Yu. M. Gufan

Phenomenological theory of phase transitions in highly piezoelectric perovskites

The recently discovered fine structure of the morphotropic phase boundaries in highly piezoelectric mixture compounds PbZr 1 - x Ti x O 3 (PZT), Pb(Mg 1 / 3 Nb 2 / 3 ) 1 - x Ti x O 3 (PMN-PT), and Pb(Zn 1 / 3 Nb 2 / 3 ) 1 - x Ti x O 3 (PZN-PT) demonstrates the importance of highly nonlinear interactions in these systems. We show that an adequate Landau-type description of the ferroelectric phase transitions in these compounds is achieved by the use of a twelfth-order expansion of the Landau potential in terms of the phenomenological order parameter. Group-theoretical and catastrophe-theory methods are used in constructing the appropriate Landau potential. A complete phase diagram is calculated in phenomenological parameter space. The theory describes both PZT and PZN-PT types of phase diagrams, including the newly found monoclinic and orthorhombic phases. Anomalously large piezoelectric coefficients are predicted in the vicinity of the phase transition lines.

Phase transitions in domain walls

Abstract It has been proved that on approaching the line of (first-order or second order) transition between ordered phases a symmetry breaking necessarily occurs within a domain wall. The symmetry breaking in the cases under investigation takes place as a second-order phase transition. Calculations were performed for a reordering which was determined both by an appearance of the second order parameter and by the second component of a multi-component order parameter. The Landau potential for the description of phase transitions in the domain wall has been calculated and the regions of existence of high- and low-symmetry domain walls on the phase diagram have been determined, as well as anomalies of heat capacity and susceptibility at transitions in the domain walls.

A new method for calculating three-particle interaction in the theory of modulus elasticity

We propose a new method for calculating the potential of multiparticle interaction. Our method considers the energy symmetry for clusters that contain N identical particles with respect to permutation of the number of atoms and free rotation in three-dimensional space. As an example, we calculate moduli of third-order rigidity for copper considering only the three-particle interaction. We analyze nine models of energy dependence on the polynomials that form the integral rational basis of invariants (IRBI) for the group G3 = O(3) ⋇ P3. In this work, we use only the simplest relation between energy and the invariants forming the IRBI:

Geometric invariant theory approach to the determination of ground states of D-wave condensates in isotropic space

\varepsilon \left( {\left. {i,k,l} \right|j} \right) = \sum\nolimits_{i,k,l} {\left[ { - A_1 r_{ik}^{ - 6} + A_2 r_{ik}^{ - 12} + Q_j I_j^{ - n} } \right]}

Dielectric, magnetoelectric, structure, and dissipative properties and the Mössbauer effect in PbFe1/2Nb1/2O3 ceramics in wide frequency and temperature ranges

, where Ij is the invariant number j (j = 1, 2,..., 9). The results are in good agreement with the experimental values. The best agreement is observed at n = 2, j = 4:

The properties of ions with intermediate valence and the theory of high-pressure phase structure

I_4 = \left( {\vec r_{ik} \vec r_{kl} } \right)\left( {\vec r_{kl} \vec r_{li} } \right) + \left( {\vec r_{kl} \vec r_{li} } \right)\left( {\vec r_{li} \vec r_{ik} } \right) + \left( {\vec r_{li} \vec r_{ik} } \right)\left( {\vec r_{ik} \vec r_{kl} } \right)