Observation of anomalies in the pressure dependence of the superconducting transition temperature of potassium-based graphite intercalation compounds

Measurements of the pressure (P) dependence of the superconducting transition temperature Tc of KC8 and KHgC4 are reported. KC8 undergoes a first-order transition from a Tc=0.13 K to a Tc=1.7 K phase over the range 215 kbar. A KHgC4 sample underwent an anomalously broad superconducting transition at ambient pressure with an onset near 1.4 K, which is nearly double previously reported Tc values. The KHgC4 transition sharpened dramatically for P1 kbar and exhibited a strongly negative dTcdP at higher pressures. KC24 was found to be nonsuperconducting to P=10 kbar and T0.07 K.