Reports

A Fluorite Isotype of SnO2 and a New Modification of TiO2: Implications for the Earth's Lower Mantle

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Science  27 Jan 1978:
Vol. 199, Issue 4327, pp. 422-425
DOI: 10.1126/science.199.4327.422

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.

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