Anharmonicity, mechanical instability, and thermodynamic properties of the Cr-Re σ-phase

Using density-functional theory in combination with the direct force method and molecular dynamics we investigate the vibrational properties of a binary Cr-Re sigma-phase. In the harmonic approximation, we have computed phonon dispersion curves and density of states, evidencing structural and chemical effects. We found that the s-phase is mechanically unstable in some configurations, for example, when all crystallographic sites are occupied by Re atoms. By using a molecular-dynamics-based method, we have analysed the anharmonicity in the system and found negligible effects (similar to 0.5 kJ/mol) on the Helmholtz energy of the binary Cr-Re sigma-phase up to 2000 K (similar to 0.8T(m)). Finally, we show that the vibrational contribution has significant consequences on the disordering of the sigma-phase at high temperature. (C) 2014 AIP Publishing LLC.