Statistical Theory for Displacement Ferroelectrics. III. Comparison with Experiment for Lithium Tantalate

The theory developed in parts I and II of the present series of papers is used to interpret the dielectric properties of LiTa${\mathrm{O}}_3$. The problem is comfortably overdetermined, allowing both for a determination of the parameters involved in the theory and for a consistency check of the theory itself. The agreement between theory and experiment is quite good using completely temperature-independent parameters, but becomes excellent if a temperature dependence of the Lorentz field parameter is allowed. When the values obtained for the parameters of the effective Hamiltonian are related to a detailed description of the system in terms of a microscopic model, it becomes apparent that a representation in terms of a set of effective point charges at the various nuclear sites is inadequate. It is suggested that a representation of each ion by two effective charges, a core charge and a shell charge, may be more appropriate.