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تسجيل مستخدم جديدComment on Molybdenum at High Pressure and Temperature: Melting from Another Solid Phase
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There has been a major controversy over the past seven years about the high-pressure melting curves of transition metals. Static compression (diamond-anvil cell: DAC) experiments up to the Mbar region give very low melting slopes dT_m/dP, but shock-wave (SW) data reveal transitions indicating much larger dT_m/dP values. Ab initio calculations support the correctness of the shock data. In a very recent letter, Belonoshko et al. propose a simple and elegant resolution of this conflict for molybdenum. Using ab initio calculations based on density functional theory (DFT), they show that the high-P/high-T phase diagram of Mo must be more complex than was hitherto thought. Their calculations give convincing evidence that there is a transition boundary between the normal bcc structure of Mo and a high-T phase, which they suggest could be fcc. They propose that this transition was misinterpreted as melting in DAC experiments. In confirmation, they note that their boundary also explains a transition seen in the SW data. We regard Belonoshko et al.s Letter as extremely important, but we note that it raises some puzzling questions, and we believe that their proposed phase diagram cannot be completely correct. We have calculated the Helmholtz and Gibbs free energies of the bcc, fcc and hcp phases of Mo, using essentially the same quasiharmonic methods as used by Belonoshko et al.; we find that at high-P and T Mo in the hcp structure is more stable than in bcc or fcc.
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ave (PAW) implementation. We present tests showing that these techniques accurately reproduce experimental data on low-temperature b.c.c. Mo, and that PAW agrees closely with results from the full-potential linearized augmented plane-wave implementation. The work attempts to overcome the uncertainties inherent in earlier DFT calculations of the melting curve of Mo, by using the ``reference coexistence technique to determine the melting curve. In this technique, an empirical reference model (here, the embedded-atom model) is accurately fitted to DFT molecular dynamics data on the liquid and the high-temperature solid, the melting curve of the reference model is determined by simulations of coexisting solid and liquid, and the ab initio melting curve is obtained by applying free-energy corrections. Our calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high pressure melting curve deduced from static compression experiments. Calculated results for the radial distribution function show that the short-range atomic order of the liquid is very similar to that of the high-T solid, with a slight decrease of coordination number on passing from solid to liquid. The electronic densities of states in the two phases show only small differences. The results do not support a recent theory according to which very low dTm/dP values are expected for b.c.c. transition metals because of electron redistribution between s-p and d states.
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