Self-Ordered Second-Component Nb Clusters in KNbxTa1?xO3 Solid Solutions and Their Physical Properties

Abstract

Semi-empirical Hartree-Fock calculations using the intermediate neglect of the differential overlap (INDO) method, for self-ordered cubic symmetry clusters of seven Nb ions in KTaO 3 are performed with the aim of verifying the cluster model [1] of second component-induced phase transitions in ferroelectric perovskite matrices. It is shown that a seven-particle cluster in KTaO 3 :Nb has two types of states with different nature. Namely, a state with a dilatation combined with an off-center displacement of the central Nb ion in [111] directions in the cluster, and a state with a full-symmetric compression without any off-center effect. The consequences of such cluster structures on the multi-well potential are discussed. In particular, the resonance tunneling between cluster excited states characterized by low frequency on-center vibrations, on the one hand, and excited states of off-center cluster vibrations, on the other, gives rise to low lying tunnel states. The latter states interact with soft TO modes and form the displacive type ferroelectric phase transition induced by impurities. This new type of displacive-like ordering co-exists with order-disorder effects with development of the central peak. All these results are in agreement with experimental data on KTaO 3 with Nb impurities. The results of the semi-empirical INDO calculations for seven Nb ions forming an impurity cluster in KTaO 3 confirm the key assumptions of the cluster model of induced phase transitions.