The Energetics of Li Off-Centering in K$_{1-x}$Li$_x$TaO$_3$; First Principles Calculations

TL;DR

This study uses first-principles calculations to explore Li-ion off-centering in KLT, revealing interactions that promote ferroelectricity and matching experimental dielectric behavior.

Contribution

It provides detailed energetics and interaction insights into Li off-centering and pairing effects in KLT using first-principles methods, advancing understanding of its ferroelectric properties.

Findings

Li off-centering instability confirmed by lattice dynamics.

Nearest neighbor Li-Li pairs favor ferroelectric alignment.

Calculated hopping barriers agree with experimental dielectric data.

Abstract

K$_{1-x}$Li$_{x}$TaO$_3$ (KLT) solid solutions exhibit a variety of interesting physical phenomena related to large displacements of Li-ions from ideal perovskite A-site positions. First-principles calculations for KLT supercells were used to investigate these phenomena. Lattice dynamics calculations for KLT exhibit a Li off-centering instability. The energetics of Li-displacements for isolated Li-ions and for Li-Li pairs up to 4th neighbors were calculated. Interactions between nearest neighbor Li-ions, in a Li-Li pair, strongly favor ferroelectric alignment along the pair axis. Such Li-Li pairs can be considered "seeds" for polar nanoclusters in KLT. Electrostriction, local oxygen relaxation, coupling to the KT soft-mode, and interactions with neighboring Li-ions all enhance the polarization from Li off-centering. Calculated hopping barriers for isolated Li-ions and for nearest neighbor Li-Li pairs are in good agreement with Arrhenius fits to experimental dielectric data.