X-ray observation of a √3¯×√3¯ superlattice inKC8at high pressure

A single-crystal x-ray study under pressure reveals a phase transition from an in-plane 2×2 commensurate potassium superstructure to a new √3 × √3 commensurate superstructure. This is the first such transition to be observed; it is related to the recently discovered staging transition of ${\mathrm{KC}}_8$ under pressure. In addition, we observe this staging transition to occur more slowly and at much lower pressures than in the reported neutron-scattering experiment on highly oriented pyrolytic graphite samples.