Sean Shieh (National Cheng Kung University) Thomas Duffy (Princeton University) and Guoyin Shen (University of Chicago)
facility: APS-GSECARS
It has been suggested that phase transitions in SiO2 may be related to seismic structure observed in the lower mantle. Stishovite is known to transform to the CaCl2 form near 50 GPa on the basis of Raman spectroscopy, x-ray diffraction, and theoretical calculations. Here we used the laser heated diamond anvil cell to investigate possible post-CaCl2-type phases in SiO2 at conditions of the deep lower mantle. We used several different starting materials (stishovite, cristobalite, silica glass) and also varied the heating environment. Samples were mixed with Pt and loaded into a hole in a Re gasket. In most experiments, Ar was used as an insulator and pressure transmitting media, but a few experiments were performed with no insulation. Experiments were carried out using the double-sided Nd:YLF laser heating system at 13-ID-D of the GSCECARS sector of the Advanced Photon Source. Alignment of the x-ray and heating spots was facilitated by the placement of YAG crystals in the sample chamber. Angle-dispersive x-ray diffraction techniques were employed together with a CCD detector. Our in situ studies show that the CaCl2 phase remains stable to about 120 GPa using both stishovite and glass starting materials. For cristobalite starting material, the alpha-PbO2 phase was observed at pressures above 54 GPa and room temperature. However upon heating at 80 GPa, the alpha-PbO2 type phase transformed into the CaCl2 type phase. A separate set of experiments on GeO2 showed that the CaCl2-alpha-PbO2 phase transition occurred near 65 GPa in this material.