In situ X-ray absorption spectroscopy study of Si(1-x)Ge(x)O2 dissolution and germanium aqueous speciation under hydrothermal conditions.

The dissolution of Si1–xGexO2 solid solutions under hydrothermal conditions was studied by in situ X-ray absorption spectroscopy. Experiments were performed at the Ge K-edge using a high-pressure cell mounted on the FAME beamline of the European Synchrotron Radiation Facility. Spectra in both transmission and fluorescence mode were collected in isobaric conditions (100 and 150 MPa) up to 475 °C. The local atomic structure around the Ge atom was investigated as a function of the temperature and in pure water and sodium hydroxide solutions. In pure water, the solubility of the cristobalite-type Si0.8Ge0.2O2 increases with the temperature and the Ge atom is in 4-fold coordination. In a sodium hydroxide aqueous solution, a complex between Ge and Na atoms forms and gives rise to precipitation of sodium germanates. Under these conditions, the Ge content in the solution decreases with increasing temperature. These results show that a sodium hydroxide aqueous solution, usually used for quartz crystal growth, is n...