High-pressure synchrotron x-ray diffraction and infrared microspectroscopy:applications to dense hydrous phases

Abstract

Synchrotron x-ray diffraction (XRD) and infrared (IR) absorption spectra ofhydrous and ‘anhydrous’ forms of phase X were measured to 30 GPa atroom temperature. Three OH stretching modes were found in the hydrousphase, and surprisingly one sharp OH mode was observed in the previouslycharacterized anhydrous phase. All OH stretching modes soften and broaden withincreasing pressure and become very weak above ∼20 GPa. XRD indicatesthat the crystal structure remains stable up to 30 GPa. Combining IRabsorption and XRD results, the behaviour is attributed to pressure-induceddistortion of the Si2O7 groups and disorder of the hydrogen atoms. The bulkmoduli of the hydrous and ‘anhydrous’ phases are in the region of 74 GPa.