Full optimization of quasiharmonic free energy with anharmonic lattice model: Application to thermal expansion and pyroelectricity of wurtzite GaN and ZnO

TL;DR

This paper develops an efficient method for structural optimization at finite temperatures within the quasiharmonic approximation, accurately modeling thermal expansion and pyroelectricity in GaN and ZnO.

Contribution

It introduces an IFC renormalization scheme for simultaneous optimization of cell shape and atomic positions, improving computational efficiency in QHA calculations.

Findings

Accurately reproduces experimental thermal expansion and pyroelectricity of GaN and ZnO.

Demonstrates the effectiveness of the scheme compared to full degrees of freedom optimization.

Provides a general approach for constrained optimizations with minimal accuracy loss.

Abstract

We present a theory and a calculation scheme of structural optimization at finite temperatures within the quasiharmonic approximation (QHA). The theory is based on an efficient scheme of updating the interatomic force constants with the change of crystal structures, which we call the IFC renormalization. The cell shape and the atomic coordinates are treated equally and simultaneously optimized. We apply the theory to the thermal expansion and the pyroelectricity of wurtzite GaN and ZnO, which accurately reproduces the experimentally observed behaviors. Furthermore, we point out a general scheme to obtain correct $T$ dependence at the lowest order in constrained optimizations that reduce the number of effective degrees of freedom, which is helpful to perform efficient QHA calculations with little sacrificing accuracy. We show that the scheme works properly for GaN and ZnO by comparing with the optimization of all the degrees of freedom.