Entropy driven stabilization of energetically unstable crystal structures explained from first principles theory

TL;DR

This paper introduces a first-principles method to calculate temperature-dependent phonon spectra, enabling the stabilization analysis of crystal structures that are unstable under harmonic approximation, such as high-temperature bcc phases.

Contribution

It develops a self-consistent phonon calculation method combining Born's approach with first-principles forces, addressing limitations in conventional harmonic phonon calculations.

Findings

Accurately reproduces high-temperature phonon frequencies of bcc Ti, Zr, and Hf.

Demonstrates stabilization of otherwise unstable crystal structures at high temperatures.

Provides a new computational tool for thermodynamic analysis of complex crystal phases.

Abstract

Conventional methods to calculate the thermodynamics of crystals evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the body-centered cubic (bcc) crystal structure when it appears as a high-temperature phase of many metals. A method for calculating temperature dependent phonon spectra self consistently from first principles has been developed to address this issue. The method combines concepts from Born's inter-atomic self-consistent phonon approach with first principles calculations of accurate inter-atomic forces in a super-cell. The method has been tested on the high temperature bcc phase of Ti, Zr and Hf, as representative examples, and is found to reproduce the observed high temperature phonon frequencies with good accuracy.