糀谷 浩

The crystal structure of the high-pressure Mg2Cr2O5 phase was studied by single-crystal X-ray diffraction (XRD) analysis for the recovered samples. The 61 parameters including anisotropic displacement parameters of each atom and site occupancies of Mg and Cr in cation sites were refined with R-1 = 1.26%, wR(2) = 4.33%, and S-fit = 1.265 for 470 unique reflections. The results show that the structure of the recovered Mg2Cr2O5 phase is the same as modified ludwigite (mLd)-type Mg2Al2O5 [space group: Pbam (no. 55)], and the lattice parameters are a = 9.6091(2), b = 12.4324(2), c = 2.8498(1) angstrom (Z = 4). The refined structure of the Mg2Cr2O5 phase has four (Mg, Cr)O-6 octahedral sites and a MgO6 trigonal prism site, and is similar to but distinct from that of CaFe3O5-type Mg2Fe2O5 phase, which has two octahedral sites and a bicapped trigonal prism site with two longer cation-oxygen bonds. The isotropic atomic displacement parameter of the trigonal prism site cation in mLd-type Mg2Cr2O5 is relatively small compared with that of CaFe3O5-type Mg2Fe2O5, suggesting that the trigonal prism site is less flexible for cation substitution than that of CaFe3O5-type structure. To stabilize mLd-type A(2)(2+)B(2)(3+)O(5) phase, it would be an important factor for the B3+ cation to have high octahedral-site preference, resulting in only A(2+) cation being accommodated in the tight trigonal prism site. Our study also suggests that mLd-type phase with (Mg, Fe2+)(2)Cr2O5 composition would crystallize as one of decomposed phases of chromitites, when the chromitites were possibly subducted into the mantle transition zone.

We determined phase relations in FeCr2O4 at 12-28 GPa and 800-1600 degrees C using a multi-anvil apparatus. At 12-16 GPa, FeCr2O4 spinel (chromite) first dissociates into two phases: a new Fe2Cr2O5 phase + Cr2O3 with the corundum structure. At 17-18 GPa, the two phases combine into CaFe2O4-type and CaTi2O4-type FeCr2O4 below and above 1300 degrees C, respectively. Structure refinements using synchrotron X-ray powder diffraction data confirmed the CaTi2O4-structured FeCr2O4 (Cmcm), and indicated that the Fe2Cr2O5 phase is isostructural to a modified ludwigite-type Mg2Al2O5 (Pbam). In situ high-pressure high-temperature X-ray diffraction experiments showed that CaFe2O4-type FeCr2O4 is unquenchable and is converted into another FeCr2O4 phase on decompression. Structural analysis based on synchrotron X-ray powder diffraction data with transmission electron microscopic observation clarified that the recovered FeCr2O4 phase has a new structure related to CaFe2O4-type. The high-pressure phase relations in FeCr2O4 reveal that natural FeCr2O4-rich phases of CaFe2O4- and CaTi2O4-type structures found in the shocked Suizhou meteorite were formed above about 18 GPa at temperature below and above 1300 degrees C, respectively. The phase relations also suggest that the natural chromitites in the Luobusa ophiolite previously interpreted as formed in the deep-mantle were formed at pressure below 12-16 GPa.

Phase transitions in NaZnF3 and NaMnF3 were examined up to 24 GPa and 1100 degrees C using a multianvil apparatus. NaZnE3 perovskite transforms to postperovskite above 11-16 GPa at 600-1000 degrees C, and the postperovskite is quenchable at ambient conditions. The NaZnE3 perovskite-postperovskite transition boundary is expressed as P (GPa) = 4.9 + 0.011T (degrees C). At 8-11 GPa and 900-1100 degrees C, NaMnF3 perovskite dissociates into two phases of Na3Mn2F7 and MnF2. The latter phase is suggested to have the structure of orthorhombic-I type ZrO2 or cotunnite. Using available experimental data on the perovskite-postperovskite transitions in thirteen compounds of A(2+)B(4+)O(3) and A(+)B(2+)F(3), several crystal-chemical characteristics of the transition are elucidated as follows. In the transition, the volume change is between -1% and -2%, and the Clapeyron slope of the boundary is 10-17 MPa/degrees C. These support reliability of recently determined Clapeyron slope of 13 MPa/degrees C in MgSiO3 which suggests that the perovskite-postperovskite boundary intersects the temperature profile twice in the D" layer. Postperovskites of ABX(3) whose enthalpies are higher by more than 70 kj/mol relative to the phase stable at I atm are unquenchable, while those by less than 15 kj/mol are quenchable to ambient conditions. Structure refinements indicate that A(+)B(2+)F(3) postperovskites quenched at 1 atm are more similar to that of MgSiO3 postperovskite at high pressure, than those of quenched A(2+)B(4+)O(3) postperovskites. With increasing pressure, octahedral tilt angles of both A(2+)B(4+)O(3) and A(+)B(2+)F(3) perovskites increase, resulting in transition to postperovskite at the angle of about 26 degrees, and fluoride perovskites are more rapidly distorted with pressure than oxide perovskites. Covalent character of B-X bonds of ABX(3) postperovskite is suggested to be favorable for stabilization of the postperovskite structure. All these features suggest that NaNiF3 is a good quenchable, low-pressure analogue compound to MgSiO3 to investigate the perovskite-postperovskite transition. (C) 2013 Elsevier B.V. All rights reserved.

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 mu C/cm(2), 88 mu C/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 mu C/cm(2), 65 mu C/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 mu C/cm(2), 76 mu C/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti4+.