Intercell Corrections for Ionic Motion in Displacement Ferroelectrics

A set of microscopic parameters determined for LiTaO₃ 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 TC. 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 TC. 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 LiTaO₃ 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.