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1.
The phase relations and compression behavior of MnTiO3 perovskite were examined using a laser-heated diamond-anvil cell, X-ray diffraction, and analytical transmission electron microscopy. The results show that MnTiO3 perovskite becomes unstable and decomposes into MnO and orthorhombic MnTi2O5 phases at above 38 GPa and high temperature. This is the first example of ABO3 perovskite decomposing into AO + AB2O5 phases at high pressure. The compression behavior of volume, axes, and the tilting angle of TiO6 octahedron of MnTiO3 perovskite are consistent with those of other A2+B4+O3 perovskites, although no such decomposition was observed in other perovskites. FeTiO3 is also known to decompose into two phases, instead of transforming into the CaIrO3-type post-perovskite phase and we argue that one of the reasons for the peculiar behavior of titanate is the weak covalency of the Ti–O chemical bonds.  相似文献   

2.
The Raman spectra of bixbyite, Mn2O3, were measured up to 40 GPa at room temperature. Mn2O3 undergoes a phase transition from the C-type rare earth structure to the CaIrO3-type (post-perovskite) structure at 16–25 GPa. The transition pressure measured in Raman spectroscopy is significantly lower than the pressure reported previously by an X-ray diffraction study. This could be due to the greater polarizability in the CaIrO3-type structure, consistent with high-pressure observation on the CaIrO3 type in MgGeO3, although it is still possible that experimental differences may cause the discrepancy. Unlike the change at the perovskite to CaIrO3-type transition, the spectroscopic Grüneisen parameter does not decrease at the C-type to CaIrO3-type transition. The spectroscopic Grüneisen parameter of the low-pressure phase (C type) is significantly lower than thermodynamic Grüneisen parameter, suggesting significant magnetic contributions to the thermodynamic property of this material. Our Raman measurements on CaIrO3-type Mn2O3 contribute to building systematic knowledge about this structure, which has emerged as one of the common structures found in geophysically important materials.  相似文献   

3.
Compression behaviors of CaIrO3 with perovskite (Pv) and post-perovskite (pPv) structures have been investigated up to 31.0(1.0) and 35.3(1) GPa at room temperature, respectively, in a diamond-anvil cell with hydrostatic pressure media. CaIrO3 Pv and pPv phases were compressed with the axial compressibility of β a > β c > β b and β b > β a > β c, respectively and no phase transition was observed in both phases up to the highest pressure in the present study. The order of axial compressibility for pPv phase is consistent with the crystallographic consideration for layer structured materials and previous experimental results. On the other hand, Pv phase shows anomalous compression behavior in b axis, which exhibit constant or slightly expanded above 13 GPa, although the applied pressure remained hydrostatic. Volume difference between Pv and pPv phases was gradually decreased with increasing pressure and this is consistent with the results of theoretical study based on the ab initio calculation. Present results, combined with theoretical study, suggest that these complicate compression behaviors in CaIrO3 under high pressure might be caused by the partially filled electron of Ir4+. Special attention must be paid in case of using CaIrO3 as analog materials to MgSiO3, although CaIrO3 exhibits interesting physical properties under high pressure.  相似文献   

4.
Lattice preferred orientations (LPO) developed in perovskite and post-perovskite structured CaIrO3 were studied using the radial X-ray diffraction technique combined with a diamond anvil cell. Starting materials of each phase were deformed from 0.1 MPa to 6 GPa at room temperature. Only weak LPO was formed in the perovskite phase, whereas strong LPO was formed in the post-perovskite phase with an alignment of the (010) plane perpendicular to the compression axis. The present result suggests that the (010) is a dominant slip plane in the post-perovskite phase and it is in good agreement with the crystallographic prediction, dislocation observations via transmission electron microscopy, and a recent result of simple shear deformation experiment at 1 GPa–1,173 K. However, the present result contrasts markedly from the results on MgGeO3 and (Mg,Fe)SiO3, which suggested that the (100) or (110) is a dominant slip plane with respect to the post-perovskite structure. Therefore it is difficult to discuss the behavior of the post-perovskite phase in the Earth’s deep interior based on existing data of MgGeO3, (Mg,Fe)SiO3 and CaIrO3. The possible sources of the differences between MgGeO3, (Mg,Fe)SiO3 and CaIrO3 are discussed.  相似文献   

5.
Low-temperature isobaric heat capacities (C p ) of MgSiO3 ilmenite and perovskite were measured in the temperature range of 1.9–302.4 K with a thermal relaxation method using the Physical Properties Measurement System. The measured C p of perovskite was higher than that of ilmenite in the whole temperature range studied. From the measured C p , standard entropies at 298.15 K of MgSiO3 ilmenite and perovskite were determined to be 53.7 ± 0.4 and 57.9 ± 0.3 J/mol K, respectively. The positive entropy change (4.2 ± 0.5 J/mol K) of the ilmenite–perovskite transition in MgSiO3 is compatible with structural change across the transition in which coordination of Mg atoms is changed from sixfold to eightfold. Calculation of the ilmenite–perovskite transition boundary using the measured entropies and published enthalpy data gives an equilibrium transition boundary at about 20–23 GPa at 1,000–2,000 K with a Clapeyron slope of −2.4 ± 0.4 MPa/K at 1,600 K. The calculated boundary is almost consistent within the errors with those determined by high-pressure high-temperature in situ X-ray diffraction experiments.  相似文献   

6.
We used an in situ measurement method to investigate the phase transition of CaGeO3 polymorphs under high pressures and temperatures. A multi-anvil high-pressure apparatus combined with intense synchrotron X-ray radiation was used. The transition boundary between a garnet and a perovskite phase at T = 900–1,650 K and P = 3–8 GPa was determined as occurring at P (GPa) = 9.0−0.0023 × T (K). The transition pressure determined in our study is in general agreement with that observed in previous high-pressure experiments. The slope, dP/dT, of the transition determined in our study is consistent with that calculated from calorimetry data.  相似文献   

7.
We have calculated the compressional, vibrational, and thermodynamic properties of Ni3S2 heazlewoodite and the high-pressure orthorhombic phase (with Cmcm symmetry) using the generalized gradient approximation to the density functional theory in conjunction with the quasi-harmonic approximation. The predicted Raman frequencies of heazlewoodite are in good agreement with room-temperature measurements. The calculated thermodynamic properties of heazlewoodite at room conditions agree very well with experiments, but at high temperatures (especially above 500 K) the heat capacity data from experiments are significantly larger than the quasi-harmonic results, indicating that heazlewoodite is anharmonic. On the other hand, the obtained vibrational density of states of the orthorhombic phase at 20 GPa reveals a group of low-frequency vibrational modes which are absent in heazlewoodite. These low-frequency modes contribute substantially to thermal expansivity, heat capacity, entropy, and Grüneisen parameter of the orthorhombic phase. The calculated phase boundary between heazlewoodite and the orthorhombic phase is consistent with high-pressure experiments; the predicted transition pressure is 17.9 GPa at 300 K with a negative Clapeyron slope of −8.5 MPa/K.  相似文献   

8.
Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO3 and CaSiO3 perovskites were calculated for pressures and temperatures of the Earth’s lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO3 at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO3 is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO3 perovskite completely in MgSiO3 perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO3 perovskite might completely dissolve in MgSiO3 perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO3 is increased by other major mantle constituents such as Fe and Al.  相似文献   

9.
We report the first study of electrical conductivities of silicate melts at very high pressures (up to 10 GPa) and temperatures (up to 2,173 K). Impedance spectroscopy was applied to dry and hydrous albite (NaAlSi3O8) glasses and liquids (with 0.02–5.7 wt% H2O) at 473–1,773 K and 0.9–1.8 GPa in a piston-cylinder apparatus, using a coaxial cylindrical setup. Measurements were also taken at 473–2,173 K and 6–10 GPa in two multianvil presses, using simple plate geometry. The electrical conductivity of albite melts is found to increase with temperature and water content but to decrease with pressure. However, at 6 GPa, conductivity increases rapidly with temperature above 1,773 K, so that at temperatures beyond 2,200 K, conductivity may actually increase with pressure. Moreover, the effect of water in enhancing conductivity appears to be more pronounced at 6 GPa than at 1.8 GPa. These observations suggest that smaller fractions of partial melt than previously assumed may be sufficient to explain anomalously high conductivities, such as in the asthenosphere. For dry melt at 1.8 GPa, the activation energy at T > 1,073 K is higher than that at T < 1,073 K, and the inflection point coincides with the rheological glass transition. Upon heating at 6–10 GPa, dry albite glass often shows a conductivity depression starting from ~1,173 K (due to crystallization), followed by rapid conductivity enhancement when temperature approaches the albite liquidus. For hydrous melts at 0.9–1.8 GPa, the activation energies for conductivity at ≥1,373 K are lower than those at <973 K, with a complex transition pattern in between. Electrical conductivity and previously reported Na diffusivity in albite melt are consistent with the Nernst–Einstein relation, suggesting the dominance of Na transport for electrical conduction in albite melts.  相似文献   

10.
High-pressure phase transitions of CaRhO3 perovskite were examined at pressures of 6–27 GPa and temperatures of 1,000–1,930°C, using a multi-anvil apparatus. The results indicate that CaRhO3 perovskite successively transforms to two new high-pressure phases with increasing pressure. Rietveld analysis of powder X-ray diffraction data indicated that, in the two new phases, the phase stable at higher pressure possesses the CaIrO3-type post-perovskite structure (space group Cmcm) with lattice parameters: a = 3.1013(1) Å, b = 9.8555(2) Å, c = 7.2643(1) Å, V m  = 33.43(1) cm3/mol. The Rietveld analysis also indicated that CaRhO3 perovskite has the GdFeO3-type structure (space group Pnma) with lattice parameters: a = 5.5631(1) Å, b = 7.6308(1) Å, c = 5.3267(1) Å, V m  = 34.04(1) cm3/mol. The third phase stable in the intermediate P, T conditions between perovskite and post-perovskite has monoclinic symmetry with the cell parameters: a = 12.490(3) Å, b = 3.1233(3) Å, c = 8.8630(7) Å, β = 103.96(1)°, V m  = 33.66(1) cm3/mol (Z = 6). Molar volume changes from perovskite to the intermediate phase and from the intermediate phase to post-perovskite are –1.1 and –0.7%, respectively. The equilibrium phase relations determined indicate that the boundary slopes are large positive values: 29 ± 2 MPa/K for the perovskite—intermediate phase transition and 62 ± 6 MPa/K for the intermediate phase—post-perovskite transition. The structural features of the CaRhO3 intermediate phase suggest that the phase has edge-sharing RhO6 octahedra and may have an intermediate structure between perovskite and post-perovskite.  相似文献   

11.
Stabilities of hexagonal new aluminous (NAL) phase and Ca-ferrite-type (CF) phase were investigated on the join NaAlSiO4-MgAl2O4 in a pressure range from 23 to 58 GPa at approximately constant temperature of 1,850 K, on the basis of in situ synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell. The results show that NAL is formed as a single phase up to 34 GPa, NAL + CF between 34 and 43 GPa, and only CF at higher pressures in 40%NaAlSiO4-60%MgAl2O4 bulk composition. On the other hand, both NAL and CF coexist below 38 and 36 GPa, and only CF was obtained at higher pressures in 60%NaAlSiO4-40%MgAl2O4 and 20%NaAlSiO4-80%MgAl2O4 composition, respectively. These results indicate that NAL appears only up to 46 GPa at 1,850 K, and CF forms continuous solid solution at higher pressures on the join NaAlSiO4-MgAl2O4. NAL has limited stability in subducted mid-oceanic ridge basalt crust in the Earth’s lower mantle and undergoes a phase transition to CF in deeper levels.  相似文献   

12.
KAlSi3O8 sanidine dissociates into a mixture of K2Si4O9 wadeite, Al2SiO5 kyanite and SiO2 coesite, which further recombine into KAlSi3O8 hollandite with increasing pressure. Enthalpies of KAlSi3O8 sanidine and hollandite, K2Si4O9 wadeite and Al2SiO5 kyanite were measured by high-temperature solution calorimetry. Using the data, enthalpies of transitions at 298 K were obtained as 65.1 ± 7.4 kJ mol–1 for sanidine wadeite + kyanite + coesite and 99.3 ± 3.6 kJ mol–1 for wadeite + kyanite + coesite hollandite. The isobaric heat capacity of KAlSi3O8 hollandite was measured at 160–700 K by differential scanning calorimetry, and was also calculated using the Kieffer model. Combination of both the results yielded a heat-capacity equation of KAlSi3O8 hollandite above 298 K as Cp=3.896 × 102–1.823 × 103T–0.5–1.293 × 107T–2+1.631 × 109T–3 (Cp in J mol–1 K–1, T in K). The equilibrium transition boundaries were calculated using these new data on the transition enthalpies and heat capacity. The calculated transition boundaries are in general agreement with the phase relations experimentally determined previously. The calculated boundary for wadeite + kyanite + coesite hollandite intersects with the coesite–stishovite transition boundary, resulting in a stability field of the assemblage of wadeite + kyanite + stishovite below about 1273 K at about 8 GPa. Some phase–equilibrium experiments in the present study confirmed that sanidine transforms directly to wadeite + kyanite + coesite at 1373 K at about 6.3 GPa, without an intervening stability field of KAlSiO4 kalsilite + coesite which was previously suggested. The transition boundaries in KAlSi3O8 determined in this study put some constraints on the stability range of KAlSi3O8 hollandite in the mantle and that of sanidine inclusions in kimberlitic diamonds.  相似文献   

13.
We have used density functional theory to investigate the stability of MgAl2O4 polymorphs under pressure. Our results can reasonably explain the transition sequence of MgAl2O4 polymorphs observed in previous experiments. The spinel phase (stable at ambient conditions) dissociates into periclase and corundum at 14 GPa. With increasing pressure, a phase change from the two oxides to a calcium-ferrite phase occurs, and finally transforms to a calcium-titanate phase at 68 GPa. The calcium-titanate phase is stable up to at least 150 GPa, and we did not observe a stability field for a hexagonal phase or periclase + Rh2O3(II)-type Al2O3. The bulk moduli of the phases calculated in this study are in good agreement with those measured in high-pressure experiments. Our results differ from those of a previous study using similar methods. We attribute this inconsistency to an incomplete optimization of a cell shape and ionic positions at high pressures in the previous calculations.  相似文献   

14.
A single crystal X-ray diffraction study on lithium tetraborate Li2B4O7 (diomignite, space group I41 cd) has been performed under pressure up to 8.3 GPa. No phase transitions were found in the pressure range investigated, and hence the pressure evolution of the unit-cell volume of the I41 cd structure has been described using a third-order Birch–Murnaghan equation of state (BM-EoS) with the following parameters: V 0  = 923.21(6) Å3, K 0  = 45.6(6) GPa, and K′ = 7.3(3). A linearized BM-EoS was fitted to the axial compressibilities resulting in the following parameters a 0  = 9.4747(3) Å, K 0a  = 73.3(9) GPa, K′ a  = 5.1(3) and c 0  = 10.2838(4) Å, K 0c  = 24.6(3) GPa, K′ c  = 7.5(2) for the a and c axes, respectively. The elastic anisotropy of Li2B4O7 is very large with the zero-pressure compressibility ratio β 0c /β 0a  = 3.0(1). The large elastic anisotropy is consistent with the crystal structure: A three-dimensional arrangement of relatively rigid tetraborate groups [B4O7]2− forms channels occupied by lithium along the polar c–axis, and hence compression along the c axis requires the shrinkage of the lithium channels, whereas compression in the a direction depends mainly on the contraction of the most rigid [B4O7]2− units. Finally, the isothermal bulk modulus obtained in this work is in general agreement with that derived from ultrasonic (Adachi et al. in Proceedings-IEEE Ultrasonic Symposium, 228–232, 1985; Shorrocks et al. in Proceedings-IEEE Ultrasonic Symposium, 337–340, 1981) and Brillouin scattering measurements (Takagi et al. in Ferroelectrics, 137:337–342, 1992).  相似文献   

15.
We present results from low-temperature heat capacity measurements of spinels along the solid solution between MgAl2O4 and MgCr2O4. The data also include new low-temperature heat capacity measurements for MgAl2O4 spinel. Heat capacities were measured between 1.5 and 300 K, and thermochemical functions were derived from the results. No heat capacity anomaly was observed for MgAl2O4 spinel; however, we observe a low-temperature heat capacity anomaly for Cr-bearing spinels at temperatures below 15 K. From our data we calculate standard entropies (298.15 K) for Mg(Cr,Al)2O4 spinels. We suggest a standard entropy for MgAl2O4 of 80.9 ± 0.6 J mol−1 K−1. For the solid solution between MgAl2O4 and MgCr2O4, we observe a linear increase of the standard entropies from 80.9 J mol−1 K−1 for MgAl2O4 to 118.3 J mol−1 K−1 for MgCr2O4.  相似文献   

16.
The viscosity of a silicate melt of composition NaAlSi2O6 was measured at pressures from 1.6 to 5.5 GPa and at temperatures from 1,350 to 1,880°C. We employed in situ falling sphere viscometry using X-ray radiography. We found that the viscosity of the NaAlSi2O6 melt decreased with increasing pressure up to 2 GPa. The pressure dependence of viscosity is diminished above 2 GPa. By using the relationship between the logarithm of viscosity and the reciprocal temperature, the activation energies for viscous flow were calculated to be 3.7 ± 0.4 × 102 and 3.7 ± 0.5 × 102 kJ/mol at 2.2 and 2.9 GPa, respectively.  相似文献   

17.
The electrical conductivity of (Mg0.93Fe0.07)SiO3 ilmenite was measured at temperatures of 500–1,200 K and pressures of 25–35 GPa in a Kawai-type multi-anvil apparatus equipped with sintered diamond anvils. In order to verify the reliability of this study, the electrical conductivity of (Mg0.93Fe0.07)SiO3 perovskite was also measured at temperatures of 500–1,400 K and pressures of 30–35 GPa. The pressure calibration was carried out using in situ X-ray diffraction of MgO as pressure marker. The oxidation conditions of the samples were controlled by the Fe disk. The activation energy at zero pressure and activation volume for ilmenite are 0.82(6) eV and −1.5(2) cm3/mol, respectively. Those for perovskite were 0.5(1) eV and −0.4(4) cm3/mol, respectively, which are in agreement with the experimental results reported previously. It is concluded that ilmenite conductivity has a large pressure dependence in the investigated P–T range.  相似文献   

18.
The stability and high-pressure behavior of perovskite structure in MnGeO3 and CdGeO3 were examined on the basis of in situ synchrotron X-ray diffraction measurements at high pressure and temperature in a laser-heated diamond-anvil cell. Results demonstrate that the structural distortion of orthorhombic MnGeO3 perovskite is enhanced with increasing pressure and it undergoes phase transition to a CaIrO3-type post-perovskite structure above 60 GPa at 1,800 K. A molar volume of the post-perovskite phase is smaller by 1.6% than that of perovskite at equivalent pressure. In contrast, the structure of CdGeO3 perovskite becomes less distorted from the ideal cubic perovskite structure with increasing pressure, and it is stable even at 110 GPa and 2,000 K. These results suggest that the phase transition to post-perovskite is induced by a large distortion of perovskite structure with increasing pressure.  相似文献   

19.
The low-temperature isobaric heat capacities (C p) of β- and γ-Mg2SiO4 were measured at the range of 1.8–304.7 K with a thermal relaxation method using the Physical Property Measurement System. The obtained standard entropies (S°298) of β- and γ-Mg2SiO4 are 86.4 ± 0.4 and 82.7 ± 0.5 J/mol K, respectively. Enthalpies of transitions among α-, β- and γ-Mg2SiO4 were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the α−β and β−γ transitions at 298 K (ΔH°298) in Mg2SiO4 are 27.2 ± 3.6 and 12.9 ± 3.3 kJ/mol, respectively. Calculated α−β and β−γ transition boundaries were generally consistent with those determined by high-pressure experiments within the errors. Combining the measured ΔH°298 and ΔS°298 with selected data of in situ X-ray diffraction experiments at high pressure, the ΔH°298 and ΔS°298 of the α−β and β−γ transitions were optimized. Calculation using the optimized data tightly constrained the α−β and β−γ transition boundaries in the P, T space. The slope of α−β transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of β−γ boundary 5.2 MPa/K at 18.7 GPa and 1,600 K. The post-spinel transition boundary of γ-Mg2SiO4 to MgSiO3 perovskite plus MgO was also calculated, using the optimized data on γ-Mg2SiO4 and available enthalpy and entropy data on MgSiO3 perovskite and MgO. The calculated post-spinel boundary with a Clapeyron slope of −2.6 ± 0.2 MPa/K is located at pressure consistent with the 660 km discontinuity, considering the error of the thermodynamic data.  相似文献   

20.
 Synthetic aegirine LiFeSi2O6 and NaFeSi2O6 were characterized using infrared spectroscopy in the frequency range 50–2000 cm−1, and at temperatures between 20 and 300 K. For the C2/c phase of LiFeSi2O6, 25 of the 27 predicted infrared bands and 26 of 30 predicted Raman bands are recorded at room temperature. NaFeSi2O6 (with symmetry C2/c) shows 25 infrared and 26 Raman bands. On cooling, the C2/c–P21/c structural phase transition of LiFeSi2O6 is characterized by the appearance of 13 additional recorded peaks. This observation indicates the enlargement of the unit cell at the transition point. The appearance of an extra band near 688 cm−1 in the monoclinic P21/c phase, which is due to the Si–O–Si vibration in the Si2O6 chains, indicates that there are two non-equivalent Si sites with different Si–O bond lengths. Most significant spectral changes appear in the far-infrared region, where Li–O and Fe–O vibrations are mainly located. Infrared bands between 300 and 330 cm−1 show unusually dramatic changes at temperatures far below the transition. Compared with the infrared data of NaFeSi2O6 measured at low temperatures, the change in LiFeSi2O6 is interpreted as the consequence of mode crossing in the frequency region. A generalized Landau theory was used to analyze the order parameter of the C2/c–P21/c phase transition, and the results suggest that the transition is close to tricritical. Received: 21 January 2002 / Accepted: 22 July 2002  相似文献   

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