Hydrogen is proposed to have (at least) two metallic states. I believe the liquid metal state has been observed for a short time under high pressure and temperature conditions, and solid metal hydrogen may have been observed at high pressure but a lower temperature.
Can one think of the difference between liquid metal hydrogen and at least some predicted form of solid metal hydrogen as the freezing of the protons from a fluid to a solid, with the protons resting on a lattice within the electron gas?
edit: In This answer by @RobJeffries about electron degeneracy in solid metal hydrogen the above is addressed - indeed it can make sense to think of the transition in those terms, so I'd like to modify my question to ask, roughly speaking, "how much like freezing" might that transition be:
If a transition between the liquid metal and some solid metal form of hydrogen occurs, would it exhibit similar thermodynamic characteristics of normal crystallization? For example, at a given pressure would there be a well defined transition temperature and energy (e.g. freezing temperature, heat of fusion) and phenomena like nucleation?
I'm guessing the answer may be yes again. This answer by @floris discusses the transition and a phase diagram in Everything about Hydrogen linked there, and mentions a certain fuzziness.
above: Screen shot from Everything about Hydrogen in which reference (12) is: W. B. Leung and N. H. March H. Motz, “Primitive Phase Diagram for Hydrogen”, Physics Letters 56A, 6 (1976), pp. 425-426 (also here)
Abstract:
Using a combination of experimental data, empirical rules and extrapolation formula, plus first principles theory, the major phase boundaries are drawn in the phase diagram of hydrogen. The relevance to laser fusion is briefly referred to.
