Some Problems in the Theory of Perovskite Ferroelectrics.

A brief review is given on the studies of ferroelectricity in perovskites. Early phenomenological theories of displacement-type ferroelectrics are considered. Two markedly different approaches based on density-functional theory are described. In one approach, the electron density of crystal is determined as a sum of Bloch functions. In the other, it is constructed as a superposition of the electron densities of individual ions. The latter approach gives a microscopic justification for the old phenomenological theories. We show that some phenomena in perovskites, which seemingly are of the order-disorder origin, can be explained by quasi-one-dimensional peculiarities of lattice dynamics. [ABSTRACT FROM AUTHOR]