Comparison of Ferroelectricity in Isomorphic Lithium Niobate and Lithium Tantalate

The statistical theory for ferroelectricity developed by the author in an earlier series of papers is used to interpret the dielectric properties of LiNb${\mathrm{O}}_3$ and to compare the findings with those obtained in the earlier papers for LiTa${\mathrm{O}}_3$. After evaluating and comparing the microscopic parameters which control the ferroelectric behavior of the two systems, it becomes evident that the major difference between their ferroelectric properties, particularly as regards the Curie temperature, centers around the greater covalency and greater strength of the tantalum-oxygen (compared with the niobium-oxygen) bond. The spontaneous polarization for both salts is found to be 45% ionic, 55% electronic; ionic charges of +1.4 and +0.8 electronic units are calculated for the formal-charge-(+5) ions niobium and tantalum, respectively.