High-pressure phase transitions in potassium and phase relations among heavy alkali metals

The optical reflectivity ($0.5 \mathrm{eV}<~\hslash \omega <~5 \mathrm{eV}$) of potassium metal has been measured to 380 kbar at room temperature. Two phase transitions occur at 114 and 188 kbar, respectively. By comparison to the high-pressure optical properties of Cs and Rb the phase K-II has fcc structure. At 188 kbar, K undergoes a considerable change in its optical response indicating the occurrence of a complicated Fermi-surface topology. The phase transitions in K, together with recent experimental investigations of Rb and Cs, reveal systematic trends in the phase stability of heavy alkali metals under pressure.