ULTRAHIGH-PRESSURE STRUCTURAL PHASE-TRANSITIONS IN CR, MO, AND W

On the basis of first-principles total-energy calculations, we predict the ultrahigh-pressure destabilization of the bcc structure in the group-VIB elements Cr, Mo, and W through a bcc --> hcp phase transition at pressures of about 7.0, 4.2, and 12.5 Mbar, respectively. In Mo and W, a subsequent hcp --> fcc transition is also predicted at about 6.2 and 14.4 Mbar, respectively. The overall driving mechanism for these transitions is a continuous sp --> d transfer of electrons upon compression, although other factors play an important quantitative role, especially the hard-core-like interaction between the large cores of these elements, which disfavors the bcc structure and serves to lower the bcc --> hcp transition pressures. While the actual predicted transition pressures are sensitive to the details of the calculations, the qualitative trends are not, and the bcc --> hcp transition in Mo should be within reach of static diamond-anvil-cell experiments. In this regard, we have also calculated accurate 300-K isotherms for bcc Cr, Mo, and W valid up to the 5-6-Mbar pressure range.