High-pressure studies on phase transitions in tetrahedral network amorphous systems

The high-pressure behavior of tetrahedral network amorphous systems has been investigated. Phase transitions and structural changes were observed through in situ diamond anvil cell experiments. The systems studied include Lead Germanate (PbGeO3), Phosphorus (P), Beryllium Fluoride (BeF2), and Beryllium Hydride (BeH2).;Structural transitions in crystalline and vitreous PbGeO3 were studied at pressures up to 20 GPa. Crystalline PbGeO3 was observed to undergo a pressure-induced amorphization (PIA) between 12-18 GPa. Vitreous PbGeO3 was found to exhibit an amorphous-to-amorphous (AAT) transition between 10-15 GPa. The structural and thermal properties of the pressure-cycled PbGeO3 materials were further studied and compared to those of thermally quenched glass and ball-milled PbGeO3 . The structure of pressure-amorphized PbGeO3 resembles that of ball-milled PbGeO3, but the thermal properties exhibited significant differences to those of thermally quenched PbGeO3 glass.;Structural transitions in beryllium-based systems BeF2 and BeH2 were also studied at high pressure. A PIA transition in BeF 2 coesite was observed between 27-33 GPa while BeF2 alpha-quartz was found to remain crystalline throughout the pressure range studied. Vitreous BeF2 was observed to undergo a reversible AAT between 3-7 GPa and a new phase was discovered at 53 GPa. The amorphous BeD2 and BeH 2 samples remained transparent up to 70 and 130 GPa respectively, and no major structural changes were observed. The main AI symmetric stretching vibrational mode in BeD2 showed a shallow increase in frequency over the pressure range studied compared to BeH2.;Structural transitions in amorphous P were studied at pressures up to 12 GPa. An amorphous-to-crystalline transition was observed in P at 7.5 (+/- 0.5) GPa. The transition was found to be irreversible and the material recovered upon pressure cycling was crystalline black P. The Equation of State (EoS) of amorphous P was measured using a direct 2-D optical imaging technique. A discontinuous change in volume in the measured EoS of P established the densification of P as the amorphous-to-crystalline transition occurred.