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

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(Ta1-xNbx)O-3 (KTN) compositions. The investigated physical characteristics, T-C and f(r), are determined via dielectric measurements performed in a wide frequency range (10(2)-10(9) 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 time rigidity and stability of the oxygen-metal network are discussed. The validity of this approach is comforted through the example of the oxyfluoride K3Li1.5Ta5O14.5F0.5.