Macroscopic quadrupole moment of dielectric solids with structural phase transitions

The macroscopic quadrupole moment is the main electronic characteristic of structural transformations between nonpolar states of dielectric solids. Several experimental methods are suggested for measuring the different components of the quadrupole moment tensor as a function of external actions. We describe the observation of quadrupole pyro-and piezoelectric, inverse piezoelectric, and electrooptic effects in centrosymmetric crystals without phase transitions, some quadrupole effects in the regions of ferroelastic and nonferroelastic phase transitions, and effects arising due to plastic deformation. The influence of defects on the quadrupole moment of real crystals is considered. Special attention is paid to the observation and study of the anomalously large quadrupole effects related to a frozen polarization wave in incommensurate phases of ferroelectrics. The experimental and theoretical data published up to 1984 are reviewed and some new results are presented.     

Isostructural phase transitions in solids

A model of electrons interacting with lattice vibrations is shown to exhibit an isostructural phase transition as a function of applied force by relating the Hamiltonian to that of an Ising model in magnetic field.     

Effect of point defects on phase transitions in ferroelectric nanocrystals

The dependences of the phase transition temperature of ferroelectric nanocrystals in a dielectric matrix on the point charged defect concentration have been obtained. The influence of point defects on the nonlinear characteristics of ferroelectric nanocrystals has been studied as a function of the strength and direction of an external electric field with the adequate inclusion of depolarizing electric fields and nonlocal effects.     

Influence of defects on structural transitions

Experiments related to the influence of defects on ordinary structural transitions or on Incommensurate phases are briefly reviewed. The reference theoretical and experimental framework of magnetic systems with frozen random field defects is also recalled.     

Photopyroelectric detection of phase transitions in solids

The recently developed photopyroelectric spectroscopy (P²ES) has been used to investigate phase transitions in solids. In our variable temperature experiments, specific heat curves for Rochelle Salt and V₂O₄ powders were obtained and found in good agreement with results obtained previously via more experimentally involved methods.     

Experimental studies of the influence of defects and impurities on structural phase transitions

A review of experiments pertaining to the influence of defects and impurities on structural phase transitions is given. The systems covered are the KH₂P0₄-family, the TGS-family, the SrTiO₃-family and, finally, the K_1-x(Li, Na) _x TaO₃-system. Emphasis is placed on the influences of impurities and defects on static and dynamic critical and precritical effects, as well as their ability to induce glass-like precursor effects.     

Structural phase transitions in ionic molecular solids

An overview is presented of our studies on the nature of structural instabilities in relatively complex ionic solids. These are based on parameter-free interionic potentials based on the Gordon-Kim modified electron gas formalism extended to molecular ions.

We describe the manner in which there emerge from these studies quite general concepts of “size” and “shape” as structural determinants. In particular, we discuss how these, and the approximate symmetries that they can produce, can provide a relatively simple structure-based explanation of the origins of incommensurate phases in these systems. However, we also emphasize that the existence of such symmetries does not guarantee an incommensurate phase. This can only be realized if long-range correlations are sufficiently strong to overcome random local disordering. Thus, either the molecular units are partially linked and/or there exist long-range Coulomb interactions between individual units.     

Kinetics of formation and growth of antiphase domains during second-order phase transitions

The kinetics of the formation and growth of antiphase domains during second-order phase transitions is investigated theoretically in the Ginzburg-Landau model using a statistical approach. It is shown that depending on the initial conditions both uniform and polydomain-type ordering can be realized in thermodynamic equilibrium. It is found that for small initial inhomogeneities the ordering process can be nonmonotonic in its initial stages. It is established that for special initial conditions long-lived ordered structures of a special type arise in the intermediate stages of the ordering process. The characteristic formation time of the domain structure is estimated. Zh. éksp. Teor. Fiz. 113, 228–239 (January 1998)     

Interaction of ultrasound with macroscopic defects in solids

The interaction between ultrasound and macroscopic defects in solids is considered using the nonlinear theory of elasticity. It is shown that, due to the resonance nature of the vibrations of the defect, considerable anharmonism occurs even when the intensity of the ultrasonic wave is weak. Because of this, effective conversion of the ultrasonic wave into internal vibrational states of the macrodefect occurs.V. D. Kuznetsov Siberian Physico-Technical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 3–9, November, 1994.     

Formation of a polarized state in polydiethylsiloxane during structural phase transitions

The interrelation between structural rearrangements and a stable induced polarization state in polydiethylsiloxane (PDES) is studied. At the temperatures of crystallization and crystal-crystal transitions in PDES, thermally-induced polarization currents are observed whose temperature dependence is determined by the magnitude of the polarizing field and the choice of polarization temperature interval. This implies the existence of a mesophase state in PDES. Fiz. Tverd. Tela (St. Petersburg) 39, 377–381 (February 1997)     

Microkinetics of the first- and second-order phase transitions

A model of the phase transition in a lattice of interacting nodes, in which each node is a statistical system with internal structure, is introduced. Configuration entropy of microscopic states of the node is defined as a basic parameter of the model. In the frame of the model the first- and second-order phase transitions are considered in details. The distinction between them on the microscopic level is analyzed. Phase diagrams have been calculated in the mean-field approximation. Changes of the phase diagrams and modifications of phase transitions under external pressure and irradiation are investigated in the frame of the microkinetic approach. Results are referred to real systems.     

Hierarchical microstructure in structural phase transitions

We consider a model in the context of martensitic materials in which hierarchical twinning near the habit plane (austenite-martensite interface) is a new and crucial ingredient. The model includes (1) a triple-well potential in local deviatoric (rectangular) strain, (2) strain gradient terms up to second order in strain and fourth order in gradient, and (3) all symmetry allowed compositional fluctuation-induced strain gradient terms. The last term favors branching of domain walls which enables communication between macroscopic and microscopic regions essential for shape memory. Below the transition temperature (T₀) we obtain the conditions under which branching of twins is energetically favorable. Above T₀ a hierarchy of branched domain walls also stabilizes tweed formation (criss-cross patterns of twins). External stress or pressure modulates (“patterns”) the spacing of domain walls. Results based on 2D time-dependent Ginzburg-Landau simulations are shown for twins, tweed and hierarchy formation.     

Inhomogeneity-induced second-order phase transitions in the Potts model on hierarchical lattices

Thermodynamics of the Potts model with an arbitrary number of states is analyzed for a class of hierarchical lattices of fractal dimension d > 1. In contrast to the case of crystal lattice, it is shown that all phase transitions on lattices of this type are of the second order. Critical exponents are determined, their dependence on structural parameters is examined, and scaling relations between them are established. A structural criterion for change in transition order is discussed for inhomogeneous systems. Application of the results to critical phenomena in phase transitions in dilute crystals and porous media is discussed.     

Translational symmetry changes in second-order phase transitions on clean crystal surfaces

The group theory approach, within the Landau-Lifshitz theory of phase transitions, is presented to predict the possible translational symmetry changes (possible surface superstructures) in second-order phase transitions on clean crystal surfaces. 80 diperiodic groups in three dimensions are used to describe the symmetry of the surface layer. A comparison of the theoretical results with experimental data is given.