Equilibrium ground state structure and phonon properties of pure and doped KNbO3 and KTaO3

Publisher Website
Google Scholar
Abstract
Electronic structures of perovskite-type ferroelectric KNbO3 and isostructural incipient ferroelectric KTaO3 have been investigated in ab initio full-potential calculations, making use of M. Methfessel's FP-LMTO code. For KNbO3, 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 KNb0.75Ta0.25O3 and KNb0.25Ta0.75O3 systems produced an indication that a single Nb atom does not have a tendency to go off-center in KTaO3. In contrast to this, Li and Na impurities in KTaO3 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 KTaO3:Na in the low concentration limit.