RACAL STRUCTURES AND THERMODYNAMIC PROPERTIES OF HIGH DENSITY AMOR PHOUS

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1. Using a large volume high pressure apparatus, quartz-type crystalline GeO_2 and Li_2O-4GeO_2 glass have been compressed up to 14 GPa at room temperature and their local structural changes have been investigated by an in-situ XAFS method. In quartz-type crystalline GeO_2, the change of the coordination number from 4 to 6 begins above 8 GPa and finishes below 12 GPa. On decompression, reversal transition begins below 8 GPa and there is a large hysteresis. Almost no sixfold coordination of Ge is preserved after releasing pressure. Change of coordination number in vitreous Li_2O-4GeO_2 begins above 6 GPa and is completed below 10 GPa. Reversal transition begins below 10 GPa and the hysteresis is smaller than that of quartz-type GeO_2. Change of coordination number in vitreous Li_2O-4GeO_2 is also reversal.2. Utilizing the high pressure differential thermal analysis (HP-DTA) system in a cubic multianvil apparatus, we measured the melting points of portlandite, Ca(OH)2, up to 6 GPa and 1273 K.We detected endothermic behavior at the temperature and pressure conditions of 1027 K and 2.5 GPa, 1042 K and 3.5 GPa, 1025 K and 4.0 GPa, 959 K and 5.0 GPa, and 869 K and 6.0 GPa, respectively., due to melting of portlandite. By in-situ X-ray studies under pressure, the melting of portlandite was observed at 1003 K and 4.32 GPa and at 913 K and 5.81 GPa, respectively. Results of both HP-DTA and X-ray studieswere consistent within experimental error. The melting is congruent and has a negative Clapeyron slope, indicating that liquid Ca(OH)2 has higher densities than crystalline portlandite in this pressure range.