Comment on 'Molybdenum at High Pressure and Temperature: Melting from Another Solid Phase'

TL;DR

This paper discusses the complex high-pressure phase diagram of molybdenum, challenging recent ab initio predictions by presenting free energy calculations that favor the hcp phase over bcc and fcc at high P and T.

Contribution

The authors provide new free energy calculations that question recent ab initio phase diagram predictions, suggesting the hcp phase is more stable than bcc or fcc at high pressure and temperature.

Findings

The hcp phase of Mo is more stable than bcc or fcc at high P and T.

The proposed phase diagram by Belonoshko et al. may be incomplete or incorrect.

The study highlights the complexity of Mo's high-pressure phase behavior.

Abstract

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.