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1.
We have investigated the evolution of the distortion of several oxide perovskites with increasing pressure, using EXAFS in the diamond anvil cell. Cubic perovskite BaZrO3 remains cubic up to 52 GPa. Orthorhombic perovskite CaGeO3 becomes less distorted as pressure increases, becomes tetragonal at about 12 GPa and evolves toward cubic structure, still not obtained at 23 GPa. The distortion of orthorhombic perovskite SrZrO3 first increases with pressure up to 8 GPa, then decreases until the perovskite becomes cubic at 25 GPa. The results are interpreted in terms of a systematics, relating the distortion to the ratio f of the volumes of the AO12 dodecahedron and the BO6 octahedron, and to the compressibilities of the polyhedra. For cubic perovskites, f=5, which may correspond to a situation where the compressibilities of octahedra and dodecahedra are equal.The behavior of SrZrO3 offers a clue to predict the evolution of the distortion of MgSiO3 at lower mantle pressures. It is suggested that the increase in distortion experimentally observed at lower pressures should stop above about 10 GPa, and the distortion decrease until the perovskite undergoes ferroelastic transitions to tetragonal and cubic phases, at pressures possibly below the pressure at the core-mantle boundary.  相似文献   

2.
New high-pressure orthorhombic (GdFeO3-type) perovskite polymorphs of MnSnO3 and FeTiO3 have been observed using in situ powder X-ray diffraction in a diamond-anvil cell with synchrotron radiation. The materials are produced by the compression of the lithium niobate polymorphs of MnSnO3 and FeTiO3 at room temperature. The lithium niobate to perovskite transition occurs reversibly at 7 GPa in MnSnO3, with a volume change of -1.5%, and at 16 GPa in FeTiO3, with a volume change of -2.8%. Both transitions show hysteresis at room temperature. For MnSnO3 perovskite at 7.35 (8) GPa, the orthorhombic cell parameters are a=5.301 (2) A, b=5.445 (2) Å, c=7.690 (8) Å and V= 221.99 (15) Å3. Volume compression data were collected between 7 and 20 GPa. The bulk modulus calculated from the compression data is 257 (18) GPa in this pressure region. For FeTiO3 perovskite at 18.0 (5) GPa, cell parameters are a=5.022 (6) Å, b=5.169 (5) Å, c=7.239 (9) Å and V= 187.94 (36) Å3. Based on published data on the quench phases, the FeTiO3 perovskite breaks down to a rocksalt + baddelyite mixture of FeO and TiO2 at 23 GPa. This is the first experimental verification of the pressure-induced breakdown of a perovskite to simple oxides.  相似文献   

3.
The adiabatic elastic moduli of a single crystal of Neighborite (NaMgF 3 perovskite) have been measured at ambient conditions using Brillouin spectroscopy. The adiabatic aggregate (Voight-Reuss-Hill) bulk modulus is K = 75.6 GPa, and shear modulus is = 46.7 GPa. The experimental results show the ratio of linear compressibilities b / a = 0.80 for neighborite. These ratios reflect the different amounts of tilting freedom of the octahedral framework along each lattice axis of the perovskite structure. It is understood that the elastic compliance S ij of the crystal can directly sense the behavior of the octahedral tilting in the structural distortion of NaMgF3 perovskite. The octahedral tilting angles are considered to be the order parameters of the ferroelastic phase transition in the perovskite structure. Single crystal elasticity data provide a basis for understanding the role of octahedral tilting in the ferroelasticity of perovskite. Together with high pressure compressional data, one can thus elucidate the relationship between crystal structure and physical properties of perovskite. A detailed assessment indicates that the dominant compression mechanism for NaMgF3 perovskite is shortening of the octahedral [MgF] bond, which is also true for orthorhombically distorted MgSiO3 perovskite.  相似文献   

4.
The effect of pressure (up to 21 GPa at room temperature) and temperature (up to 1570 K at room pressure) on the Raman spectrum of CaTiO3 is presented. No significant changes, which could be attributed to a major structural change, are observed in the spectra up to 22 GPa. The pressure shifts of the Raman modes can be related to a significant compression of the Ti-O bond. Discontinuous changes in the spectra upon heating may be related to phase changes observed by calorimetry and X-ray diffraction. The important temperature shifts of some low-frequency modes can be related to an increase in the Ti-O-Ti angle in agreement with the X-ray data showing a decrease of the structural distortion with increasing temperature. These data are compared to those available for MgSiO3-perovskite and show that CaTiO3 is a good structural analogue for MgSiO3-perovskite. The present spectroscopic data are used to calculate the specific heat and entropy of CaTiO3. The role of the low frequency modes in the calculations is emphasized. Good agreement is observed between calculated and experimentally determined values in the 0–1300 K temperature range. A similarly defined model is proposed for MgSiO3-perovskite. It is found that the entropy lies between 57 and 64 J/mol/K at 298 K and between 190 and 200 J/mol/K at 1000 K in agreement with the values inferred from experimental equilibrium data. Finally we briefly discuss the values of the Grüneisen parameters of both perovskites inferred from macroscopic and microscopic data.  相似文献   

5.
Experiments using laser-heated diamond anvil cells combined with synchrotron X-ray diffraction and SEM–EDS chemical analyses have confirmed the existence of a complete solid solution in the MgSiO3–MnSiO3 perovskite system at high pressure and high temperature. The (Mg, Mn)SiO3 perovskite produced is orthorhombic, and a linear relationship between the unit cell parameters of this perovskite and the proportion of MnSiO3 components incorporated seems to obey Vegard’s rule at about 50 GPa. The orthorhombic distortion, judged from the axial ratios of a/b and \( \sqrt{2}\,a/c, \) monotonically decreases from MgSiO3 to MnSiO3 perovskite at about 50 GPa. The orthorhombic distortion in (Mg0.5, Mn0.5)SiO3 perovskite is almost unchanged with increasing pressure from 30 to 50 GPa. On the other hand, that distortion in (Mg0.9, Mn0.1)SiO3 perovskite increases with pressure. (Mg, Mn)SiO3 perovskite incorporating less than 10 mol% of MnSiO3 component is quenchable. A value of the bulk modulus of 256(2) GPa with a fixed first pressure derivative of four is obtained for (Mg0.9, Mn0.1)SiO3. MnSiO3 is the first chemical component confirmed to form a complete solid solution with MgSiO3 perovskite at the P–T conditions present in the lower mantle.  相似文献   

6.
The relative stability of MgSiO3-ilmenite, MgSiO3-perovskite and (periclase+stishovite) assemblage phases as a function of the pressure is investigated with the periodic quantum mechanical ab initio HartreeFock program CRYSTAL. For the first time, the structure of MgSiO3-ilmenite is fully optimized. Basis set effects are explored. It turns out that relatively small basis sets reproduce correctly experimental geometries. However, larger basis sets (triple zeta quality, plus polarization d functions) are needed to yield significant thermochemical results. All contributions to the 0 K enthalpy are discussed. On the basis of the present highest level calculations, it appears that in the explored range of pressure (0P< 60=" gpa)=" the=" mineralogical=" assemblage=" periclase+stishovite=" has=" higher=" enthalpy=" than=">3-ilmenite or perovskite, and that ilmenite transforms to orthorhombic perovskite around to 29.4 GPa in good agreement with experimental data extrapolated down to 0 K.  相似文献   

7.
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   

8.
The structural changes of CaSnO3, a GdFeO3-type perovskite, have been investigated to 7 GPa in a diamond-anvil cell at room temperature using single-crystal X-ray diffraction. Significant changes are observed in both the octahedral Sn–O bond lengths and tilt angles between the SnO6 octahedra. The octahedral (SnO6) site shows anisotropic compression and consequently the distortion of SnO6 increases with pressure. Increased pressure also results in a decrease of both of the inter-octahedral angles, Sn–O1–Sn and Sn–O2–Sn, indicating that octahedral tilting increases with increasing pressure, chiefly equivalent to rotation of the SnO6 octahedra about the pseudocubic <001>p axis. The distortion in the CaO12 and SnO6 sites, along with the octahedral SnO6 tilting, is attributed to the SnO6 site being less compressible than the CaO12 site.Acknowledgments The authors acknowledge with gratitude the financial support for this work from NSF grant EAR-0105864. Ruby pressure measurements were conducted with the Raman system in the Vibrational Spectroscopy Laboratory in the Department of Geosciences at Virginia Tech with the help of Mr. Charles Farley.  相似文献   

9.
The pressure derivatives of elastic moduli (∂M/∂P; M=KS and G) for a suite of polycrystalline oxide perovskites (2 titanates, 1 stannate and 2 aluminates) have been measured up to 3 GPa using the ultrasonic interferometry method combined with a buffer rod technique. Two empirical systematic relationships (∂G/∂P vs KS/G and ∂KS/∂P vs KS (/ρ)1/3) have been used to investigate the elasticity systematics of this suite of perovskites and to estimate ∂M/∂P of MgSiO3 perovskite. The pressure derivatives ∂G/∂P and ∂KS/∂P for this suite of perovskites scatter between well-defined linear trends for the rutile, rocksalt and spinel structures. The more diffuse trends observed for the perovskites might reflect greater flexibility in the response of its corner-connected octahedral framework structure to changing pressure. The pressure derivatives of the elastic moduli for MgSiO3 perovskite estimated by the “perovskite bands” are ∂G/∂P=1.6–2.2 and ∂KS/∂P=3.9–4.2. Received: 13 November 1997 / Revised, accepted: 31 August 1998  相似文献   

10.
The stable polymorph of MnTiO3 at room temperature and pressure has the ilmenite structure. At high temperatures and pressures, MnTiO3 ilmenite transforms to a LiNbO3 structure through a cation reordering process (Ko and Prewitt 1988). Single crystals of both phases have been studied with X-ray diffraction to 5.0 GPa. We have obtained the first experimental verification of the close relationship between the LiNbO3 and perovskite structures, first postulated by Megaw (1968). MnTiO3 with the LiNbO3 structure (MnTiO3 II) transforms directly to an orthorhombic perovskite structure (MnTiO3 III) between 2.0 and 3.0 GPa. The transition involves a change of volume of -5%, is reversible and has pronounced hysteresis. Only pressure is required to drive the transition because it involves no breaking of bonds; it simply involves rotation of the [TiO6] octahedra about their triad axes accompanied by displacement of the Mn cations to the distorted twelve-coordinated sites formed by the rotations. An unusual aspect of this transition is that twinned MnTiO3 II crystals transform to untwinned MnTiO3 III crystals with increasing pressure. The twin plane of MnTiO3 II, , corresponds to the (001) mirror plane of the orthorhombic perovskite structure. MnTiO3 III examined at 4.5 GPa is very distorted from the ideal cubic perovskite structure. The O(2)-O(2)-O(2) angle describing the tilting in the ab plane is 133.3(7)°, in contrast to 180° for a cubic perovskite and the O(2)-O(2)-O(2) angle describing the tilting in the ac plane is 109.3(4)°, as opposed to 90° in a cubic perovskite.  相似文献   

11.
Synthetic clinoenstatite (MgSiO3) has been converted to a single phase with the perovskite structure in complete reactions at approx. 300 kbar in experiments that utilize the laser-heated diamond-anvil pressure apparatus. The structure of this phase after quenching was determined by powder X-ray diffraction intensity measurement to be similar to that of the distorted rare-earth, orthoferrite-type, orthorhombic perovskites, but it is suggested that such distortion from ideal cubic perovskite would diminish at high pressure. The unit cell dimensions and density of perovskite-type MgSiO3 at ambient conditions (1 bar, 25°C) are a=4.780(1) Å, b=4.933(1) Å, c=6.902(1) Å, V=162.75 Å3, and ρ=4.098(1) g/cm3. This phase is 3.1% denser than the isochemical oxide mixture [periclase (MgO)+stishovite (SiO2)]. The small crystal-field stabilization energy of the cation site in the perovskite structure may play an important role in limiting the high-pressure stability field of perovskites that contain transition metal cations. Approximate calculations of the crystal-field effects indicate that perovskite of pure FeSiO3 composition may become stable at 400–600 kbar; pressures greater than 800 kbar would be required to stabilize CoSiO3 or NiSiO3 perovskite.  相似文献   

12.
Single crystal X-ray diffraction study of MgSiO3 perovskite has been completed from 77 to 400 K. The thermal expansion coefficient between 298 and 381 K is 2.2(8) × 10-5 K-1. Above 400 K, the single crystal becomes so multiply twinned that the cell parameters can no longer be determined.From 77 to 298 K, MgSiO3 perovskite has an average thermal expansion coefficient of 1.45(9) × 10-5 K-1, which is consistent with theoretical models and perovskite systematics. The thermal expansion is anisotropic; the a axis shows the most expansion in this temperature range (a = 8.4(9) × 10-6 K-1) followed by c(c = 5.9(5) × 10-6 K-1) and then by b, which shows no significant change in this temperature range. In addition, the distortion (i.e., the tilting of the [SiO6] octahedra) decreases with increasing temperature. We conclude that the behavior of MgSiO3 perovskite with temperature mirrors its behavior under compression.  相似文献   

13.
  相似文献   

14.
The stability field of Mg3Al2Si3O12-pyrope was examined for the first time under hydrostatic pressure conditions in a CO2-laser heated diamond cell in the pressure range 21–30 GPa between 2300 and 3200 K. The phases were characterized using Raman and fluorescence spectroscopy. With increasing pressure pyrope transforms to an ilmenite phase above ∼21.5 GPa, to perovskite plus ilmenite above ∼24 GPa, and to perovskite above 29 GPa. The pressures of the first occurrence of perovskite in this study are about 2 GPa above the corresponding phase boundary between end-member MgSiO3-ilmenite and perovskite. A small amount of Al2O3 coexists with perovskite up to 43 GPa, as evident from fluorescence spectra resembling those of ruby, but above 43 GPa the entire Al2O3 content of the pyrope starting material is accommodated in the perovskite structure. Received: 6 March 1997 / Revised, accepted: 23 July 1997  相似文献   

15.
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   

16.
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   

17.
Phase transitions in MgGeO3 and ZnGeO3 were examined up to 26 GPa and 2,073 K to determine ilmenite–perovskite transition boundaries. In both systems, the perovskite phases were converted to lithium niobate structure on release of pressure. The ilmenite–perovskite boundaries have negative slopes and are expressed as P(GPa)=38.4–0.0082T(K) and P(GPa)=27.4−0.0032T(K), respectively, for MgGeO3 and ZnGeO3. Enthalpies of SrGeO3 polymorphs were measured by high-temperature calorimetry. The enthalpies of SrGeO3 pseudowollasonite–walstromite and walstromite–perovskite transitions at 298 K were determined to be 6.0±8.6 and 48.9±5.8 kJ/mol, respectively. The calculated transition boundaries of SrGeO3, using the measured enthalpy data, were consistent with the boundaries determined by previous high-pressure experiments. Enthalpy of formation (ΔH f°) of SrGeO3 perovskite from the constituent oxides at 298 K was determined to be −73.6±5.6 kJ/mol by calorimetric measurements. Thermodynamic analysis of the ilmenite–perovskite transition boundaries in MgGeO3 and ZnGeO3 and the boundary of formation of SrSiO3 perovskite provided transition enthalpies that were used to estimate enthalpies of formation of the perovskites. The ΔH f° of MgGeO3, ZnGeO3 and SrSiO3 perovskites from constituent oxides were 10.2±4.5, 33.8±7.2 and −3.0±2.2 kJ/mol, respectively. The present data on enthalpies of formation of the above high-pressure perovskites were combined with published data for A2+B4+O3 perovskites stable at both atmospheric and high pressures to explore the relationship between ΔH f° and ionic radii of eightfold coordinated A2+ (R A) and sixfold coordinated B4+ (R B) cations. The results show that enthalpy of formation of A2+B4+O3 perovskite increases with decreasing R A and R B. The relationship between the enthalpy of formation and tolerance factor ( R o: O2− radius) is not straightforward; however, a linear relationship was found between the enthalpy of formation and the sum of squares of deviations of A2+ and B4+ radii from ideal sizes in the perovskite structure. A diagram showing enthalpy of formation of perovskite as a function of A2+ and B4+ radii indicates a systematic change with equienthalpy curves. These relationships of ΔH f° with R A and R B can be used to estimate enthalpies of formation of perovskites, which have not yet been synthesized.  相似文献   

18.
The crystal structures and energies of SiO2 stishovite, MgO periclase, Mg2SiO4 spinel, and MgSiO3 perovskite were calculated as a function of pressure with the polarization-included electron gas (PEG) model. The calculated pressures of the spinel to perovskite phase transitions in the Mg2SiO4 and MgSiO3 systems are 26.0 GPa and 27.0 GPa, respectively, compared to the experimental zero temperature extrapolations of 27.4 GPa and 27.7 GPa. The two oxide phases are found to be the most stable form in the pressure range 24.5 GPa to 31.5 GPa, compared to the experimental zero temperature extrapolation of 26.7 GPa to 28.0 GPa. The volume changes associated with the phase transitions are in good agreement with experiment. The transition pressures calculated with the PEG model, which allows the ions to distort from spherical symmetry, are in much better agreement with experiment than those calculated with the modified electron gas (MEG) model, which constrains the ions to be spherical.  相似文献   

19.
In situ X-ray diffraction measurements of Fe- and Al-bearing MgSiO3-rich perovskite (FeAl-Pv), which was synthesized from a natural orthopyroxene, were performed at pressures of 19–32 GPa and temperatures of 300–1,500 K using a combination of a Kawai-type apparatus with eight sintered-diamond anvils and synchrotron radiation. Two runs were performed using a high-pressure cell with two sample chambers, and both MgSiO3 perovskite (Mg-Pv) and FeAl-Pv were synthesized simultaneously in the same cell. Thus we were able to measure specific volumes (V/V 0) of Mg-Pv and FeAl-Pv at the same P−T conditions. At all the measurement conditions, values of the specific volume of FeAl-Pv are consistent with those of Mg-Pv within 2 Standard Deviation, strongly suggesting that effect of incorporation of iron and aluminum on the thermoelastic properties of magnesium silicate perovskite is undetectable in this composition, pressure, and temperature range. Two additional runs were performed using a high-pressure cell that has one sample chamber and unit-cell volumes of FeAl-Pv were measured at pressures and temperatures up to 32 GPa and 1,500 K, respectively. All the unit-cell volume data of FeAl-Pv perovskite were fitted to the high temperature Birch–Murnaghan equation of state and a complete set of thermoelastic parameters of this perovskite was determined with an assumption of K′ 300,0 = 4. The determined parameters are K 300,0 = 243(3) GPa, (∂K T,0/∂T) P = −0.030(8) GPa/K, a 0 = 2.78(18) × 10−5 K−1, and b 0 = 0.88(28) × 10−8 K−2, where a 0 and b 0 are the coefficients of the following expression describing the zero-pressure thermal expansion: α T,0 = a 0 + b 0 T. The equation-of-state parameters of FeAl-Pv are in good agreement with those of MgSiO3 perovskite at the conditions corresponding to the uppermost part of the lower mantle.  相似文献   

20.
We have determined the partitioning of a wide range of trace elements between silicate melts and CaSiO3 and MgSiO3 perovskites using both laser ablation-ICPMS and ion microprobe techniques. Our results show that, with the exception of Sc, Zr, and Hf, all trace elements we considered are incompatible in MgSiO3 perovskite, from highly incompatible for U, Th, Ba, La, Sr and monovalent elements to slightly incompatible for heavy rare earth elements. MgSiO3 perovskite-melt partition coefficients increase slightly with Al content in the perovskite. These observations contrast strongly with partitioning between CaSiO3 perovskite and silicate melts. In the latter case, all rare earth elements are clearly compatible as are U and Th. Our data also suggest that, contrary to pressure and temperature, melt composition can significantly affect CaSiO3 perovskite-melt partitioning; partition coefficients for rare earth elements and U and Th increase with decreasing CaO melt content. The presence of ∼0.4 wt% water in melt makes little difference, however. Partitioning of trace elements into the large site of both MgSiO3 and CaSiO3 perovskites follows the near-parabolic dependence on ionic radius predicted from the lattice strain model. The peaks of the parabolae are much higher for the CaSiO3 phase, perhaps suggesting that the mechanisms of charge compensation for heterovalent substitution are different in the two cases. Our partitioning data have been used to assess the potential effect of perovskite fractionation into the lower mantle during early Earth history. Crystallisation of less than 8% of a mixture of CaSiO3 and MgSiO3 perovskites could have led to a ‘layer’ enriched in U and Th without disturbing the chondritic pattern of refractory lithophile elements in the primitive upper mantle. The resultant reservoir could have high Sm/Nd, U/Pb, Sr/Rb, Lu/Hf ratios similar to the HIMU component of ocean island basalts, but would not balance the observed depletion of the primitive upper mantle in Si and Nb.  相似文献   

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