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 suggest that the quantum rotor behaviour disappears with pressurisation, without any apparent phase transition. Here we report molecular dynamics (MD) calculations which show that all these phenomena can be understood in a classical model with simple treatment of exchange-correlation. We also discuss the thermodynamics of the phase boundaries of Phase I.
Highlights
In 1935 Wigner and Huntington proposed a metallic modification of hydrogen as an atomic phase at high pressure
As pressure is increased the molecules interact with each other and J ceases to be a good quantum number. At low temperature this leads to a “broken symmetry” Phase II, where the rotation has stopped
EQQ is sufficient to cause long-range ordering in the broken-symmetry phase II, with a binding of up to 30meV, enough to be the cause of its stabilisation below 100K
Summary
In 1935 Wigner and Huntington proposed a metallic modification of hydrogen as an atomic phase at high pressure. The melt line shows a maximum around 900K/70GPa[2, 3, 4]; if pressure is increased further the melting temperature drops, meaning the liquid is denser than the close-packed Phase I. At high pressures, where the mechanical work of compression (P V ) approaches the molecular binding energy, complex phases are observed Theory predicts that these are based around a new motif - weakly bound molecules arranged into hexagonal trimers[5, 6, 7]. AAF, RDF, MSD graphs confirm the broken symmetry - free rotor - liquid sequence with temperature and the weakening of the vibron with increased pressure, followed by extreme broadening and further softening, plus suppression of molecular rotation in Phase III. This, and the PV equation of state, is in good agreement with the experiment[20]
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