Structure Optimization and Frozen Phonons in LiNbO3

TL;DR

This paper uses first-principles calculations to optimize the structure of LiNbO3 and predict phonon frequencies, providing detailed insights into its ferroelectric and paraelectric phases with good experimental agreement.

Contribution

It presents a comprehensive first-principles study of LiNbO3's structure and phonons, including predictions for Raman inactive modes and isotope effects.

Findings

Optimized structural parameters match experimental data.

Satisfactory agreement with experimental phonon frequencies for A1 modes.

Predicted frequencies and eigenvectors for A2 modes.

Abstract

The equilibrium ground-state structure of LiNbO3 in the paraelectric and ferroelectric phases is fully optimized in a first-principles calculation using the full-potential linearized augmented plane wave method. The equilibrium volume, c/a ratio and all (four, in the ferroelectric phase) internal parameters are found to be in good agreement with the experimental data. Frozen phonon calculations are performed for TO-Gamma phonons corresponding to the A1 and A2 irreducible representations of the R3c space group in the ferroelectric phase. The comparison with available experimental frequencies for the A1 modes is satisfactory (including the 6Li isotope effect), and the displacement patterns are unambiguously attributed. For the (Raman inactive) A2 modes, phonon frequencies and eigenvectors are predicted.