Finite-temperature lattice dynamics of FeV at high pressure from first principles

The phonon dispersions of FeV in the B2 crystal structure computed using the finite displacement method and density functional theory (DFT) data show that the system becomes mechanically unstable when the lattice parameter is smaller than 2.78 Å. The instability coincides with a pressure-induced electronic topological transition and is accompanied by a charge transfer from the s to the (directional) d orbitals that occurs preferentially at the Fe sites. A method was developed to efficiently sample the phase space of molecular dynamics (MD) simulations and was used to extract harmonic force constants from finite-temperature DFT MD runs. The phonon dispersions obtained from such force constants for the FeV system at 2.77 Å show that temperature stabilizes the system at high enough temperature even in the absence of magnetism, and that the system is mechanically stable even at 10K when magnetism is considered, hinting at the importance of phonon anharmonicity in the phase stability of the system. X-ray diffraction patterns of FeV in a diamond-anvil cell at 300K and up to 80 GPa did not show any phase transitions.