Hiroshi Kojitani

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.

gamma-Ca-3(PO4)(2), naturally known as tuite, is regarded as an important potential reservoir for rare earth elements and large ion lithophile elements. It is a high-pressure polymorph of beta-Ca-3(PO4)(2) whitlockite and a decomposed product of apatite under high-pressure and temperature. Drop-solution enthalpies of beta- and gamma-Ca-3(PO4)(2) were obtained as 298.59 +/- 3.02 and 278.74 +/- 2.98 kJ/mol, respectively, by the drop-solution calorimetry with 2PbO center dot B2O3 solvent at 978 K. Thus the enthalpy of transition from beta- to gamma-Ca-3(PO4)(2) at 298 K (Delta H-tr,298(o)) was 19.85 +/- 4.24 kJ/mol. The isobaric heat capacities of beta- and gamma-Ca-3(PO4)(2) were measured at temperature range of 300-770 K by differential scanning calorimetry, and compared with the results calculated from the Kieffer model. The equilibrium phase boundary between beta- and gamma-Ca-3(PO4)(2) was calculated using present measured data combined with other available thermochemical and thermoelastic data. The calculated boundary gave a phase transition boundary with a dP/dT slope of 4.7 +/- 0.2 MPa/K in the temperature range of 900-2000 K. Based on the phase relationship, the occurrences of tuite and whitlockite in meteorites are discussed. (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+.

BaIrO3 adopts the 9R polytype structure if it is synthesized at ambient pressure. High-pressure and high-temperature treatments up to 10 GPa have led to other 5H and 6H polytype phases, which are quenchable to the ambient condition. However, the single-phase 3C perovskite, the end member of polytypes, has not been obtained so far. Here we report the high-pressure synthesis of BaIrO3 perovskite under 25 GPa and 1150 degrees C. Rietveld refinement on the powder x-ray diffraction pattern revealed that this new compound crystallizes in a tetragonal I4/mcm structure rather than the expected cubic Pm-3m structure. The new perovskite is a Pauli paramagnetic metal with a Fermi liquid ground state, i.e., rho proportional to T-2 below 6 K. The availability of BaIrO3 perovskite also allows completion of the structure-physical property evolution in the AIrO(3) perovskites (A = Ca, Sr, Ba). The Fermi energy located in bands originated from Ir: 5d and O: 2p electrons are found to be very sensitive to the change of crystal structure.

The phonon dispersions and vibrational density of state (VDoS) of the K2SiSi3O9-wadeite (Wd) have been calculated by the first-principles method using density functional perturbation theory. The vibrational frequencies at the Brillouin zone center are in good correspondence with the Raman and infrared experimental data. The calculated VDoS was then used in conjunction with a quasi-harmonic approximation to compute the isobaric heat capacity (C (P) ) and vibrational entropy (), yielding C (P) (T) = 469.4(6) - 2.90(2) x 10(3) T (-0.5) - 9.5(2) x 10(6) T (-2) + 1.36(3) x 10(9) T (-3) for the T range of 298-1,000 K and = 250.4 J mol(-1) K-1. In comparison, these thermodynamic properties were calculated by a second method, the classic Kieffer's lattice vibrational model. On the basis of the vibrational mode analysis facilitated by the first-principles simulation result, we developed a new Kieffer's model for the Wd phase. This new Kieffer's model yielded C (P) (T) = 475.9(6) - 3.15(2) x 10(3) T (-0.5) - 8.8(2) x 10(6) T (-2) + 1.31(3) x 10(9) T (-3) for the T range of 298-1,000 K and = 249.5(40) J mol(-1) K-1, which are in good agreement both with the results from our first method containing the component of the first-principles calculation and with some calorimetric measurements in the literature.

The phonon dispersions and vibrational density of state (VDoS) of the K2SiSi3O9-wadeite (Wd) have been calculated by the first-principles method using density functional perturbation theory. The vibrational frequencies at the Brillouin zone center are in good correspondence with the Raman and infrared experimental data. The calculated VDoS was then used in conjunction with a quasi-harmonic approximation to compute the isobaric heat capacity (C (P) ) and vibrational entropy (), yielding C (P) (T) = 469.4(6) - 2.90(2) x 10(3) T (-0.5) - 9.5(2) x 10(6) T (-2) + 1.36(3) x 10(9) T (-3) for the T range of 298-1,000 K and = 250.4 J mol(-1) K-1. In comparison, these thermodynamic properties were calculated by a second method, the classic Kieffer's lattice vibrational model. On the basis of the vibrational mode analysis facilitated by the first-principles simulation result, we developed a new Kieffer's model for the Wd phase. This new Kieffer's model yielded C (P) (T) = 475.9(6) - 3.15(2) x 10(3) T (-0.5) - 8.8(2) x 10(6) T (-2) + 1.31(3) x 10(9) T (-3) for the T range of 298-1,000 K and = 249.5(40) J mol(-1) K-1, which are in good agreement both with the results from our first method containing the component of the first-principles calculation and with some calorimetric measurements in the literature.

High-pressure micro-Raman spectroscopic measurements of calcium ferrite-type MgAl2O4 and CaAl2O4 were made using a diamond-anvil cell high-pressure apparatus. The pressure dependence of frequencies of 18 Raman peaks for calcium ferrite-type MgAl2O4 and 26 Raman peaks for calcium ferrite-type CaAl2O4 were determined up to 20 GPa at ambient temperature. The mode Gruneisen parameter for each observed Raman mode was obtained from the pressure dependence of frequencies. Vibrational mode calculations by first principles using density functional theory were also performed for assignment of Raman peaks and for estimating frequencies of Raman inactive modes. From the obtained mode Gruneisen parameters and the results of the vibrational mode calculations, thermal Gruneisen parameters were determined to be 1.50(5) for calcium ferrite-type MgAl2O4 and 1.31(3) for calcium ferrite-type CaAl2O4. These thermal Gruneisen parameters were applied to heat capacity and vibrational entropy calculations using Kieffer model.

High-pressure micro-Raman spectroscopic measurements of calcium ferrite-type MgAl2O4 and CaAl2O4 were made using a diamond-anvil cell high-pressure apparatus. The pressure dependence of frequencies of 18 Raman peaks for calcium ferrite-type MgAl2O4 and 26 Raman peaks for calcium ferrite-type CaAl2O4 were determined up to 20 GPa at ambient temperature. The mode Gruneisen parameter for each observed Raman mode was obtained from the pressure dependence of frequencies. Vibrational mode calculations by first principles using density functional theory were also performed for assignment of Raman peaks and for estimating frequencies of Raman inactive modes. From the obtained mode Gruneisen parameters and the results of the vibrational mode calculations, thermal Gruneisen parameters were determined to be 1.50(5) for calcium ferrite-type MgAl2O4 and 1.31(3) for calcium ferrite-type CaAl2O4. These thermal Gruneisen parameters were applied to heat capacity and vibrational entropy calculations using Kieffer model.

Calcium ferrite-type MgAl2O4 is an important endmember of the calcium ferrite phase, which is a high-pressure constituent mineral of mid-ocean ridge basalt at lower mantle conditions. Drop-solution enthalpies of the calcium ferrite-type MgAl2O4 and a mixture of MgO and alpha-Al2O3 with a mol ratio of 1:1 were measured as 109.18 +/- 1.10 and 140.41 +/- 0.85 kJ/mol, respectively, by drop-solution calorimetry with 2PbO center dot B2O3 solvent at 978 K. As determined by differences in the drop-solution enthalpies, the formation enthalpy of calcium ferrite-type MgAl2O4 from oxides at 298 K (Delta H-f(298)degrees) was 31.23 +/- 139 kJ/mol. To thermodynamically calculate the phase boundary between calcium ferrite-type MgAl2O4 and MgO + alpha-Al2O3, the Delta H-f(298)degrees was used together with the other thermochemical and thermoelastic data for calcium ferrite-type MgAl2O4, i.e., heat capacity and entropy estimated by the lattice vibrational calculation, thermal expansivity calculated from the Gruneisen equation, and isothermal bulk modulus and its pressure and temperature derivatives determined by Sueda et al. (2009). The calculated boundary gave a phase transition pressure of 26.6 GPa at 1873K with a slope of -0.006 GPa/K. This phase boundary is potentially applicable to a high-pressure calibration standard. (C) 2012 Elsevier B.V. All rights reserved.

We precisely determined detailed phase relations of upper continental crust (UCC) at 20-28 GPa and 1200-1800 degrees C across the 660-km discontinuity conditions with a high-pressure multi-anvil apparatus. We used multi-sample chambers packed with both of UCC and pressure marker, and they were kept simultaneously at the same high-pressure and high-temperature conditions in each run. The high-pressure experiments were carried out in pressure and temperature intervals of about 1 GPa and 200 degrees C, respectively. At 22-25 GPa and 1600-1800 degrees C, UCC transformed from the assemblage of CaAl4Si2011-rich phase (CAS)+clinopyroxene+garnet+hollandite+stishovite to that of calcium ferrite+calcium perovskite+hollandite+stishovite via the assemblage of CAS+calcium ferrite+calcium perovskite+garnet+hollandite+stishovite. No CAS was observed at 1200 degrees C. The textures and grain sizes in the run products suggested that hollandite (II) (monoclinic symmetry) was stable above 24-25 GPa and transformed to hollandite (I) (tetragonal symmetry) during decompression. We calculated the density of UCC at high pressure and high temperature from the mineral proportions which were calculated from the mineral compositions. UCC has a higher density than PREM up to 23.5 GPa in the range of 1200-1800 degrees C. Above 24 GPa, the density of UCC is lower than that of PREM at 1600-1800 degrees C, but is almost equal to that at 1400 degrees C and higher than PREM at temperature below 1400 inverted perpendicular C. Therefore, we suggest that the subducted UCC may penetrate the 660-km discontinuity into the lower mantle, when its temperature is lower than 1400 degrees C at around 660 km depth. (C) 2012 Elsevier B.V. All rights reserved.

Calcium ferrite-type MgAl2O4 is an important endmember of the calcium ferrite phase, which is a high-pressure constituent mineral of mid-ocean ridge basalt at lower mantle conditions. Drop-solution enthalpies of the calcium ferrite-type MgAl2O4 and a mixture of MgO and alpha-Al2O3 with a mol ratio of 1:1 were measured as 109.18 +/- 1.10 and 140.41 +/- 0.85 kJ/mol, respectively, by drop-solution calorimetry with 2PbO center dot B2O3 solvent at 978 K. As determined by differences in the drop-solution enthalpies, the formation enthalpy of calcium ferrite-type MgAl2O4 from oxides at 298 K (Delta H-f(298)degrees) was 31.23 +/- 139 kJ/mol. To thermodynamically calculate the phase boundary between calcium ferrite-type MgAl2O4 and MgO + alpha-Al2O3, the Delta H-f(298)degrees was used together with the other thermochemical and thermoelastic data for calcium ferrite-type MgAl2O4, i.e., heat capacity and entropy estimated by the lattice vibrational calculation, thermal expansivity calculated from the Gruneisen equation, and isothermal bulk modulus and its pressure and temperature derivatives determined by Sueda et al. (2009). The calculated boundary gave a phase transition pressure of 26.6 GPa at 1873K with a slope of -0.006 GPa/K. This phase boundary is potentially applicable to a high-pressure calibration standard. (C) 2012 Elsevier B.V. All rights reserved.

We precisely determined detailed phase relations of upper continental crust (UCC) at 20-28 GPa and 1200-1800 degrees C across the 660-km discontinuity conditions with a high-pressure multi-anvil apparatus. We used multi-sample chambers packed with both of UCC and pressure marker, and they were kept simultaneously at the same high-pressure and high-temperature conditions in each run. The high-pressure experiments were carried out in pressure and temperature intervals of about 1 GPa and 200 degrees C, respectively. At 22-25 GPa and 1600-1800 degrees C, UCC transformed from the assemblage of CaAl4Si2011-rich phase (CAS)+clinopyroxene+garnet+hollandite+stishovite to that of calcium ferrite+calcium perovskite+hollandite+stishovite via the assemblage of CAS+calcium ferrite+calcium perovskite+garnet+hollandite+stishovite. No CAS was observed at 1200 degrees C. The textures and grain sizes in the run products suggested that hollandite (II) (monoclinic symmetry) was stable above 24-25 GPa and transformed to hollandite (I) (tetragonal symmetry) during decompression. We calculated the density of UCC at high pressure and high temperature from the mineral proportions which were calculated from the mineral compositions. UCC has a higher density than PREM up to 23.5 GPa in the range of 1200-1800 degrees C. Above 24 GPa, the density of UCC is lower than that of PREM at 1600-1800 degrees C, but is almost equal to that at 1400 degrees C and higher than PREM at temperature below 1400 inverted perpendicular C. Therefore, we suggest that the subducted UCC may penetrate the 660-km discontinuity into the lower mantle, when its temperature is lower than 1400 degrees C at around 660 km depth. (C) 2012 Elsevier B.V. All rights reserved.

Phase relations in the system TiO2-ZrO2 were examined in the pressure range of 3.5-12 GPa at 1,800 degrees C, using multianvil apparatus. At 1,800 degrees C, TiO2 rutile transforms to alpha PbO2 structure at 10 GPa, and the alpha PbO2-type solid solution is stable in compositional range between TiO2 and about (Ti-0.6, Zr-0.4)O-2 at 3.5-12 GPa. Combination of the present results with the published data at 0-3 GPa demonstrates that continuous solid solution with the alpha PbO2-type structure is stable between TiO2 and (Ti1-x , Zr (x) )O-2 (x approximate to 0.6) at 0-12 GPa. This indicates that both the alpha PbO2-type TiO2 and srilankite Ti2ZrO6 with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO2 content, lattice parameters of a- and c-axes of the alpha PbO2-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the alpha PbO2-type solid solution dissociates into two phases, alpha PbO2-type phase and tetragonal zirconia. Srilankite with more TiO2-rich composition than Ti2ZrO6 might be found in natural rocks derived from the deep upper mantle.

Phase relations in the system TiO2-ZrO2 were examined in the pressure range of 3.5-12 GPa at 1,800 degrees C, using multianvil apparatus. At 1,800 degrees C, TiO2 rutile transforms to alpha PbO2 structure at 10 GPa, and the alpha PbO2-type solid solution is stable in compositional range between TiO2 and about (Ti-0.6, Zr-0.4)O-2 at 3.5-12 GPa. Combination of the present results with the published data at 0-3 GPa demonstrates that continuous solid solution with the alpha PbO2-type structure is stable between TiO2 and (Ti1-x , Zr (x) )O-2 (x approximate to 0.6) at 0-12 GPa. This indicates that both the alpha PbO2-type TiO2 and srilankite Ti2ZrO6 with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO2 content, lattice parameters of a- and c-axes of the alpha PbO2-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the alpha PbO2-type solid solution dissociates into two phases, alpha PbO2-type phase and tetragonal zirconia. Srilankite with more TiO2-rich composition than Ti2ZrO6 might be found in natural rocks derived from the deep upper mantle.

Isobaric heat capacities (C-p) of Mg2SiO4 forsterite and ringwoodite were measured by differential scanning calorimetry in the temperature range of 306-833 K. The measured C-p of Mg2SiO4 forsterite was consistent with those reported by previous studies. On the other hand, the present C-p of Mg2SiO4 ringwoodite was about 3-5% larger than those measured by previous researchers. The calorimetric data of Mg2SiO4 ringwoodite were extrapolated to 2500 K using a lattice vibrational model calculation, which well reproduced the low-temperature C-p data measured by thermal relaxation method. The calculated C-p shows good agreement with the present calorimetric data. The obtained C-p was expressed by the polynomial of temperature: C-p = 164.30 + 1.0216 x 10(-2)T + 7.6665 x 10(3)T(-1) - 1.1595 x 10(7)T(-2) + 1.3807 x 10(9)T(-3) [J/(mol-K)] in the range of 250-2500 K.

Isobaric heat capacities (C-p) of Mg2SiO4 forsterite and ringwoodite were measured by differential scanning calorimetry in the temperature range of 306-833 K. The measured C-p of Mg2SiO4 forsterite was consistent with those reported by previous studies. On the other hand, the present C-p of Mg2SiO4 ringwoodite was about 3-5% larger than those measured by previous researchers. The calorimetric data of Mg2SiO4 ringwoodite were extrapolated to 2500 K using a lattice vibrational model calculation, which well reproduced the low-temperature C-p data measured by thermal relaxation method. The calculated C-p shows good agreement with the present calorimetric data. The obtained C-p was expressed by the polynomial of temperature: C-p = 164.30 + 1.0216 x 10(-2)T + 7.6665 x 10(3)T(-1) - 1.1595 x 10(7)T(-2) + 1.3807 x 10(9)T(-3) [J/(mol-K)] in the range of 250-2500 K.

NaNiF3 perovskite was found to transform to post-perovskite at 16-18 GPa and 1273-1473 K. The equilibrium transition boundary is expressed as P (GPa)= -2.0+0.014 x T (K). Structure refinements indicated that NaNiF3 perovskite and post-perovskite have almost regular NiF6 octahedra consistent with absence of the first-order Jahn-Teller active ions. Both NaNiF3 perovskite and post-perovskite are insulators. The perovskite underwent a canted antiferromagnetic transition at 156 K, and the post-perovskite antiferromagnetic transition at 22 K. Magnetic exchange interaction of NaNiF3 post-perovskite is smaller than that of perovskite, reflecting larger distortion of Ni-F-Ni network and lower dimension of octahedral arrangement in post-perovskite than those in perovskite. (C) 2012 Elsevier Inc. All rights reserved.

NaNiF3 perovskite was found to transform to post-perovskite at 16-18 GPa and 1273-1473 K. The equilibrium transition boundary is expressed as P (GPa)= -2.0+0.014 x T (K). Structure refinements indicated that NaNiF3 perovskite and post-perovskite have almost regular NiF6 octahedra consistent with absence of the first-order Jahn-Teller active ions. Both NaNiF3 perovskite and post-perovskite are insulators. The perovskite underwent a canted antiferromagnetic transition at 156 K, and the post-perovskite antiferromagnetic transition at 22 K. Magnetic exchange interaction of NaNiF3 post-perovskite is smaller than that of perovskite, reflecting larger distortion of Ni-F-Ni network and lower dimension of octahedral arrangement in post-perovskite than those in perovskite. (C) 2012 Elsevier Inc. All rights reserved.

High-pressure structural phase transitions in NaNiF3 and NaCoF3 were investigated by conducting in situ synchrotron powder X-ray diffraction experiments using a diamond anvil cell. The perovskite phases (GdFeO3 type) started to transform into postperovskite phases (CaIrO3 type) at about 11-14 GPa, even at room temperature. The transition pressure is much lower than those of oxide perovskites. The anisotropic compression behavior led to heavily tilted octahedra that triggered the transition. Unlike oxide postperovskites, fluoropostperovskites remained after decompression to 1 atm. The postperovskite phase in NaCoF3 broke down into a mixture of unknown phases after laser heating above 26 GPa, and the phases changed into amorphous ones when the pressure was released. High-pressure and high-temperature experiments using a multianvil apparatus were also conducted to elucidate the phase relations in NaCoF3. Elemental analysis of the recovered amorphous samples indicated that the NaCoF3 postperovskite disproportionated into two phases. This kind of disproportionation was not evident in NaNiF3 even after laser heating at 54 GPa. In contrast to the single postpostperovskite phase reported in NaMgF3, such a postpostperovskite phase was not found in the present compounds.

High-pressure structural phase transitions in NaNiF3 and NaCoF3 were investigated by conducting in situ synchrotron powder X-ray diffraction experiments using a diamond anvil cell. The perovskite phases (GdFeO3 type) started to transform into postperovskite phases (CaIrO3 type) at about 11-14 GPa, even at room temperature. The transition pressure is much lower than those of oxide perovskites. The anisotropic compression behavior led to heavily tilted octahedra that triggered the transition. Unlike oxide postperovskites, fluoropostperovskites remained after decompression to 1 atm. The postperovskite phase in NaCoF3 broke down into a mixture of unknown phases after laser heating above 26 GPa, and the phases changed into amorphous ones when the pressure was released. High-pressure and high-temperature experiments using a multianvil apparatus were also conducted to elucidate the phase relations in NaCoF3. Elemental analysis of the recovered amorphous samples indicated that the NaCoF3 postperovskite disproportionated into two phases. This kind of disproportionation was not evident in NaNiF3 even after laser heating at 54 GPa. In contrast to the single postpostperovskite phase reported in NaMgF3, such a postpostperovskite phase was not found in the present compounds.

A high-pressure phase of CaRhO3 stable between perovskite and post-perovskite in P-T space was synthesized at 17 GPa and 1650 degrees C using a multi-anvil apparatus. The crystal structure of CaRhO3 was solved by the structure prediction evolutionary algorithm and was refined by Rietveld analysis of the synchrotron powder X-ray diffraction pattern, along with transmission electron microscopy observations. The structure is monoclinic with lattice parameters of a = 12.5114(1) angstrom, b = 3.1241(1) angstrom, c= 8.8579(1) angstrom, beta = 103.951 (1)degrees, V= 336.01(1) angstrom(3) with space group P2(1)/m. The structure contains edge-sharing RhO6 octahedral chains along the b-axis. The six RhO6 octahedral chains make a unit, which stacks up alternatively with the CaO8 polyhedral layer along the [101] direction to form the structure of CaRhO3 intermediate phase.

A high-pressure phase of CaRhO3 stable between perovskite and post-perovskite in P-T space was synthesized at 17 GPa and 1650 degrees C using a multi-anvil apparatus. The crystal structure of CaRhO3 was solved by the structure prediction evolutionary algorithm and was refined by Rietveld analysis of the synchrotron powder X-ray diffraction pattern, along with transmission electron microscopy observations. The structure is monoclinic with lattice parameters of a = 12.5114(1) angstrom, b = 3.1241(1) angstrom, c= 8.8579(1) angstrom, beta = 103.951 (1)degrees, V= 336.01(1) angstrom(3) with space group P2(1)/m. The structure contains edge-sharing RhO6 octahedral chains along the b-axis. The six RhO6 octahedral chains make a unit, which stacks up alternatively with the CaO8 polyhedral layer along the [101] direction to form the structure of CaRhO3 intermediate phase.

We precisely compared phase boundaries of post-spinet transition in pyrolite and Mg2SiO4 and of akimotoite-perovskite transition in MgSiO3 at 21-28 GPa and 1400-1800 degrees C by detailed phase relation experiments using a multi-anvil apparatus. We used a multi-sample cell technique, in which pyrolite. Mg2SiO4 and MgSiO3 were kept simultaneously at the same pressure-temperature conditions in each run. The experiments were performed in pressure and temperature intervals of 03 GPa and 100 degrees C, respectively. The post-spinel transition boundary in Mg2SiO4 is located at higher pressure by about 0.8 GPa than the akimotoite-perovskite transition boundary in MgSiO3. Both the transition boundaries have the same slope of -0.002 GPa/degrees C. In pyrolite, the post-spinel transition occurs in a pressure interval within 0.4 GPa at lower pressure by about 02-1.0 GPa than that in Mg2SiO4 at 1400-1800 degrees C. The Clapeyron slope of the post-spinel transition boundary in pyrolite is -0.001 GPa/degrees C, which is half of -0.002 GPa/degrees C of Mg2SiO4. When we assume that both the transition zone and the uppermost lower mantle have approximately pyrolitic composition, the above results imply that the Clapeyron slope of the transition boundary in pyrolite is more appropriate than that of Mg2SiO4 to evaluate effects of the post-spinel transition on mantle dynamics and the 660-km discontinuity topography. In pyrolite, the akimotoite-perovskite transition and the post-spinet transition occur at the same pressure at 1400 degrees C. Above 1700 degrees C, a part of ringwoodite in pyrolite transforms to garnet + magnesiowustite at pressure below the post-spinet transition, and abundances of garnet and magnesiowustite increase with increasing temperature. (C) 2011 Elsevier B.V. All rights reserved.

We precisely compared phase boundaries of post-spinet transition in pyrolite and Mg2SiO4 and of akimotoite-perovskite transition in MgSiO3 at 21-28 GPa and 1400-1800 degrees C by detailed phase relation experiments using a multi-anvil apparatus. We used a multi-sample cell technique, in which pyrolite. Mg2SiO4 and MgSiO3 were kept simultaneously at the same pressure-temperature conditions in each run. The experiments were performed in pressure and temperature intervals of 03 GPa and 100 degrees C, respectively. The post-spinel transition boundary in Mg2SiO4 is located at higher pressure by about 0.8 GPa than the akimotoite-perovskite transition boundary in MgSiO3. Both the transition boundaries have the same slope of -0.002 GPa/degrees C. In pyrolite, the post-spinel transition occurs in a pressure interval within 0.4 GPa at lower pressure by about 02-1.0 GPa than that in Mg2SiO4 at 1400-1800 degrees C. The Clapeyron slope of the post-spinel transition boundary in pyrolite is -0.001 GPa/degrees C, which is half of -0.002 GPa/degrees C of Mg2SiO4. When we assume that both the transition zone and the uppermost lower mantle have approximately pyrolitic composition, the above results imply that the Clapeyron slope of the transition boundary in pyrolite is more appropriate than that of Mg2SiO4 to evaluate effects of the post-spinel transition on mantle dynamics and the 660-km discontinuity topography. In pyrolite, the akimotoite-perovskite transition and the post-spinet transition occur at the same pressure at 1400 degrees C. Above 1700 degrees C, a part of ringwoodite in pyrolite transforms to garnet + magnesiowustite at pressure below the post-spinet transition, and abundances of garnet and magnesiowustite increase with increasing temperature. (C) 2011 Elsevier B.V. All rights reserved.

The low-temperature isobaric heat capacity (C-p) of synthetic stishovite was measured between 2 and 311 K by the thermal relaxation method. The measured C-p is considerably smaller than that of Holm et al. (1967) over the whole temperature range studied. The standard entropy, S-298.15(o), of stishovite obtained is 24.0 J/(mol.K), which is 3.8 J/(mol.K) lower than that of Holm et al. (1967). Using the measured C-p, the Debye temperature and thermal Gruneisen parameter at 298.15 K were calculated to be 1109 K and 1.68, respectively. The equilibrium coesite-stishovite transition boundary was calculated using the S-298.15(o) of stishovite from published thermodynamic data. The calculated boundary has a slope of 3.2 +/- 0.1 MPa/K at 1200-1600 K, which is larger than the slope determined by the high-pressure in situ X-ray diffraction study by Zhang et al. (1996).

Two hexagonal aluminous phases, which could serve as potential Na- and K-host minerals in the lower mantle, with compositions K1.00Mg2.00Al4.80Si1.15O12 and Na1.04Mg1.85Al4.64Si1.32O12 were synthesized at 22-25 GPa and 1500 degrees C. The K-rich hexagonal aluminous phase was synthesized for the first time. Crystal structures of both hexagonal aluminous phases were refined using the Rietveld method. Obtained interatomic distances and bond angles were compared to published data on the hexagonal aluminous phase CaMg2Al6O12. The general chemical formula of the hexagonal aluminous phase is represented as [M3][M2](2)[M1](6)O-2, where the small-, middle-, and large-sized cations occupy the M1, M2, and M3 sites, respectively. Changes of size and shape of M1O(6) octahedra by the substitution of Si4- for Al3- in the M1 site make it possible to adjust the size of the M2 and the M3 sites to accommodate Na- and Mg2- in the M2 sites and Na- and K+ in the M3 sites, respectively. The stability of hexagonal aluminous phases in a relatively wide compositional range of 30-50 mol% in NaAlSiO4 component along the NaAlSiO4-MgAl2O4 join can be explained by possible replacement of Mg2- by Na- in the M2 site and by shrinkage and deformation of M1O(6) octahedra with the coupled substitution: Mg-M3(2+) + (M1) Al3- -> Na-M2(-) + Si-M1(4+).

The low-temperature isobaric heat capacity (C-p) of synthetic stishovite was measured between 2 and 311 K by the thermal relaxation method. The measured C-p is considerably smaller than that of Holm et al. (1967) over the whole temperature range studied. The standard entropy, S-298.15(o), of stishovite obtained is 24.0 J/(mol.K), which is 3.8 J/(mol.K) lower than that of Holm et al. (1967). Using the measured C-p, the Debye temperature and thermal Gruneisen parameter at 298.15 K were calculated to be 1109 K and 1.68, respectively. The equilibrium coesite-stishovite transition boundary was calculated using the S-298.15(o) of stishovite from published thermodynamic data. The calculated boundary has a slope of 3.2 +/- 0.1 MPa/K at 1200-1600 K, which is larger than the slope determined by the high-pressure in situ X-ray diffraction study by Zhang et al. (1996).

Two hexagonal aluminous phases, which could serve as potential Na- and K-host minerals in the lower mantle, with compositions K1.00Mg2.00Al4.80Si1.15O12 and Na1.04Mg1.85Al4.64Si1.32O12 were synthesized at 22-25 GPa and 1500 degrees C. The K-rich hexagonal aluminous phase was synthesized for the first time. Crystal structures of both hexagonal aluminous phases were refined using the Rietveld method. Obtained interatomic distances and bond angles were compared to published data on the hexagonal aluminous phase CaMg2Al6O12. The general chemical formula of the hexagonal aluminous phase is represented as [M3][M2](2)[M1](6)O-2, where the small-, middle-, and large-sized cations occupy the M1, M2, and M3 sites, respectively. Changes of size and shape of M1O(6) octahedra by the substitution of Si4- for Al3- in the M1 site make it possible to adjust the size of the M2 and the M3 sites to accommodate Na- and Mg2- in the M2 sites and Na- and K+ in the M3 sites, respectively. The stability of hexagonal aluminous phases in a relatively wide compositional range of 30-50 mol% in NaAlSiO4 component along the NaAlSiO4-MgAl2O4 join can be explained by possible replacement of Mg2- by Na- in the M2 site and by shrinkage and deformation of M1O(6) octahedra with the coupled substitution: Mg-M3(2+) + (M1) Al3- -> Na-M2(-) + Si-M1(4+).

High-pressure phase relations in the system NaAl3Si3O11-CaAl4Si2O11 were examined at 13-23 GPa and 1600-1900 degrees C, using a multianvil apparatus. A Ca-aluminosilicate with CaAl4Si2O11 composition, designated CAS phase, is stable above about 13 GPa at 1600 degrees C. In the system NaAl3Si3O11-CaAl4Si2O11, the CAS phase dissolving NaAl3Si3O11 component coexists with jadeite, corundum and stishovite below 22 GPa, above which the CAS phase coexists with Na-rich calcium ferrite, corundum and stishovite. At 1600 degrees C, the solubility of NaAl3Si3O11 component in the CAS solid solution increases with increasing pressure up to about 50 mol% at about 22 GPa, above which the solubility decreases with pressure. The maximum solubility of NaAl3Si3O11 component in the CAS phase increases with temperature up to around 70 mol% at 1900 degrees C at 22 GPa. The dissociation of NaAlSi2O6 jadeite to NaAlSiO4 Calcium ferrite plus stishovite occurs at about 22 GPa. Lattice parameters of the CAS phase with the hexagonal Ba-ferrite structure change with increase of the NaAl3Si3O11 component: a-axis decreases and c-axis slightly increases, resulting in decrease of molar volume. Enthalpies of the CAS solid solutions were measured by high-temperature drop-solution calorimetry techniques. The results show that enthalpy of hypothetical NaAl3Si3O11 CAS phase is much higher than the mixture of NaAlSi2O6 jadeite, corundum and stishovite and is close to that of the mixture of NaAlSiO4 Calcium ferrite, corundum and stishovite. When we adopt the Na:Ca ratio of 75:25 of the natural Na-rich CAS phase in a shocked Martian meteorite, Zagami, the phase relations determined above suggest that the natural CAS phase crystallized from melt at pressure around 22 GPa and temperature close to or higher than 2000-2200 degrees C. The inferred P, T conditions are consistent with those estimated using other high-pressure minerals in the shocked meteorite. (C) 2009 Elsevier B.V. All rights reserved.

CaRuO3 post-perovskite which has quasi-two-dimensional lattice shows one dimensional antiferromagnetism such as Bonner-Fisher type above 400 K. The Neel temperature, T-N, is around 270 K.

High-pressure phase transitions in CaRhO3 were examined using a multianvil apparatus up to 27 GPa and 1930 degrees C. CaRhO3 perovskite transforms to post-perovskite via a monoclinic intermediate phase with increasing pressure. Volume changes for the transitions of perovskite - intermediate phase and of intermediate phase - post-perovskite are -1.1 and -0.7 %, respectively. CaRhO3 post-perovskite is the fourth quenchable post-perovskite oxide found so far. By high-temperature calorimetric experiments, enthalpy of the perovskite - post-perovskite transition in CaRuO3 was measured as 15.2 +/- 3.3 kJ/mol. Combining the datum with those of CaIrO3, it is shown that CaIrO3 perovskite is energetically less stable than CaRuO3 perovskite. This is consistent with the fact that orthorhombic distortion of CaIrO3 perovskite is larger than CaRuO3, as indicated with the tilt-angle of octahedral framework of perovskite structure. The transition pressure from perovskite to post-perovskite in CaBO3 (B = Ru, Rh, Ir) increases almost linearly with decreasing the tilt-angle, suggesting that the perovskite - post-perovskite transition may result from instability of the perovskite structure with pressure.

High-pressure high-temperature phase relation experiments in MgAl2O4 were performed in the pressure and temperature ranges of 18-27 GPa and 1400-2500 degrees C using a Kawai-type multi-anvil high-pressure apparatus. It was clarified that MgAl2O4 spinel directly transforms to the Mg2Al2O5 + Al2O3 assemblage at about 20 GPa and temperature higher than 2100 degrees C. The experimental results indicates that the phase assemblage of Mg2Al2O5 + Al2O3 is stable in the P-T region of 20 < P < 25 GPa and T > 2000 degrees C. The crystal structure of the Mg2Al2O5 phase was refined by the Rietveld method using a structure model based on that of ludwigite. The calculated density of rho(calc) = 3.801(1) g/cm(3) for the Mg2Al2O5 phase is very consistent with the phase relations determined by the high-pressure high-temperature experiments.

High-pressure phase relations in the system NaAl3Si3O11-CaAl4Si2O11 were examined at 13-23 GPa and 1600-1900 degrees C, using a multianvil apparatus. A Ca-aluminosilicate with CaAl4Si2O11 composition, designated CAS phase, is stable above about 13 GPa at 1600 degrees C. In the system NaAl3Si3O11-CaAl4Si2O11, the CAS phase dissolving NaAl3Si3O11 component coexists with jadeite, corundum and stishovite below 22 GPa, above which the CAS phase coexists with Na-rich calcium ferrite, corundum and stishovite. At 1600 degrees C, the solubility of NaAl3Si3O11 component in the CAS solid solution increases with increasing pressure up to about 50 mol% at about 22 GPa, above which the solubility decreases with pressure. The maximum solubility of NaAl3Si3O11 component in the CAS phase increases with temperature up to around 70 mol% at 1900 degrees C at 22 GPa. The dissociation of NaAlSi2O6 jadeite to NaAlSiO4 Calcium ferrite plus stishovite occurs at about 22 GPa. Lattice parameters of the CAS phase with the hexagonal Ba-ferrite structure change with increase of the NaAl3Si3O11 component: a-axis decreases and c-axis slightly increases, resulting in decrease of molar volume. Enthalpies of the CAS solid solutions were measured by high-temperature drop-solution calorimetry techniques. The results show that enthalpy of hypothetical NaAl3Si3O11 CAS phase is much higher than the mixture of NaAlSi2O6 jadeite, corundum and stishovite and is close to that of the mixture of NaAlSiO4 Calcium ferrite, corundum and stishovite. When we adopt the Na:Ca ratio of 75:25 of the natural Na-rich CAS phase in a shocked Martian meteorite, Zagami, the phase relations determined above suggest that the natural CAS phase crystallized from melt at pressure around 22 GPa and temperature close to or higher than 2000-2200 degrees C. The inferred P, T conditions are consistent with those estimated using other high-pressure minerals in the shocked meteorite. (C) 2009 Elsevier B.V. All rights reserved.

CaRuO3 post-perovskite which has quasi-two-dimensional lattice shows one dimensional antiferromagnetism such as Bonner-Fisher type above 400 K. The Neel temperature, T-N, is around 270 K.