A Fluorite Isotype of SnO 2 and a New Modification of TiO 2 : Implications for the Earth's Lower Mantle

Abstract

The existence of a cubic fluorite-type SnO₂ and a hexagonal TiO₂ (which may be related to the fluorite structure) have been demonstrated by an in situ x-ray diffraction study in which a diamond-anvil pressure cell was used after the samples had been heated by a continuous yttrium-aluminum-garnet laser. At room temperature, the lattice parameter for SnO₂ (fluorite) is a = 4.925 ± 0.005 angstroms and those for TiO₂ (fluorite-related) are a = 9.22 ± 0.01 angstroms and c = 5.685 ± 0.006 angstroms at about 250 kilobars. The volume change associated with the transition from rutile to fluorite (or related structure) is about –8 percent for SnO₂ and –10.5 percent for TiO₂ at transition. Upon release of pressure, both the fluorite-type SnO₂ and the TiO₂ reverted to the α-PbO₂ structure at room temperature. The hypothesis that the earth's lower mantle is composed of oxide phases might be feasible if it were possible for SiO₂ to possess the fluorite structure or its related forms at high pressure, as shown for SnO₂ and TiO₂ in this study. The oxide hypothesis proposed here differs from that postulated by Birch in that the primary coordination of silicon is 6 for Birch's hypothesis and 8 for the hypothesis presented here.