A fresh look at dense hydrogen under pressure. IV. Two structural models on the road from paired to monatomic hydrogen, via a possible non-crystalline phase

In this paper, we examine the transition from a molecular to monatomic solid in hydrogen over a wide pressure range. This is achieved by setting up two models in which a single parameter delta allows the evolution from a molecular structure to a monatomic one of high coordination. Both models are based on a cubic Bravais lattice with eight atoms in the unit cell; one belongs to space group Pa (3) over bar, the other to space group R (3) over barm. In Pa (3) over bar one moves from effective 1-coordination, a molecule, to a simple cubic 6-coordinated structure but through a very special point (the golden mean is involved) of 7-coordination. In R (3) over barm, the evolution is from 1 to 4 and then to 3 to 6-coordinate. If one studies the enthalpy as a function of pressure as these two structures evolve (delta increases), one sees the expected stabilization of minima with increased coordination (moving from 1 to 6 to 7 in the Pa (3) over bar structure, for instance). Interestingly, at some specific pressures, there are in both structures relatively large regions of phase space where the enthalpy remains roughly the same. Although the structures studied are always higher in enthalpy than the computationally best structures for solid hydrogen those emerging from the Pickard and Needs or McMahon and Ceperley numerical laboratories - this result is suggestive of the possibility of a microscopically non-crystalline or soft phase of hydrogen at elevated pressures, one in which there is a substantial range of roughly equi-enthalpic geometries available to the system. A scaling argument for potential dynamic stabilization of such a phase is presented. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3679751]