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

American Association for the Advancement of Science (AAAS) - Tập 199 Số 4327 - Trang 422-425 - 1978
Lin-gun Liu1
1Seismological Laboratory, California Institute of Technology, Pasadena 91125

Tóm tắt

The existence of a cubic fluorite-type SnO 2 and a hexagonal TiO 2 (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 SnO 2 (fluorite) is a = 4.925 ± 0.005 angstroms and those for TiO 2 (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 SnO 2 and –10.5 percent for TiO 2 at transition. Upon release of pressure, both the fluorite-type SnO 2 and the TiO 2 reverted to the α-PbO 2 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 SiO 2 to possess the fluorite structure or its related forms at high pressure, as shown for SnO 2 and TiO 2 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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American Association for the Advancement of Science (AAAS)

Tập 199 Số 4327

422-425

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