Abstract
Phase I of hydrogen has several peculiarities. Despite having a close-packed crystal structure, it is less dense than either the low temperature Phase II or the liquid phase. At high pressure, it transforms into either phase III or IV, depending on the temperature. Moreover, spectroscopy suggests that the quantum rotor behaviour disappears with pressurisation, without any apparent phase transition. Here we present a simple thermodynamic model for this behaviour based on packing atoms and molecules and discuss the thermodynamics of the phase boundaries. We also report first principles molecular dynamics calculations for a more detailed look at the same phase transitions.
Highlights
The theory of molecular dynamics (MD) is well established and MD remains the best way to study high temperature behaviour of materials, especially dynamical processes such as phase transitions, anharmonic vibrations and diffusion
It is interesting that energy minimization finds two metastable variants of Cmca: molecular dynamics can provide a good test as to whether the they should be regarded as part of the same “phase” at high temperature
At 400GPa, Cmca4(low) should should be stable in ab initio molecular dynamics (AIMD) and unstable in path integral molecular dynamics (PIMD), no transition or phase fluctuation is observed in either case, presumably due to kinetic barriers
Summary
The theory of molecular dynamics (MD) is well established and MD remains the best way to study high temperature behaviour of materials, especially dynamical processes such as phase transitions, anharmonic vibrations and diffusion. At the high-N limit, the distribution of frequencies becomes a continuous density of states g(ω) and the quantum free energy becomes: 2 Centroid dynamics is a marked improvement on standard MD, for hydrogen molecules the assumptions break down.
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