Defects from phonons: Atomic transport by concerted motion in simple crystalline metals

TL;DR

This paper reveals that concerted atomic motions frequently create point defects in body-centered cubic metals at temperatures much lower than melting, due to lattice anharmonicity and flat energy landscapes, impacting diffusion understanding.

Contribution

It demonstrates that defect formation via concerted atomic motion is common in BCC metals at moderate temperatures, linked to lattice anharmonicity, a novel insight into defect dynamics.

Findings

Defect formation occurs frequently in BCC metals at ~50% melting temperature.

Anharmonic lattice vibrations facilitate defect formation.

Explains anomalous diffusion in group 4 transition metals.

Abstract

Point defects play a crucial role in crystalline materials as they do not only impact the thermodynamic properties but are also central to kinetic processes. While they are necessary in thermodynamic equilibrium spontaneous defect formation in the bulk is normally considered highly improbable except for temperatures close to the melting point. Here, we demonstrate by means of atomistic simulations that processes involving concerted atomic motion that give rise to defect formation are in fact frequent in body-centered cubic metals even down to about 50% of the melting temperature. It is shown that this behavior is intimately related to the anharmonicity of the lattice vibrations and a flat energy landscape along certain crystallographic directions, a feature that is absent in, e.g., face-centered cubic lattice structures. This insight has implications for our general understanding of these materials and furthermore provides a complementary explanation for the so-called anomalous diffusion in group 4 transition metals.