Origin of negative thermal expansion and pressure induced amorphization in zirconium tungstate from machine-learning potential

TL;DR

This study uses machine-learning potentials to uncover the atomistic mechanisms behind negative thermal expansion and pressure-induced amorphization in zirconium tungstate, highlighting the role of nonbridging oxygen atoms in these phenomena.

Contribution

The paper reveals the atomistic origin of NTE and PIA in ZrW2O8 using a machine-learning potential, linking nonbridging oxygen atoms to these properties for the first time.

Findings

Nonbridging O atoms enable flexible vibrations leading to NTE.

Migration of nonbridging O atoms causes pressure-induced amorphization.

Identification of a hidden phase transition in the amorphous phase.

Abstract

Understanding various macroscopic pressure-volume-temperature properties of materials on the atomistic level has always been an ambition for physicists and material scientists. Particularly, some materials such as zirconium tungstate (ZrW2O8), exhibit multiple exotic properties including negative thermal expansion (NTE) and pressure-induced amorphization (PIA). Here, using machine-learning based deep potential, we trace both of the phenomena in ZrW2O8 back to a common atomistic origin, where the nonbridging O atoms play a critical role. We demonstrate that the nonbridging O atoms confer great flexibility to vibration of polyhedrons, and kinetically drive volume shrinking on heating, or NTE. In addition, beyond a certain critical pressure, we find that the migration of nonbridging O atoms leads to additional bond formation that lowers the potential energy, suggesting that the PIA is a potential-driven first-order phase transition. Most importantly, we identify a second critical pressure beyond which the amorphous phase of ZrW2O8 undergoes a hidden phase transition from a reversible phase to an irreversible one.