Off-center lithium ions in KTaO3

Abstract

We present a polarizable point-charge model for the KTa${\mathrm{O}}_3$ lattice that permits the calculation of the off-center displacement $d$ of Li ions that substitute for K. We find $d=1.35\mathrm{}$ Å in good agreement with a value $d=1.26\mathrm{}$ Å derived from NMR measurements. The permanent dipole created by the off-center Li ion is found to be screened by the dipoles induced in the lattice: The "screening volume" is strongly anisotropic, reminiscent of the correlation volume for polarization fluctuations. The dielectric susceptibility of the Li-dipole system is considerably enhanced by local-field effects.