Frustration of tilts and A-site driven ferroelectricity in KNbO_3-LiNbO_3 alloys

TL;DR

This study uses density functional calculations to reveal that A-site disorder in K_{0.5}Li_{0.5}NbO_3 induces strong ferroelectricity driven by Li off-centering, despite a small tolerance factor and absence of stereochemically active ions.

Contribution

It demonstrates that A-site disorder can induce significant ferroelectricity and stabilize a tetragonal ground state in KNbO_3-LiNbO_3 alloys, challenging traditional expectations.

Findings

A-site disorder frustrates tilt instabilities

Large Li off-centerings drive ferroelectricity

Tetragonal ground state stabilized without strain

Abstract

Density functional calculations for K_{0.5}Li_{0.5}NbO_3 show strong A-site driven ferroelectricity, even though the average tolerance factor is significantly smaller than unity and there is no stereochemically active A-site ion. This is due to the frustration of tilt instabilities by A-site disorder. There are very large off-centerings of the Li ions, which contribute strongly to the anisotropy between the tetragonal and rhombohedral ferroelectric states, yielding a tetragonal ground state even without strain coupling.