16:30 〜 16:45
[SIT03-09] High-pressure decomposition of FeTiO3, MgTiO3 and ZnTiO3 perovskites
キーワード：ペロブスカイト, チタン酸塩, 高圧実験, ポストペロブスカイト
It is generally accepted that transition of MgSiO3-rich perovskite (bridgmanite) to CaIrO3-type postperosvkite is responsible for formation of D” layer in the lowermost mantle. Since discovery of the perovskite-postperovskite transition in MgSiO3, numerous studies have been made to clarify high-pressure transitions of ABX3 perovskites. FeTiO3 and MgTiO3 are endmembers of natural ilmenite, and ZnTiO3 is an analogue for these compounds. Previous studies revealed that at 15-20 GPa range FeTiO3 and MgTiO3 ilmenites transform to perovskites, which are recovered as LiNbO3-type phases at ambient conditions. In contrast to many perovskite-type oxides including MgSiO3 perovskite, FeTiO3 and MgTiO3 perovskites do not transform to CaIrO3-type postperovskite but decompose into two-phase assemblages. However, the transition behaviors of these perovskites are still in controversy, and little has been known on phase transition of ZnTiO3.
We have examined high-pressure transitions in ZnTiO3 and MgTiO3 to about 25 GPa and those of FeTiO3 to about 33 GPa using multianvil apparatus. Pressure was calibrated against press-load using pressure-fixed points at room temperature including GaP and Zr. The pressure was further corrected at high temperature using transition boundaries including dissociation of pyrope into Mg-rich perovskite + corundum. Quenched samples were examined by powder X-ray diffractometers and a scanning electron microscope with an energy-dispersive X-ray spectrometer. Some in-situ X-ray observations on ZnTiO3 phases have also been made using a diamond anvil cell with synchrotron X-ray diffraction method.
The results by quench and in-situ experiments on ZnTiO3 indicate that at 1200 oC ZnTiO3 ilmenite transforms to perovskite at 10 GPa, which dissociates into rocksalt-type ZnO + baddeleyite-type TiO2 at 22 GPa. On release of pressure, perovskite-type ZnTiO3, rocksalt-type ZnO and baddeleyite-type TiO2 are converted into LiNbO3-type, wurtzite-type and αPbO2-type phases, respectively. MgTiO3 perovskite decomposes into MgO + baddeleyite-type TiO2 at 20 GPa and 1400 oC. FeTiO3 perovskite dissociates first into CaTi2O4-type Fe2TiO4 + TiO2 phase at about 28 GPa. At about 30 GPa, this assemblage further changes into Fe3Ti2O7 phase + TiO2 phase below about 1100 oC, above which CaTi2O4-type Fe2TiO4 + FeTi2O5 phase were synthesized. These two assemblages have not yet been found in the previous studies. Combination of our results on ZnTiO3, MgTiO3 and FeTiO3 with those of other titanate perovskites suggests that transition pressure of titanate perovskite increases with tolerance factor or ionic radius of divalent cation. It is also indicated that all the titanate perovskites studied dissociate into two-phase assemblages which are denser than hypothetical CaIrO3-type postperovskites. This is consistent with the previously suggested tendency that ABX3 perovskites with relatively ionic B-X bonds do not transform to postperovskites.
We have examined high-pressure transitions in ZnTiO3 and MgTiO3 to about 25 GPa and those of FeTiO3 to about 33 GPa using multianvil apparatus. Pressure was calibrated against press-load using pressure-fixed points at room temperature including GaP and Zr. The pressure was further corrected at high temperature using transition boundaries including dissociation of pyrope into Mg-rich perovskite + corundum. Quenched samples were examined by powder X-ray diffractometers and a scanning electron microscope with an energy-dispersive X-ray spectrometer. Some in-situ X-ray observations on ZnTiO3 phases have also been made using a diamond anvil cell with synchrotron X-ray diffraction method.
The results by quench and in-situ experiments on ZnTiO3 indicate that at 1200 oC ZnTiO3 ilmenite transforms to perovskite at 10 GPa, which dissociates into rocksalt-type ZnO + baddeleyite-type TiO2 at 22 GPa. On release of pressure, perovskite-type ZnTiO3, rocksalt-type ZnO and baddeleyite-type TiO2 are converted into LiNbO3-type, wurtzite-type and αPbO2-type phases, respectively. MgTiO3 perovskite decomposes into MgO + baddeleyite-type TiO2 at 20 GPa and 1400 oC. FeTiO3 perovskite dissociates first into CaTi2O4-type Fe2TiO4 + TiO2 phase at about 28 GPa. At about 30 GPa, this assemblage further changes into Fe3Ti2O7 phase + TiO2 phase below about 1100 oC, above which CaTi2O4-type Fe2TiO4 + FeTi2O5 phase were synthesized. These two assemblages have not yet been found in the previous studies. Combination of our results on ZnTiO3, MgTiO3 and FeTiO3 with those of other titanate perovskites suggests that transition pressure of titanate perovskite increases with tolerance factor or ionic radius of divalent cation. It is also indicated that all the titanate perovskites studied dissociate into two-phase assemblages which are denser than hypothetical CaIrO3-type postperovskites. This is consistent with the previously suggested tendency that ABX3 perovskites with relatively ionic B-X bonds do not transform to postperovskites.