Andrei P. Mirgorodsky

Phonon spectra evolution and soft-mode instabilities of zirconia during the c–t–m transformation

Abstract The joint lattice-dynamical model treatment of c-, t-, and m-polymorph of ZrO2 is conducted with an emphasis on the evolution of their vibrational spectra during the structure variation. The soft mode mechanisms are established for both c–t and t–m phase transitions and some challenging points of relevant Raman-scattering evidence are discussed. The results obtained propose the assignment of the bands observed in Raman spectra of t-ZrO2 other than those conventionally accepted in the literature, explain the stabilization mechanism of the doped t-ZrO2, and clarify some points in the interpretations of the Raman and infrared spectra of m-ZrO2.

CRYME: A program for simulating structural, vibrational, elastic, piezoelectric and dielectric properties of materials within a phenomenological model of their potential functions

Abstract The CRYME (CRYstal MEchanics) code is written in FORTRAN and adapted to IBM PC. It enables to use a wide scope of phenomenological approaches worked out for description of potential functions of crystals and theoretical investigation of solids. These functions range from the simplest short-range ones like the Valence-Force or Born-von Karman fields, to the most elaborated versions of the Shell-Model. The investigations include lattice vibrations and related properties, elastic, dielectric and piezoelectric tensors, thermodynamical characteristics, a lattice equilibrium geometry minimizing the internal energy of the crystal potential and strength of the electrostatic field of atomic sites. CRYME enables to calculate the above-mentioned properties for a stress-free lattice, or for a crystal under hydrostatic compression, anisotropic macroscopic tension or under an external electric field.

Lattice vibrations and bulk properties of ceramics-forming materials: Piezoeffect in SiC and elasticity of ZrO2

The role of lattice vibrations in the crystal properties related to the homogeneous strains is discussed. Such properties depend on the individual phonon characteristics measurable by vibrational spectroscopy. So, being applicable to powderlike samples, it can provide an indirect information on those bulk characteristics which cannot be directly measured for such samples. The piezoelectricity of cubic SiC, and the elasticity of tetragonal ZrO2 are considered along with the phonon spectra. Thus the mechanism of the polarization of sphalerite-like materials is elucidated; a contradiction between the elastic properties of the t-ZrO2 lattice and the conventional interpretation of its Raman spectra is revealed.

Structural instabilities of YBa2Cu3O7−δ lattices under hydrostatic compression

Abstract Lattice-dynamical calculation results are presented in which YBa2Cu3O7−δ (δ = 1.0) lattices become unstable at high hydrostatic compression. For YBa2Cu3O1, an incommensurate instability is predicted via the vanishing of the A′ mode near the (0.2, 0.2, 0.2) point of the Brillouin zone. For YBa2Cu3O7, the prognosticated destabilization is found to be associated with the softening of a whole dispersionless phonon branch corresponding to zig-zag vibrations of the CuOCu chain.

On the mechanism of the cubic-tetragonal transition in ZrO2

Abstract This paper concerns the problem of the driving forces of the nonferroelectric structural phase transitions in cubic ZrO2, treated within the framework of lattice dynamics. In a simple model, we consider a short-range microscopic mechanism for the vanishing of the lattice vibration in this crystal. The most important calculational result is that the dispersion branch originating from the highest T2g vibration undergoes a very sharp decrease and provides the lowest-frequency X− 2 phonon (41cm−1) at the Brillouin Zone (BZ) boundary. The model explains why and how the lattice can lose its stability on cooling, and thus confirms the hypothesis of the X− 2 phonon vanishing mechanism.