COVALENT AND LIQUID-LIKE AMORPHOUS PHASES IN AL-GE ALLOYS

We report the formation of a liquid-like amorphous metallic phase in t he AI-Ge alloy system at high pressures. We used Menill-Bassett diamon d anvil cells to apply pressures from 0 to 200kbar (from 0 to 20 GPa) to nanosize two-phase mixtures of aluminium and germanium. At a pressu re of approximately 25 kbar, we observed the formation of a metastable high-pressure hexagonal gamma phase. On releasing the pressure, the g amma phase transforms to a tetrahedrally bonded continuous random netw ork structure, amorphous A1Ge-I, similar to that found under high-pres sure indents in crystalline pure germanium. On further increasing the pressure from 25 kbar, the gamma phase remains stable up to 80 kbar. A t 80 kbar, the gamma phase transforms to a dense liquid-like phase, am orphous A1Ge-II. This transformation is accompanied by an increase in density of about 15%. Both amorphous phases are metastable at ambient conditions of pressure and temperature and this facilitates the observ ations of the phases by transmission electron microscopy.