New considerations on the role of covalency in ferroelectric niobates and tantalates
Institut de Chimie de la Matière Condensée de Bordeaux
(ICMCB-CNRS), avenue du Dr. A. Schweitzer, 33608 Pessac, France
Corresponding author: a email@example.com
Revised: 22 June 1998
Accepted: 16 July 1998
Published online: 15 December 1998
Values of Curie temperature, TC, and microwave relaxation frequency, fr, 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(TaNbx)O3 (KTN) compositions. The investigated physical characteristics, TC and fr, 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 K3Li1.5Ta5O14.5F0.5.
PACS: 71.20.-b – Electron density of states and band structure of crystalline solids / 77.80.-e – Ferroelectricity and antiferroelectricity / 77.84.Dy – Niobates, titanates, tantalates, PZT ceramics, etc.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 1998