Potassium graphite under pressure: Optical reflectivity of superdense phases

Stage-1 potassium graphite has been prepared by solid-state reaction in a diamond-anvil high-pressure cell and the optical reflectivity has been measured at 300 K as a function of pressure. At 3.5 kbar and in the presence of excess K we observe a discontinuous blue shift of the visible plasma reflection edge by 0.35 eV which indicates a reaction from ${\mathrm{KC}}_8$ to ${\mathrm{KC}}_6$. In the range 4 to ∼70 kbar the plasma frequency is nearly independent of pressure as would be expected for full charge transfer from K to graphite. Temperature cycling at pressures above 20 kbar induces a further blue shift of the plasma edge, which we associate with a sluggish reaction towards a compound with limiting stoichiometry close to ${\mathrm{KC}}_{4.4}$. At 300 K the changes in optical properties are reversible.