The structural and dynamical stability of BeF2 at finite temperatures studied by machine learning molecular dynamics.

Abstract

BeF2 is a key component of fluoride coolants in molten salt reactors. The structural phase transition between its α- and β-quartz phases at high temperatures directly affects the mechanical and thermal properties. In this work, by combining first-principles calculations and machine learning molecular dynamics simulations, we systematically investigate the structural stability, elastic properties, and anharmonic lattice dynamics of BeF2 at finite temperatures. A displacive second-order phase transition from the α to the β phase driven by anharmonic effects is revealed. At low temperatures, F atoms are confined in a multi-well potential landscape, and their off-center displacements induce a multi-peak distribution of Be atoms through the Be-F network coupling. As the temperature increases, enhanced anharmonic atomic motion and dynamic averaging gradually smear out the multi-well energy barriers, releasing F atoms from their local confinement and weakening the Be-F vibrational coupling, which ultimately stabilizes the high-symmetry β phase. The high temperature β phase exhibits higher bulk and Young's moduli, indicating enhanced resistance to compression. The imaginary phonon modes present in the harmonic phonon spectrum disappear at high temperatures due to anharmonic renormalization, confirming the dynamical stability of the β phase at elevated temperatures. The temperature evolution of the phonon density of states further shows that the Be-F vibrational coupling weakens with increasing temperature, and the Be-related modes in the high-frequency region undergo red shifts and broadening, consistent with the enhanced anharmonicity and increased local disorder. This study provides a systematic theoretical basis for understanding the microscopic mechanism of the quartz-type phase transition in BeF2 and offers guidance for optimizing the high-temperature service performance of fluoride materials in molten salt reactors.