Equilibrium ground state structure and phonon properties of pure and doped knb03 and kta03

Abstract

Electronic structures of perovskite-type ferroelectric KNb03 and isostructural incipient ferroelectric KTaO3, have been investigated in ab initio full-potential calculations, making use of M. Methfessel's FP-LMTO code. For KNb03, the total energy supercell calculations for an “impurity” Nb atom being displaced independently of “bulk” Nb atoms made it possible to prove the local potential well at the Nb site and to conclude that the cubic to tetragonal ferroelectric phase transition involves cooperative displacive motion of Nb atoms. Similar calculations for KNb0 75Ta0.2503 and KNb0.25Tao.7503 systems produced an indication that a single Nb atom does not have a tendency to go off-center in KTa03. In contrast to this, Li and Na impurities in KTa03 are displaced off-center due to short-range forces, irrespectively of the macroscopic electrostatic field in the crystal. This explains the observation of spin glass-type behavior in KTaO3,:Li and KTa03.Na in the low concentration limit. © 1995, Taylor & Francis Group, LLC. All rights reserved.