Local structure changes and structural transformations of melt, glass and gel under the Earth's mantle conditions.

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Using a large-volume high-pressure apparatus, phases of crystals, melts, glasses and gel in the system of some germanates, fluorides, halogenides and silicates have been compressed up to 15 GPa at 300-1500K. The local structure changes around the specific kind of atom have been investigated by an in situ XAFS(x-ray absorption fine structure) method using synchrotron radiation. The structures have also been characterized by means of diffraction and spectroscopic methods. Dynamical atomic motion for each atom in these phases has been studied by determined Debye-Waller factors. The atomic effective pair potentials, the phonon energies and Gruneisen parameters have been estimated in this study. For example, the coordination number of Ge in GeO2 glass changes from four to six between 8 and 10 GPa. In contrast to the case for the glass, the coordination environment around Ge in GeO2 gel gradually increases over a pressure range from 2 to 12 GPa. Liquid germinate consisting of tetrahedrally coordinated Ge contracts with increasing pressure without significant changes in the local structure up to 2.5 GPa at 1300 K and then shows an abrupt fourfold-to-sixfold coordination change around 3 GPa. The coordination change is completed below 4 GPa where upon a high-density liquid consisting of octahedrally coordinated Ge becomes stable. By analogy with this liquid germanate, it is expected that a sharp transition occurs in liquid silicate at a higher pressure. Geophysical implications derived from this study will be discussed with detailed data of our studies.