Structural, mechanical, and thermodynamic properties of solid BeF <b>2</b> at finite temperatures based on machine learning force field simulations

Abstract

As a primary component of molten salt reactor (MSR) fuel and coolant mixtures, BeF2 exhibits critical mechanical and thermodynamic behaviors at high temperatures that are essential for MSR design. This study systematically investigates the structural, mechanical, and thermodynamic properties of solid BeF2 at finite temperatures using first-principles calculations and machine learning force field molecular dynamics simulations. BeF2 is a wide-bandgap transparent compound with a bandgap of 10.4 eV, slightly lower than that of LiF. At low temperatures, BeF2 exhibits anisotropic and brittle elastic properties, with lower resistance to external pressure and shear compared to LiF, attributed to its lower symmetry and weaker bonding strength. As the temperature rises, the elastic moduli show enhanced isotropy and ductility. The Be–F bond demonstrates strong stability at high temperatures, revealed by the atomic vibration displacement distribution and the pair correlation function, while the Be–Be and F–F distributions become more dynamically averaged. Due to the tetrahedral structure and covalent bonding characteristics, BeF2 displays a significantly higher thermal expansion coefficient than the ionic crystal LiF. Furthermore, BeF2 has a broader phonon vibration frequency distribution compared to LiF, with a notable density of states peak in the high-frequency region of 20–28 THz. This provides additional contributions to the heat capacity in the elevated temperature range, making the isobaric heat capacity of BeF2 much higher than that of LiF. These findings deepen the understanding of the fundamental properties of BeF2 and provide theoretical insights for the design and optimization of MSR fuel mixtures.