High-pressure studies of ammonia and silver perrhenate

The equation of state of solid ammonia (NH{dollar}sb3{dollar}) was determined by x-ray diffraction using synchrotron radiation in a diamond anvil cell between 3 and 56 GPa. A phase transition from a face centered cubic to a hexagonal phase occurs between 3 and 4.2 GPa. The hexagonal phase is stable up to 56 GPa, the highest pressure reached. The x-ray data do not allow a unique determination of the hexagonal space group because of the weak scattering of ammonia, severe grain orientation effects, and the spectral characteristics of the synchrotron radiation. Two possible structures are discussed: P6{dollar}sb3{dollar}/mmc with four molecules per unit cell, and P6{dollar}sb3{dollar}mc with two molecules per unit cell. For the latter structure, a Birch-Murnaghan equation of state fit gives the parameters V{dollar}sb0{dollar} = 19.94 {dollar}pm{dollar} 0.04 cm{dollar}sp{lcub}-3{rcub}{dollar}, B{dollar}sb{lcub}rm o{rcub}{dollar} = 7.56 {dollar}pm{dollar} 0.06 GPa, and B{dollar}sb{lcub}rm o{rcub}spprime{dollar} = 5.29 {dollar}pm{dollar} 0.03. The phases of ammonia at low temperatures and pressures are discussed in terms of transitions between two-dimensional structures with interlayer coupling by the hydrogen bonds.; The structure and optical properties of silver perrhenate (AgReO{dollar}sb4{dollar}) are studied in a diamond anvil cell with synchrotron radiation, Raman scattering, optical absorption and optical reflectivity under pressure to investigate the possibility of metallization by charge transfer Ag{dollar}sp{lcub}rm I{rcub}{dollar} Re{dollar}sp{lcub}rm VII{rcub}{dollar} {dollar}to{dollar} Ag{dollar}sp{lcub}rm III{rcub}{dollar} Re{dollar}sp{lcub}rm V{rcub}{dollar}. The compressibility of AgReO{dollar}sb4{dollar} is strongly pressure dependent (B{dollar}sb{lcub}rm o{rcub}{dollar} = 30.9 {dollar}pm{dollar} 4 GPa for B{dollar}sb{lcub}rm o{rcub}spprime{dollar} = 30.9 {dollar}pm{dollar} 6). The decrease in the c/a ratio with pressure and the weak pressure dependence of the internal Raman active modes (particularly {dollar}nusb1{dollar}A{dollar}sb{lcub}rm g{rcub}{dollar}) suggest that the ReO{dollar}sb4{dollar} tetrahedra are incompressible, while the Ag polyhedra compress anisotropically. The energy gap decreases by 75 meV/GPa. At a first-order phase transition from the scheelite structure at 13 {dollar}pm{dollar} 1 GPa, the energy gap drops by {dollar}sim{dollar}0.19 eV. The number of Bragg reflections in the high pressure phase, and the similarity of the Raman spectrum to that in the scheelite phase suggest another high symmetry structure, but we have not determined that phase. The energy gap continues to decrease gradually above the phase transition. The rate is 58 meV/GPa up to 25 GPa, and then reduces strongly to 47 GPa. Reflectivity measurements show that AgReO{dollar}sb4{dollar} remains a poor semiconductor to at least 52 GPa. No evidence of a valence change in Ag due to charge transfer was found.