Chemistry - Does exotic potassium exist?

Solution 1:

First, a point that is more like a comment: I'm not sure that just because electrons get put in to d-orbitals (and thinking of crystal band structures in terms of atomic orbitals is of questionable value anyway) that it would not be a metal. Many elements (one could even say most elements) with d-electrons are quite happy as metals.

You might want to start with L.F. Lundegaard et al., Phys Rev B80 020101(R) (2009), where they do DFT calculations on potassium at different pressures. I will quote from their conclusions section:

The observation of the oP8 structure in K reveals it to have a pressure-induced structural behavior like that of Na, as well as having behavior like Rb and Cs as indicated by the observation of the tI4 and oC16 phases. The similarity to Na is unexpected in relation to the recent electronic structure calculations of Ma et al.$^{19}$ As noted, they found K, Rb, and Cs to have the same high-pressure structural sequence, and they attributed this to these heavier alkalis’ being dominated by d electrons arising from s-d transfer, whereas the ultrahigh pressure behavior of Na arises, they suggest, from p-d hybridization. Thus our results question this picture of a simple dependence of structural stability on d-electron dominance or p-d hybridization.

Solution 2:

Another interesting paper (arXiv:1310.4718 [cond-mat.mtrl-sci]) on this topic. I think you should be able to access this one, but I found three passages worth noting in the article.

Peculiarities in structural behaviour of K under pressure are based on specific features of its electron configuration: K opens in the Periodic table the first long period with 3d transition metals. The empty 3d electron band in K is just above the valence electron 4s level and under pressure there is s – d transition [16]. In heavy alkalis Rb and Cs that have d-levels in their cores electron transitions start at lower pressures than in K. In lighter element Na the 3d level is well above the 3s valence electron level therefore fcc in Na is stable up to a very high pressure of ~100 GPa.

It seems that this is saying that the structural change away from FCC marks where the electron transition occurs. In the paper, they say that the FCC structure stops being the most stable for $\ce{K}$ after $\pu{20 GPa}$ (giga or billion pascal) at $\pu{300K}$.

We suggest that in potassium at densities higher than V/Vo ≈ 0.3 there is not only s – d electron transfer but also core electrons start to participate in the valence band and K becomes a polyvalent metal with the number of valence electrons counting to 2 in oP8 and even to 4 in oC16.

This is saying that for some of the even higher pressures, that even some of the core electrons are moving to the valence band.

This passage from the introduction of the article above does seem to support your view that, for some cases, compression can make a metal less metallic.

Na, one the best metal in terms of conductivity and reflectivity, becomes insulating and transparent at around 2 Mbar [4]. Lithium becomes superconducting [5] below 20K at pressures around 0.5 Mbar and at higher pressures shows a series of complex structures even with a semiconducting behaviour [6,7].

We wouldn't expect metal to exhibit insulating behavior.