Free energy of the order-disorder phase transition in FeV from molecular dynamics

We conducted an analysis of the free energy and vibrational entropy of chemically ordered and disordered body-centered cubic-based equiatomic iron-vanadium (FeV) using classical molecular dynamics simulations and harmonic Born-von Kármán (BvK) force constant fits to the molecular dynamics data. Our investigations spanned a range of temperatures, from 0K to 1300 K, and 40 distinct volumes, providing insights into the structural and thermodynamic properties of the material. To perform these calculations, we generated supercells consisting of 2000 atoms, with B2 ordering for the chemically ordered structures, and randomly occupied bcc lattice sites for the disordered ones. We calculated phonon dispersions and partial density-of-state (DOS) curves from the dynamical matrix obtained using the BvK force constant matrices. The results were then employed to determine the temperature-dependent vibrational entropies. We find that the vibrational entropy is larger for the ordered phase at low temperatures, in agreement with experiments, but at high temperatures, it is larger for the disordered phase. The configurational entropy drives the order-disorder phase transition.