Neutron diffraction study of potassium-graphite intercalation compounds at high pressure: Staging transitions and one-dimensional disorder

Detailed (00L) neutron scattering profiles have been measured versus hydrostatic pressure (0P20 kbar) at room temperature for potassium-graphite intercalation compounds KCₓ with 8x40. The average density of filled galleries along the c axis decreases with increasing P, implying a concomitant increase in the (average) areal K density within a gallery. We observe both large P regions exhibiting continuous evolution of different stages, and narrow P regions where the average stage changes abruptly. The Hendricks-Teller model of one-dimensional disorder is used to fit entire diffraction profiles over extended q ranges. Some of the high-P staging and disorder effects are attributable to P-induced in-plane intercalate densification, the sequence of gallery occupancy and its perfection being a secondary consequence. Other features are entirely controlled by interlayer interactions: e.g., a pure stage-4 phase evolves asymptotically with increasing P at 6--7 kbar for 36x39, whereas a commensurate 2×{}2 in-plane density would be compatible with a pure stage-4 phase only for x=32.0. We demonstrate from the fits that the fractional stage-(3/2) phase in KC₈, reported earlier by us to occur between 15 and 19 kbar, exhibits c-axis correlations no worse than other high-P phases, indicating that this unusual structure represents a true equilibrium phase and does not result simply from random layer sequencing.