Phonon Instabilities in fcc and bcc Tungsten

Abstract

The lattice dynamics of bcc and fcc W is studied as a function of pressure using the density-functional linear-response theory. At high pressures and $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ K, bcc W has a higher enthalpy than the fcc and hcp phases and it develops phonon softening anomalies related to this thermodynamic instability; however, it remains dynamically stable. In contrast, the widely unstable shear modes of fcc W at zero pressure (when ${H}_{\mathrm{W}}^{\mathrm{fcc}}g{H}_{\mathrm{W}}^{\mathrm{bcc}}$) stabilize with increasing pressure before ${H}_{\mathrm{W}}^{\mathrm{fcc}}l{H}_{\mathrm{W}}^{\mathrm{bcc}}$. Hence the thermodynamic and dynamic instabilities are uncorrelated.