Phonon Instabilities in fcc and bcc Tungsten

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=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 HfccW>HbccW) stabilize with increasing pressure before HfccW<HbccW. Hence the thermodynamic and dynamic instabilities are uncorrelated.