Off-center displacements and hydrostatic pressure induced phase transition in perovskites

Abstract

Pressure has a profound effect on the paraelectric and ferroelectric properties of perovskite crystals. In this paper we theoretically investigate the effect of pressure on the cubic-to-tetragonal phase transition and on the soft mode dynamics of some classical perovskite crystals: BaTiO(3), PbTiO(3), and KNbO(3). We use a model consisting of three subsystems: electrons, phonons, and off-center displacements treated as spins. Experiments show that pressure has a large effect on the tunneling and hopping of the off-center displacements, that in turn strongly affect the pressure dependence of the transition temperature and the soft mode frequency. This model, with a very small number of adjustable parameters, accounts quantitatively for the experimentally measured nonlinear pressure dependence of the cubic-to-tetragonal phase transition temperature, up to the critical pressure where the transition temperature is zero. It also accounts quantitatively for the pressure dependence of the soft mode frequency, which is finite at the phase transition in spite of the fact that the phase transition at elevated pressures is second order, and for the pressure dependence of the electronic gap energy.

Thermal disorder and correlation effects in anti-perovskite-type copper nitride

“…Many of the unique properties of perovskite-type materials are a result of local distortions and peculiar atomic dynamics in such materials. Importance of such effects as anisotropy and anharmonicity of atomic vibrations and the presence of strong correlations in atomic displacements is acknowledged, for instance, in many studies of phase-transitions [7,8,9], ferroelectric properties [7,10,11] and negative thermal expansion effect [12,13,14,15], observed in perovskites.…”

The role of mechanical boundary conditions in the soft mode dynamics of PbTiO3

“…Interestingly, despite their importance, these relationships are only known for a very few specific cases of boundary conditions and are mostly limited to the soft mode dynamics under hydrostatic pressure. For example, in case of PbTiO 3 the dependence of the soft-mode frequency on the hydrostatic pressure has been studied quite extensively using both experimental [6][7][8][9] and theoretical approaches [10,11]. These studies revealed that in the tetragonal ferroelectric phase the frequency of the soft transverse optical mode, ν, can be well described by the Curie-Weiss pressure law ν 2 = A P (P − P 0 ) (1) where A P is the Curie-Weiss pressure constant, P is the hydrostatic pressure and P 0 is the pressure at which the mode frequency extrapolates to zero.…”