New considerations on the role of covalency in ferroelectric niobates and tantalates

Abstract:

Values of Curie temperature, T_C, and microwave relaxation frequency, f_r, of ferroelectric niobates and tantalates are found to be closely related to the metal-oxygen network covalency. Correlations previously evidenced in perovskite-type relaxors (PSN, PST) are confirmed here for K(Ta_1-xNb_x)O₃ (KTN) compositions. The investigated physical characteristics, T_C and f_r, are determined via dielectric measurements performed in a wide frequency range (10²-10⁹ Hz) and as a function of temperature (250-800 K). On a theoretical point of view, bond covalencies are evaluated through tight-binding band structure calculations. The complex role of covalency on the metal potential is precised. Two antagonist effects acting on both the short-range interatomic repulsions and the rigidity and stability of the oxygen-metal network are discussed. The validity of this approach is comforted through the example of the oxyfluoride K₃Li_1.5Ta₅O_14.5F_0.5.