Phonon-assisted diffusion in bcc phase of titanium and zirconium from first-principles

TL;DR

This study reveals that anharmonic phonon-phonon interactions are crucial for accurately modeling and understanding the fast diffusion observed in the bcc phases of titanium and zirconium, which standard harmonic models fail to predict.

Contribution

The paper introduces a novel approach combining ab initio molecular dynamics and lattice dynamics to incorporate anharmonic effects into diffusion modeling in bcc metals.

Findings

Anharmonic effects explain large diffusion coefficients.

Harmonic models underestimate diffusivity.

Validated predictions match experimental data.

Abstract

Diffusion is the underlying mechanism for many complicated materials phenomena, and understanding it is basic to the discovery of novel materials with desired physical and mechanical properties. Certain groups of solid phases, such as the bcc phase of IIIB and IVB metals and their alloys, which are only stable when they reach high enough temperatures and experience anharmonic vibration entropic effects, exhibit "anomalously fast diffusion". However, the underlying reason for the observed extraordinary fast diffusion is poorly understood and due to the existence of harmonic vibration instabilities in these phases the standard models fail to predict their diffusivity. Here, we indicate that the anharmonic phonon-phonon coupling effects can accurately describe the anomalously large macroscopic diffusion coefficients in the bcc phase of IVB metals, and therefore yield a new understanding of the underlying mechanism for diffusion in these phases. We utilize temperature-dependent phonon analysis by combining ab initio molecular dynamics with lattice dynamics calculations to provide a new approach to use the transition state theory beyond the harmonic approximation. We validate the diffusivity predictions for the bcc phase of titanium and zirconium with available experimental measurements, while we show that predictions based on harmonic transition state theory severely underestimates diffusivity in these phases.