Origin of Increased Curie Temperature in Lithium-Substituted Ferroelectric Niobate Perovskite: Enhancement of the Soft Polar Mode

TL;DR

This study uses first-principles calculations to reveal how lithium substitution enhances the Curie temperature in ferroelectric niobates by modifying polar modes and phase stability, offering insights for designing more thermally stable ferroelectrics.

Contribution

It uncovers the atomic-scale mechanisms by which lithium substitution increases Curie temperature in ferroelectric niobates, including effects on polar modes and phase stability.

Findings

Li substitution introduces compressive chemical pressure reducing Nb-O hybridization.

Large off-center displacement of Li enhances the soft polar mode, raising Curie temperature.

Li substitution stabilizes the tetragonal phase and reveals a metastable anti-phase polar state.

Abstract

The functionality of ferroelectrics is often constrained by their Curie temperature, above which depolarization occurs. Lithium (Li) is the only experimentally known substitute that can increase the Curie temperature in ferroelectric niobate-based perovskites, yet the mechanism remains unresolved. Here, the unique phenomenon in Li-substituted KNbO3 is investigated using first-principles density functional theory. Theoretical calculations show that Li substitution at the A-site of perovskite introduces compressive chemical pressure, reducing Nb-O hybridization and associated ferroelectric instability. However, the large off-center displacement of the Li cation compensates for this reduction and further enhances the soft polar mode, thereby raising the Curie temperature. In addition, the stability of the tetragonal phase over the orthorhombic phase is predicted upon Li substitution, which reasonably explains the experimental observation of a decreased orthorhombic-to-tetragonal phase transition temperature. Finally, a metastable anti-phase polar state in which the Li cation displaces oppositely to the Nb cation is revealed, which could also contribute to the variation of phase transition temperatures. These findings provide critical insights into the atomic-scale mechanisms governing Curie temperature enhancement in ferroelectrics and pave the way for designing advanced ferroelectric materials with improved thermal stability and functional performance.