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 $2<~P<~15$ 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 $T>~0.07$ K.