Intercell Corrections for Ionic Motion in Displacement Ferroelectrics

Abstract

A set of microscopic parameters determined for LiTa${\mathrm{O}}_3$ in an earlier paper by comparing dielectric properties with the results of a statistical calculation are analyzed in terms of a point-charge-plus-electronic-polarizability model for the system. The results indicate the existence of very strong intercell correlations for ionic motion to temperatures considerably in excess of the Curie point $T_C$. These correlations are weak functions of temperatures to the extent that they do not produce deviations from effective-field (e.g., Curie-Weiss) behavior near $T_C$. Neglect of correlation effects leads to many inconsistencies and indicates that the use of random-phase Lorentz fields in statistical theories for displacement ferroelectrics is qualitatively unsound. Spontaneous polarization for LiTa${\mathrm{O}}_3$ is found to be 45% ionic, 55% electronic; the tantalum-oxygen bond is very markedly covalent, with estimated ionic charges of + 0.8 and - 0.6 electronic units, respectively.