The remaining unsettled issue for C4KHg is the question of the difference between the pink and gold types of C4KHg. The experimentally verified differences between the two types of specimens are summarized in Table II. As mentioned above, extensive characterization of the specimens used for the low-temperature measurements[10] suggests that the only normal-state difference between the gold, low-Tc samples and the pink, Tc = 1.5 K samples is that the gold specimens contain both the alpha and beta phases, while the pink specimens contain only the alpha phase. While this finding is of great interest, it is far from an explanation of the Tc difference between the pink and gold samples. The suppression of the bulk Tc of a superconductor by the presence of a small amount of a second phase is unusual if the second phase is not magnetic. Certainly no magnetic ordering is anticipated in the beta phase of C4KHg.
Previously reported experiments on the hydrogenation[45] and application of pressure[15] to C4KHg may help to clarify the question of the two types of C4KHg. The hydrogenation experiments[45] showed that a minute amount of hydrogen gas can increase the Tc of the gold mixed-phase specimens to 1.5 K, and dramatically narrow the superconducting transition.[45] The results of the applied pressure experiments were strikingly similar:[15] a small hydrostatic pressure of 0.8 kbar narrows an initially broad superconducting transition and increases Tc. The most obvious explanation for the hydrogenation and pressure results would seem to be that application of pressure or exposure to hydrogen converts the beta phase to alpha phase, raising Tc. However, this hypothesis is contradicted by neutron diffraction experiments performed by Kim et al. ,[34] which show that the fraction of alpha and beta phases in a mixed-phase sample is not changed by pressures up to 13.8 kbar.
DeLong and Eklund proposed two ideas to explain the
Tc(P) data.[15]
One proposal is that the pressure induces a disorder-order
structural transformation which increases Tc and
narrows the superconducting transition. A problem with this
hypothesis has been pointed out by Clarke and Uher,[11] who note that the
reversibility of the pressure-induced transformation at 
K is inconsistent with a disorder-order
transformation.
The second proposal of DeLong and Eklund to explain the effect of pressure is charge-density wave (CDW) suppression.[15] Rapid increases of Tc with small applied pressures have been observed in several of the transition metal dichalcogenides (TMDC's).[23] These increases are widely believed to be due to suppression of the CDW that is known from other experiments to exist in many of the TMDC's.[58] Equally suggestive is the fact that low-level hydrogenation also rapidly increases Tc in several of the TMDC's that can support a CDW.[41]
Though none of these similarities between the TMDC's and C4KHg is direct evidence for the presence of a CDW, the possibility of a CDW instability has previously been mentioned on the basis of band-structure calculations for C8K.[28] The periodic lattice distortion that is associated with CDW formation has never been observed in x-ray[20] or neutron diffraction[31] experiments on C4KHg. This is not proof of the non-existence of a CDW, though, since the periodic lattice distortion is usually easily observed only in electron diffraction[58] or scanning tunnelling microscopy experiments.[12] In light of the evidence which suggests the presence of a CDW in C4KHg, temperature-dependent microscopy studies of C4KHg are highly desirable. A search for resistivity and susceptibility anomalies associated with CDW formation in Tc = 0.8 K samples would also be of interest.