Models of structural disorder in ferroelectric perovskites. first principles cluster calculations for BaTiO3 and KNbO3

Miorosoopic models of the structural disorder in Barium Titanate and Potassium Niobate have been studied using the cluster ab initio MO LCAO SCF method. It is shown that on-site potentials for Ti and Nb atoms in oubio phase of these compounds are of single-well type. The electric field gradient tensor at Ti site in tetragonal Barium Titanate measured by NMR method is reproduced within off-center model of oxygen disorder at a reasonable value of the off-center displacement (0.21A) of the oxygen atoms.