I just answered a question about boiling water which concerned the vaporization enthalpy of water — the energy needed to evaporate it, ceteris paribus —, which is substantial.
I was wondering whether there are exotic materials which have a negative enthalpy of vaporization.
Apparently there is one for the other phase change, "fusion": The wikipedia article for the enthalpy of fusion states helium as the only known exception to the usually positive enthalpy for that phase change, at very low temperatures.
The physics would be quite exotic, because the enthalpy of a fluid or solid can surely be roughly equated with the difference of the energy of the bonds between the atoms or molecules in each state. Ordinarily those are obviously stronger for solids than fluids, and for fluids than vapors.
Such a substance would need to be cooled to evaporate.
Does one exist? If so, why does it have that property? I'd also be curious to understand what the negative fusion enthalpy of helium means in terms of helium's behavior around its condensation point close to 0 K, and why its enthalpy is is negative. (These are actually two questions, but they are closely related, so I leave them in one post for now.)