Phase-Transition Studies of Germanium to 1.25 Mbar

Abstract

New phase transitions in Ge were observed by energy-dispersive x-ray diffraction techniques for pressures up to 125 GPa (1.25 Mbar) as follows: the $\ensuremath{\beta}$-Sn structure to the simple hexagonal (sh) phase at 75 \ifmmode\pm\else\textpm\fi{} 3 GPa and to the double hexagonal close-packed structure (dhcp) at 102 \ifmmode\pm\else\textpm\fi{} 5 GPa. These are the highest pressures for which a crystalline structure change has been directly observed by x-ray diffraction. Total-energy pseudopotential calculations predict 84 \ifmmode\pm\else\textpm\fi{} 10 GPa for the $\ensuremath{\beta}$-Sn to sh phase transition and 105 \ifmmode\pm\else\textpm\fi{} 2 GPa for sh to hcp (not dhcp) transition. The role of 3 $d$ core electrons in increasing the transformation pressures in Ge, as compared to Si, is emphasized.