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 H-W(fcc) > H-W(bcc)) stabilize with increasing pressure before H-W(fcc) < H-W(bcc). Hence the thermodynamic and dynamic instabilities are uncorrelated.