Introduction

The superconducting graphite intercalation compounds (GIC's) are among the most intensively studied layered superconductors.[11,19] Initially most research efforts centered on the compounds C₈M, where M is a heavy alkali metal.[25,37] The occurrence of superconductivity in these GIC's is surprising, since neither of the starting constituents (graphite or the alkali metal) is superconducting.[25] In recent years the focus of GIC superconductivity research has shifted to the ternary compounds[48] in which the intercalant is made up of an alkali metal and a heavy metal.

An extensive study of the MHg- and MTl-GIC's (M = heavy alkali metal) was performed by Iye and Tanuma.[30] Superconductivity of the second-stage compound C₈KHg was also studied by Pendrys et al. /[44] Several unusual features were found:[30,44,29] extended linearity and/or positive curvature in the temperature dependence of the upper critical field H_c2, and critical field anisotropy ratios as large as 47. Positive curvature of H_c2(T) is contrary to the usual low-temperature saturation of H_c2(T) seen in most type II superconductors.[26,42] Yet positive curvature or extended linearity of H_c2(T) appear to be common features of nearly all layered superconductors.[59] An understanding of this anomalous critical field behavior is particularly of interest because the high-T_c oxide superconductors are also, in a sense, layered materials.

The superconducting transition temperature of C₄KHg has been reported[30,53,45] to be anywhere in the range from 0.7[30] to 1.5 K. Understanding this T_c variation may well be an important first step in figuring out the stage dependence of T_c in the KHg-GIC's. Raman scattering[53] and transmission electron microscopy (TEM)[52] have been employed by Timp et al. to examine the differences between these two types of C₄KHg, but no full comparison of the superconducting properties has been made. In the present work a detailed study of the superconductivity of the two types of C₄KHg is reported, including neutron diffraction, critical field, and hydrogenation experiments. Preliminary results have been reported elsewhere.[9,45]