Pressure-temperature phase boundaries in K${\mathrm{C}}_{24}$: Evidence for a kinetically-hindered low-temperature staging transition

The pressure-temperature phase diagram of K${\mathrm{C}}_{24}$ was mapped out using in-plane resistivity anomalies to identify phase boundaries. Surprisingly, the 300-K staging transitions at 3.4 and 6.4 kbar are found to connect to the 1-bar ordering-stacking transitions at $T_u=123\mathrm{}$ K and $T_l=96\mathrm{}$ K, respectively, whereas the loci of $P$, $T$ anomalies cannot be equilibrium phase boundaries since the low-$P$, low-$T$ transitions do not involve stage transformations. We suggest that this unusual behavior is due to kinetic hindering of the low-$T$ staging transitions, predicted by simple equilibrium models and observed in the analogous Li compound. A possible origin of the slow staging kinetics at low $T$ in K${\mathrm{C}}_{24}$ is the pinning of discommensurations at Daumas-Herold boundaries.