Introduction

One particularly active area of research in recent years has been the study of the effects of hydrogen absorption on metals.[1] Among the many results of hydrogen uptake are changes in superconductivity. For example, in some transition metals hydrogen absorption causes an increase in the superconducting transition temperature, T_c, while in others T_c decreases upon hydrogenation.[2] The introduction of hydrogen has previously been shown to increase the transition temperature of one graphite intercalation compound, C₈K.[3] The transition temperature depends strongly on the details of the band structure near the Fermi level. Therefore one cannot generalize from a single example about the effect of hydrogenation on other graphite intercalation compounds, just as it was difficult to generalize about the transition metals.

The stage 1 potassium-mercury graphite intercalation compounds (KHg-GIC's) are especially interesting from the standpoint of superconductivity. Discussions of the nature of the superconductivity in these compounds have been greatly complicated by widely varying values reported for the superconducting transition temperature, T_c, which have ranged from the 0.8 K initially found[4] up to 1.5 K both with[5] and without[6] the application of external pressure. Structural and stoichiometric distinctions between samples have been cited as the source of variation among them. Previous attempts to understand the differences between samples focused on their intercalation conditions.[7] These experiments were limited by the indirect nature of the connection between the preparation parameters and the final properties of the GIC.

In this work we describe the application of hydrogen-doping as a new probe of the relationship between structure and superconductivity in stage 1 KHg-GIC's. In contrast to other experiments relevant to the superconductivity of this compound, exposure to hydrogen yields a result which is reproducible not only among similarly prepared samples, but among stage 1 KHg-GIC's with a wide range of initial properties. Therefore this technique may properly be used to study the characteristics of stage 1 KHg-GIC's (denoted by C₄KHg when stoichiometric[8]) without reference to the history of a particular sample. It is remarkable in this context to note that the uniformity of superconducting properties is achieved not by degrading all compounds to the level of the most disordered GIC, but by elevating all of them to the quality of the best ordered ones.

Generally the stage 1 KHg-GIC has been discussed in conjunction with the corresponding stage 2 compound because of the surprising finding that stage 2 has a higher T_c than stage 1, despite specific heat measurements which indicate that stage 2 has a lower density of states at the Fermi level.[9] In this context, a comparison with the potassium hydride graphite intercalation compounds should also prove instructive. One reason is that C₈KH_2/3 has a triple-intercalate-layer structure[10] similar to that of potassium-mercury GIC's.[8] Also significant as far as superconductivity is concerned is that the T_c of C₈KH_0.19 is 0.22 K,[3] fifty percent higher than that of its parent compound, C₈K, with a T_c of 0.15 K.[11]