Mass- and charge-density modulation of graphite in potassium-graphite intercalates

X-ray evidence on a stage-II potassium-graphite crystal indicates that below the upper critical temperature, $T_U\cong 123$ K, the disordered unregistered potassium layers order and rotate with respect to the graphite. We postulate that the alkali ordering is accompanied by both a static displacement and charge-density modulation of the adjacent graphite planes in which the potassium periodicity is impressed. We then make preliminary estimates of these modulation amplitudes. A simple $\overrightarrow{\mathrm{k}}$-space geometrical argument relates the rotation angle to the alkali ion size in K, Rb, and Cs compounds, and a universal curve is thereby generated which fits well the available data.