The contribution of structural disorder to diffuse phase transitions in ferroelectrics

Simple crystal-chemical arguments were used to suggest that in the ferroelectric perovskite lead-scanium-tantalate (PbSc_0.5Ta_0.5O₃) the B-site cations in this simple ABO₃ structure should be close to the boundary between order and disorder. Both polycrystal ceramic and single crystal materials of this composition have been grown. In this study X-ray powder diffraction is used to identify the strong superlattice reflections associated with the ordering of scandium and tantalum ions in the B-site, and to demonstrate quantitatively how the degree of ordering can be modified by suitable thermal treatment. Thermal changes associated with the ferroelectric Curie temperature have been measured by differential scanning calorimetry and show very clearly the manner in which the diffuse (broadened) transition in this crystal is sharpened by increase in the B-cation ordering.     

Phase transitions in superplasticity

Existing ideas about reasons for the occurrence and phenomenology of development of the superplasticity effect are analyzed. It is shown that within the scope of the thermodynamic approach superplasticity depends on a diffuse phase transition accomplished with heating and deformation. Traditional evaluation of super-plasticity from the value of the rate sensitivity factor is considered. Conditions are determined which should be fulfilled with transfer of material into a super-plastic condition, in particular for wrought aluminum alloys without a previously prepared structure.Translated from Problemy Prochnosti, No. 10, pp. 50–54, October, 1990.     

Phase field simulation of domain structures in cracked ferroelectrics

The fracture of ferroelectrics is a complex process which is influenced by various factors, among which are the domain switching near the crack tip, the crack face boundary conditions and the applied electric field. Domain switching near crack tips induces major local nonlinearity, while the crack face boundary conditions vary considerably due to different working conditions. In this work, a phase field model and a generalization of the configurational force theory into this model are used to investigate the microstructure around the crack tip and to quantitatively study the influence of the applied electric field and the crack face boundary conditions (permeable, impermeable, semi-permeable and energetically consistent). Evaluation of the fracture properties is done by the nodal configurational force at the crack tip based on the generalized configurational force theory. Results show that the induced domain structure relies significantly on the loading and on the surface boundary conditions. Among the four different conditions considered, the energetically consistent conditions lead to the smallest crack driving force, and the permeable conditions lead to the largest crack driving force. Calculations also show that positive electric fields tend to inhibit fracture, whereas negative electric fields tend to promote fracture.     

Phase transitions in doped silicon

The temperature dependence of the heat capacity of silicon doped with impurity atoms with deep levels has been investigated in the temperature interval of 15–300 K. Phase transitions induced by impurity centers have been discovered.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 3, pp. 459–463, September, 1988.     

Recent progress in relaxor ferroelectrics with perovskite structure

Relaxor ferroelectrics were discovered almost 50 years ago among the complex oxides with perovskite structure. In recent years this field of research has experienced a revival of interest. In this paper we review the progress achieved. We consider the crystal structure including quenched compositional disorder and polar nanoregions (PNR), the phase transitions including compositional order-disorder transition, transition to nonergodic (probably spherical cluster glass) state and to ferroelectric phase. We discuss the lattice dynamics and the peculiar (especially dielectric) relaxation in relaxors. Modern theoretical models for the mechanisms of PNR formation and freezing into nonergodic glassy state are also presented.     

Dynamic behavior of electro-optic and strain-optic effects in ferroelectrics with diffuse phase transitions

The transverse electro-optic effect under the step-function electric field in PMN single crystals and PLZT ceramics has been investigated. The time and temperature dependences of the electro-optic coefficients g₁₁–g₁₂ and R₁₁–R₁₂ have been determined. The strain-optic contribution to the total electro-optic effect has been evaluated. The influence of the polar phase regions on the electro-optic properties and their clamping is discussed.     

Phase interface perturbations in phase transitions

The linear problem of viscous phase interface perturbations in intensive condensation/evaporation processes (at great transversal mass fluxes) is numerically solved. In limit cases, transition to the classic Helmholtz and Landau instability problems is ensured. The effects of viscosity, thermal conductivity, and surface tension are taken into account. The limit case of long-wavelength perturbations is considered using the method of matched asymptotic expansions.     

Phase transitions in alkali halide crystals

The paper is devoted to the study of the characteristics of structural phase transitions occurring in ionic crystals under an extremely high pressure. The calculations are made for crystals of infinite size at absolute zero temperature. The pressure ofB1-B2 polymorphic transformation is calculated for a number of alkali halide crystals using pair interaction potentials obtained self-consistently within the framework of inhomogeneous electron gas theory. In so doing, the effects of seven coordination spheres are taken into account in the thermodynamic potential. The changes of the cohesion characteristics of crystals (cohesion energy, lattice constant) are calculated for the transition from theB1 structure (NaCl type) to theB2 structure (CsCl type). Based on the obtained results, conclusions are made on the stability of one or another crystal structure in the given range of external pressure. For crystals in the B1 phase, the moduli of elasticity and relative changes of the volume at the phase transition pressure are calculated. It is shown that the inclusion of the effect of higher coordination spheres affects considerably the values of the elastic characteristics of ionic crystals.     

Phase transitions in crystalline host-guest suprastructures

One characteristic feature that differentiates inorganic host-guest systems of supramolecular chemistry from conventional architectures, which typically build on the use of organic or organoelement ligands, is the necessity of taking into account phase transitions, which may be caused by changes in charges and thermodynamic conditions. This is left out of consideration in the existing postulates of supramolecular chemistry. This paper examines the feasibility of applying the ideology of supramolecular chemistry to inorganic systems.     

One-dimensionally disordered structure and polytypism in SiC

Almost perfect, one-dimensionally disordered structures in SiC are found by transmission electron microscopy in a CVD material and in an annealed reaction-sintered material at 1900°C. The observed common existence of one-dimensionally disordered structure in SiC supports the concept that the structure of SiC can be conveniently represented by a close packing of spheres with nearest neighbor interactions.     

Phase transitions in Na2TeO4 ceramics

Polycrystalline samples of NaTeO₄ were prepared by conventional solid-state reaction technique at low temperature (600° C). X-ray powder diffraction (XRD) technique was used to check the formation of single phase NaTeO₄ compound with cell parametersa = 10·602(1) åb = 70·622(1) å andc = 8·506(1) å in orthorhombic crystal system. Detailed studies of dielectric constant (ε) (and loss tangent (tan δ) as a function of frequency (400 Hz-10 kHz) and temperature (?120°C–260°C) show that the compound has two phase transitions in the ferroelectric phase.     

Phase transitions in copper(II) orthovanadate

Data on the polymorphs of copper(II) orthovanadate are reported. The Cu₃V₂O₈ phase synthesized in this laboratory exhibits phase transitions between 460° and 560°C. These phase transitions are identified through detailed DTA and high temperature XRD techniques; it is observed that these structural transitions are rapid and reversible. The crystal structure of Cu₃V₂O₈ is similar to that of Mg₃V₂O₈, Zn₃V₂O₈, Co₃V₂O₈ and Ni₃V₂O₈.     

Phase transitions in cuprous sulphide evaporated thin films

The films of Cu_xS(1x2), prepared by evaporation in vacuum and condensation of stoichiometric sulphides of copper, change their phases by heat treatment in vacuum. Cu₂S chalcocite appears as the most stable phase in the range 270 to 450° C and the other sulphur rich phases transform into chalcocite presumably by a loss of sulphur. By electron diffraction we have confirmed the low temperature monoclinic chalcocite phase, the high temperature hexagonal chalcocite phase, the monoclinic djurleite phase, the cubic digenite low and high temperature forms and the hexagonal covellite phase. The low temperature digenite phase presents a superstructure which is similar to that observed by Kazinetset al. on digenite prepared by evaporation in vacuum on a single crystal substrate of NaCl at temperatures of 350 to 400° C. Electron diffraction data of chalcocite (Cu₂S), djurleite (Cu_1.93S), digenite (Cu_1,8S) are presented.     

Phase transitions and quantum effects in adsorbed monolayers

Phase transitions in absorbed (two-dimensional) fluids and in absorbed layers of linear molecules are studied with a combination of path integral Monte Carlo (PIMC), Gibbs ensemble Monte Carlo (GEMC), and finite size scaling techniques. For a classical (nonadditive) hard-disk fluid the critical nonadditivities, where the entropy-driven phase separations set in, are presented. For a fluid with internal quantum states the gas-liquid coexistence region, tricritical, and triple points can be determined, and a comparison with density functional (DFT) results shows good agreement for the freezing densities. LinearN ₂ molecules adsorbed on graphite (in the 3 × 3 structure) show a transition from a high-temperature phase to a low-temperature phase withherringbone ordering of the orientational degrees of freedom. The order of the transition is determined in the anisotropic planar rotor model as a weak first-order transition. The effect of quantum fluctuations on the herringbone transition is quantified by PIMC and classical simulational methods. The values of the order parameter at low temperatures and the transition temperature are both lowered by roughly 10% due to quantum effects. Rounding effects of the phase transition in adsorbed layers of (N₂) _x (CO)₁, for× < 7% are analyzed by Monte Carlo ( MC) methods, and the ground state ordering for the transition in the adsorbed pure CO system is discussed, from ab initio potentials.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Phase transitions and fluctuations in classical statistical mechanics

It is shown that allowance for fluctuations leads to the appearance of additional terms in the Hamiltonian of the system and in the thermodynamic parameters of the substance. As a result, it becomes possible to describe the critical phenomena and phase transitions of liquid–vapor within classical statistical mechanics (without taking into account fluctuations, this is impossible).     

Phase transitions in an ionic conductor CuTeBr crystal

The dilatometric and pressure studies were carried out for CuTeBr crystal. The P–T phase diagram was constructed within the range of 3kbar. A triple point was found at 70 °C and 0.3 kbar, beyond which a new phase appears.