Pressure-temperature phase diagrams of stage-2–4 potassium-graphite intercalation compounds deduced from anomalies in the basal-plane resistivity

The basal-plane resistivities ρ of stage-2–4 potassium-graphite intercalation compounds (K-GIC’s) have been measured as functions of temperature T and pressure p using a contactless method. The pressure range investigated was 0–1.4 GPa. For stage-2 K-GIC (${\mathrm{KC}}_{24}$) measurements were made in the temperature range 90–300 K, while for the higher-stage materials the range was mainly limited to 245–300 K. In all cases ρ increases with p through the anomalous regime near the phase transitions, whereas dρ/dp is always negative in the stable high-p (high-stage) phases. Approximate p-T phase diagrams are deduced from anomalies in ρ corresponding to the staging and in-plane order-disorder transitions, respectively. In all cases the slopes dp/dT of the phase boundaries for the high-pressure staging transitions are such that the transitions extrapolate to positive T at atmospheric p, although no such transitions have been reported. The low-p staging-ordering phase boundary of ${\mathrm{KC}}_{24}$ is particularly complex, involving segments with both dp/dT→0 (125p-T diagrams are compared with recent structural experiments and models. We suggest that the complexity of the ${\mathrm{KC}}_{24}$ phase diagram is attributable to the weakly incommensurate in-plane structure at low p and T.