat capacity at constant pressure and thermodynamic properties of phase transitions in PbMO3 (M=Ti, Zr and Hf)

The heat capacity of PbMO3 (M=Ti, Zr and Hf) at constant pressure was measured using a differential scanning calorimeter (DSC) from room temperature up to 870 K. Large anomalies were found in the heat capacity curves, corresponding to the ferroelectricparaelectric phase transition in PbTiO3 (PT), the antiferroelectric-paraelectric phase transitions in PbZrO3 (PZ) and PbHfO3 (PH). The transition entropies were estimated as 7.3 J K−1 mol−1 (PT), 9.9 J K−1 mol−1 (PZ) and 9.3 J K−1 mol−1 (PH). These values of transition entropies are much larger than that of a typical displacive-type phase transition.

The pressure influence on the incommensurate–commensurate ferroelectric phase transition in [4-NH 2C 5H 4NH][SbCl 4

The dielectric properties of the [4-NH 2C 5H 4NH] SbCl 4 (abbreviated as 4-APCA) crystal were investigated under hydrostatic pressure up to 300 Mpa. The pressure–temperature phase diagram was given. The paraelectric–ferroelectric phase transition (II→III) temperature ( T c) increases linearly with increasing pressure with a slope d T c/d p=21×10 −2 K/MPa. The pressure dependence of Curie–Weiss constants has been evaluated also. In the paraelectric phase (II) the Curie constant ( C +) was pressure dependent whereas the C − constant over the ferroelectric phase (III) was almost constant. The results are interpreted in terms of improper and displacive type phase transition model with a soft phonon at a zone boundary.

Displacive Character of the Paraelectric–Ferroelectric Phase Transition in K2ZnI4

The crystal structure of K 2 ZnI 4 is refined at 24 temperatures above and below the paraelectric–ferroelectric phase transition temperature by means of X-ray diffraction to reinvestigate the mechanism of the phase transition in more detail. The temperature dependence of the translation of K(1) and K(2) along the b -axis and the rotational angle of the ZnI 4 tetrahedron about the axis nearly parallel to the a -axis is obtained by the structural analyses. It is concluded that the mechanism of this phase transition is not the order–disorder type but the displacive one. The temperature variation of the anisotropic temperature parameter U 22 of K(1), K(2), I(1) and I(2) is coincident with the theoretical prediction for the displacive type phase transition proposed by Fujii and Matsubara [Prog. Theor. Phys. 78 (1987) 177].

Distortion of Local Order Parameter Caused by Impurities in Displacive Phase Transition*

Effect of impurities on structural phase transition of displacive type was examined through molecular dynamics simulation. Anharmonic oscillators were arranged on a two-dimensional square lattice with harmonic nearest-neighbor interaction. Local order parameters were calculated with impurities of different strength. Weak impurities affect more strongly than strong inpurities in relative magnitude.

Influence of molecular geometry on the absorption spectra of p-quaterphenyl crystals and phase transition

The fluorescence and the absorption spectra of the p-quaterphenyl crystals have been measured at 300-14 K. We found the Gaussian bands in the absorption edge region and its peak shift with the temperature below 233 K is explained by the displacive-type phase transition.

Analysis of Raman Spectra of LiB3O5 Single Crystals

Polarized Raman spectra of LBO single crystal were recorded in the temperature range 300–870 K. At room temperature, 26A1, 27A2, 26B1 and 26B2 modes expected from group theoretical analysis are observed by various scattering geometries. Among them, the internal bands are tentatively assigned to vibrations of the BO3 triangle and BO4 tetrahedron in terms of factor group analysis. Based on the temperature dependence of the wavenumbers, the Grüneisen parameters of A1(TO), B1(TO) and B2(TO) modes are calculated and discussed. No soft modes are observed in the above temperature range, implying that no displacive-type phase transition takes place there. The crystal structure of LBO below 870 K is stable.

Rhodes-Wohlfarth ratio and anomalous specific heat in displacive type ferroelectrics

The Rhodes-Wohlfarth plot for ferroelectrics introduced and theoretically discussed in terms of the self-consistent phonon (SCP) theory by Tokunaga is briefly reviewed. Specific heat is known to show no anomaly in the displacive type phase transition as far as definition is concerned. The larger Rhodes-Wohlfarth ratio is known to result in the smaller anomaly of specific heat within the framework of the SCP theory.

Study of phonon scattering processes in displacive ferroelectrics by means of heat conductivity measurements

Abstract The results of measurements of heat conductivity coefficient for the set of displacement type phase transition crystal is presented and discussed in the framework of τ-approximation.

Heat transport properties of ferroelectrics and related materials

Abstract A review of experimental data on heat transport properties of crystals with ferroelectric and ferrodistortive phase transitions is presented. Special attention is paid to the temperature dependence of the thermal conductivity coefficient in order-disorder and displacive type phase transitions. An interpretation and a qualitative approach to the description of the heat transport coefficients anomalies are discussed.

On the Pb thermal vibrations in PbHfO3 crystals

In PbHfO3 single crystals the amplitude of the thermal vibrations for Pb ions in the high-temperature Pm3m phase, calculated from the Debye-Waller factor, reached unexpectedly large values. The random displacements of Pb ions were assumed and the new thermal vibration amplitude, which is more reasonable from the crystal lattice dynamics point of view, was calculated. A possible displacive-type phase transition was observed in the vicinity of 400 degrees C.

Observation of the silent soft phonon in beta -quartz by means of hyper-Raman scattering.

By means of hyper-Raman scattering, a silent soft phonon was observed in \ensuremath{\beta}-quartz for the first time. The soft phonon was found to become underdamped at high temperatures. Both the square of the frequency ${\mathrm{\ensuremath{\omega}}}_{0}^{2}$ and the integrated intensity ${\mathit{I}}_{0}$ of the soft phonon were found to obey Landau-Cochran's soft-mode theory. The \ensuremath{\alpha}-\ensuremath{\beta} phase transition of \ensuremath{\beta}-quartz is found to be well described by a simple displacive-type phase transition with the silent soft phonon, in contrast to many studies which suggest an order-disorder phase transition.

Raman Scattering Study of Lithium Diborate (Li2B4O7) Single Crystal

The temperature dependence of Raman spectra of lithium diborate (Li 2 B 4 O 7 ) was measured at temperature range from 300 K to 900 K. At room temperature, 12 A 1 , 12 B 1 , 15 B 2 and 22 E modes were observed by various scattering geometries. No soft mode was observed in the above temperature range, implying that no displacive-type phase transition takes place there.

構造解析からみた強誘電体の相転移

After the early 1970's, structural studies on the ferroelectric phase transition have been developed on the basis of the crystal structure analysis. The atomic order parameter, which characterizes atomic occupancies on local equilibrium positions, describes the order-disorder type phase transition. In case of the displacive type phase transition, fluctuations of the atomic positions, calculated from the temperature factor, become prominent with approaching the phase transition point. Several examples are presented for recently refined ferroelectric structures. [J. Cryst. Soc. Jpn, 28, 247 (1986) ] .

On the lattice dynamics of solid solutions with a quantum paraelectric and a ferroelectric component

Abstract The lattice dynamics of displacive type phase transitions in solid solutions with a quantum paraelectric and a ferroelectric component is considered. Using the self-consistent phonon approximation and the coherent potential approximation the phonon spectrum of the disordered lattice is obtained and the concentration dependence of the phase transition temperature is calculated. Good agreement with experimental results for (Ba, Sr) TiO3 is obtained.