Pressure-Induced Staging Transitions in KC8: Observation of a Fractional Stage

An elastic neutron-scattering study of the "saturated" graphite compound K${\mathrm{C}}_8$ under hydrostatic pressure up to 20 kbar reveals two reversible staging transitions which require an increase of K in-plane density relative to the usual 2 × 2 superlattice. Between 15 and 19 kbar a mixture of integral stage-1 ($\dots \mathrm{C}M\dots$) and fractional stage-$\frac{3}{2}$ ($\dots \mathrm{C}M\mathrm{CC}M\dots$) stacking sequences is observed. The fractional staging confirms theoretical predictions of its existence in systems with long-range interlayer repulsion.