Temperature Effect on Phonon Dispersion Stability of Zirconium by Machine Learning-driven Atomistic Simulations

TL;DR

This paper develops a machine learning interatomic potential for Zirconium to accurately predict phonon dispersion and phase stability at high temperatures, overcoming limitations of traditional methods and empirical potentials.

Contribution

It introduces a novel machine learning-based interatomic potential for Zirconium that captures anharmonic effects and phase stability, enabling accurate finite-temperature phonon calculations.

Findings

Successfully modeled phonon dispersion at high temperature (1188 K).

Revealed the origin of BCC phase instability from the double-well potential energy surface.

Demonstrated stabilization of BCC phase due to atomic vibrations and dynamical averaging.

Abstract

It is well known that conventional harmonic lattice dynamics cannot be applied to energetically unstable crystals at 0 K, such as high temperature body centered cubic (BCC) phase of crystalline Zr. Predicting phonon spectra at finite temperature requires the calculation of force constants to the third, fourth and even higher orders, however, it remains challenging to determine to which order the Taylor expansion of the potential energy surface for different materials should be cut off. Molecular dynamics, on the other hand, intrinsically includes arbitrary orders of phonon anharmonicity, however, its accuracy is severely limited by the empirical potential field used. Using machine learning method, we developed an inter-atomic potential for Zirconium crystals for both the hexagonal closed-packed (HCP) phase and the body centered cubic phase. The developed potential field accurately captures energy-volume relationship, elastic constants and phonon dispersions. The instability of BCC structure is found to originate from the double-well shape of the potential energy surface where the local maxima is located in an unstable equilibrium position. The stabilization of the BCC phase at high temperature is due to the dynamical average of the low-symmetry minima of the double well due to atomic vibrations. Molecular dynamics simulations are then performed to stochastically sample the potential energy surface and to calculate the phonon dispersion at elevated temperature. The phonon renormalization in BCC-Zr is successfully captured by the molecular dynamics simulation at 1188 K.