Dynamical and thermodynamical instabilities in the disordered RexW1-x system

The dynamical and thermodynamical stability of the bcc and fee disordered R exW1-x system is studied within the density-functional theory. The configur ational part of the free energy is obtained from abinitio electron structur e calculations together with the cluster expansion and the cluster variatio n formalism. Electronic excitations are accounted for through the temperatu re-dependent Fermi-Dirac distribution. The lattice dynamics of Re and W is studied using the density-functional linear-response theory. The calculated dispersion curves show that fee Re is dynamically stable while bcc Re exhi bits phonon instabilities in large parts of the Brillouin zone, similar to previous results for fee W. Interestingly, the phonon dispersion curves for fee Re show pronounced phonon anomalies characteristic of superconductors such as TaC and NbC. Due to the instabilities in bcc Re and fee W the vibra tional entropy, and therefore the free energy, is undefined. In order to pr edict the regions where the disordered RexW1-x alloy is unstable we calcula te the phonon dispersion curves in the virtual crystal approximation. Then we apply a concentration-dependent nonlinear interpolation to the force con stants, which are calculated through a Born-von Karman fit to the ab initio obtained dynamical matrices. The vibrational free energy is calculated in the stable regions for the phases as a function of concentration. The compl ete analysis gives a region where the bcc phase would become thermodynamica lly unstable towards a phase decomposition into disordered bcc and fee phas es. [S0163-1829(99)11537-6].