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1.
A high-pressure single-crystal x-ray diffraction study of perovskite-type MgSiO3 has been completed to 12.6 GPa. The compressibility of MgSiO3 perovskite is anisotropic with b approximately 23% less compressible than a or c which have similar compressibilities. The observed unit cell compression gives a bulk modulus of 254 GPa using a Birch-Murnaghan equation of state with K set equal to 4 and V/V 0 at room pressure equal to one. Between room pressure and 5 GPa, the primary response of the structure to pressure is compression of the Mg-O and Si-O bonds. Above 5 GPa, the SiO6 octahedra tilt, particularly in the [bc]-plane. The distortion of the MgO12 site increases under compression. The variation of the O(2)-O(2)-O(2) angles and bondlength distortion of the MgO12 site with pressure in MgSiO3 perovskite follow trends observed in GdFeO3type perovskites with increasing distortion. Such trends might be useful for predicting distortions in GdFeO3-type perovskites as a function of pressure.  相似文献   

2.
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   

3.
High-temperature X-ray diffraction studies of SrZrO3 and BaZrO3 perovskites have been carried out to 1200° C. The diffraction patterns are analyzed with Rietveld method so as to refine the unit cell dimensions. The volumetric thermal expansion coefficient are observed to be 2.98*10-5K-1 for orthorhombic Pbnm phase, 3.24*10-5K-1 for orthorhombic Cmcm phase, 3.75*10-5K-1 for tetragonal I4/mcm phase of SrZrO3 perovskite, and 2.06*10-5K-1 for cubic Pm3m phase of BaZrO3 perovskite, respectively. The linear thermal expansion coefficients of SrZrO3 perovskite show considerable anisotropy of α a >α c >α b for orthorhombic Pbnm phase, which reflect the decrease of distortion of the perovskite. It is demonstrated that thermal expansion of the centrosymmetrically distorted ABX3 perovskite can be empirically expressed as a combination of the changes of [B-X] bond length and tilting angle of BX6 octahedral framework. The octahedral tilting is considered to be the primary order parameter for the ferroelastic type of structural phase transitions in perovskite. Thermodynamically, the tilting induced volume change denotes the “excess volume” and the corresponding thermal expansion represents the “excess thermal expansion” for the lower symmetry phase with respect to its prototype of the cubic perovskite.  相似文献   

4.
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5.
Elasticity of CaTiO3, SrTiO3 and BaTiO3 perovskites has been experimentally investigated as a function of pressure up to 3.0 GPa in a liquid-medium piston cylinder apparatus using a high precision ultrasonic interferometric technique. Specimens used are hot-pressed fine-grained (3–10 μm) polycrystalline aggregates with low porosity (<1.5%). Compressional and shear wave velocities and their pressure derivatives have been measured. The results are compared with previous studies on other perovskites and the role of structural transitions is examined. We find that the role of Ti-O6 polyhedral tilting (such as observed in CaTiO3) is small in the sense that a single well-defined general trend exists in perovskites with a wide range of tilting angles, although there is suggestion that cubic perovskites have slightly higher bulk modulus than orthorhombic perovskites. In contrast, cation-anion displacement that changes crystal symmetry from cubic to tetragonal in BaTiO3 has very large effects on elasticity. This distortion significantly reduces the bulk modulus (but not much the shear modulus) and results in an unusually large pressure derivative of bulk modulus (dK/dP～10). A large change in elasticity in BaTiO3 associated with the structural transition (without a significant volume change) is a clear example of the breakdown of the Birch's law between densities and elastic wave velocities.  相似文献   

6.
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   

7.
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   

8.
An exploratory high-pressure study of the join CaTiO3-FeTiO3 has uncovered two intermediate perovskites with the compositions CaFe3Ti4O12 and CaFeTi2O6. These perovskites have ordering of Ca2+ and Fe2+ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe2+ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe3Ti4O12, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn7O12. Fe2+ is in a square-planar A site (similar to Mn3+ in NaMn7O12) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi2O6 crystallizes in a unique tetragonal double perovskite structure (SG P42/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO3 recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe3Ti4O12 is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.  相似文献   

9.
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   

10.
Using density functional simulations, within the generalized gradient approximation and projector-augmented wave method, we study structures and energetics of CaSiO3 perovskite in the pressure range of the Earths lower mantle (0–150 GPa). At zero Kelvin temperature the cubic CaSiO3 perovskite structure is unstable in the whole pressure range, at low pressures the orthorhombic (Pnam) structure is preferred. At 14.2 GPa there is a phase transition to the tetragonal (I4/mcm) phase. The CaIrO3-type structure is not stable for CaSiO3. Our results also rule out the possibility of decomposition into oxides.
Daniel Y. JungEmail: Phone: +41-44-6323744Fax: +41-44-6321133
  相似文献   

11.
The elastic properties of CaSnO3 perovskite have been measured by both ultrasonic interferometry and single-crystal X-ray diffraction at high pressures. The single-crystal diffraction data collected using a diamond-anvil cell show that CaSnO3 perovskite does not undergo any phase transitions at pressures below 8.5?GPa at room temperature. Ultrasonic measurements in the multianvil press to a maximum pressure of ～8?GPa at room temperature yielded S- and P-wave velocity data as a function of pressure. For a third-order Birch-Murnaghan EoS the adiabatic elastic moduli and their pressure derivatives determined from these velocity data are K S0=167.2±3.1?GPa, K ′ S0=4.89±0.17, G 0=89.3±1.0?GPa, G ′ 0=0.90±0.02. The quoted uncertainties include contributions from uncertainties in both the room pressure length and density of the specimen, as well as uncertainties in the pressure calibration of the multianvil press. Because the sample is a polycrystalline specimen, this value of K S0 represents an upper limit to the Reuss bound (conditions of uniform stress) on the elastic modulus of CaSnO3 perovskite. If the value of αγT is assumed to be 0.01, the value of K S0 corresponds to K T0=165.5±3.1?GPa. The 10 P-V data obtained by single-crystal diffraction were fit with a third-order Birch–Murnaghan equation-of-state to obtain the parameters V 0=246.059±0.013 Å3, K T0=162.6±1.0?GPa, K ′ T0=5.6±0.3. Because single-crystal measurements under hydrostatic conditions are made under conditions of uniform stress, they yield bulk moduli equivalent to the Reuss bound on a polycrystalline specimen. The results from the X-ray and ultrasonic experiments are therefore consistent. The bulk modulus of CaSnO3 perovskite lies above the linear trend of K 0 with inverse molar volume, previously determined for Ca perovskites. This prevents an estimation of the bulk modulus of CaSiO3 perovskite by extrapolation. However, our value of G 0 for CaSnO3 perovskite combined with values for CaTiO3 and CaGeO3 forms a linear trend of G 0 with octahedral tilt angle. This allows a lower bound of 150?GPa to be placed on the shear modulus of CaSiO3 by extrapolation.  相似文献   

12.
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   

13.
We have obtained Raman spectra for a number of orthorhombic perovskites CaBO3, where B=Ti, Ge, Zr or Sn. The room temperature Raman spectrum of CaTiO3 was compared with cubic SrTiO3 to assign first- and second-order features. Partially polarized micro-Raman spectra were obtained for CaTiO3 perovskite. The CaBO3 perovskites showed a sequence of increasing complexity in their Raman spectra with increasing degree of orthorhombic distortion from the ideal cubic structure. The spectral changes cannot easily be correlated with changes in chemistry or structure of the perovskite. High temperature micro-Raman spectra for CaGeO3 perovskite were obtained by laser-heating a 15 μm sample. None of the low frequency Raman modes were soft, but showed only normal anharmonicity up to approximately 700 K.  相似文献   

14.
Polycrystalline specimens in the CaTiO3–CaSiO3 perovskite system have been hot-pressed in a 2000-ton uniaxial split-sphere apparatus (USSA-2000) at pressures up to 15 GPa and temperature of 1550°C, for the compositions CaTiO3, Ca(Ti0.75Si0.25)O3, Ca(Ti0.5Si0.5)O3. For the specimens with the bulk densities within 1% of the X-ray density, compressional and shear wave velocity measurements have been conducted using ultrasonic interferometry. The measured adiabatic bulk moduli (K s ) for the CaTiO3 and Ca(Ti0.5Si0.5)O3 perovskites are 175(1) and 188(1) GPa and shear moduli (G) of 106(1) and 109(1) GPa. In situ X-ray diffraction studies at high pressure and temperature resulted in isothermal values for K 0 of 170(5) and 185(5) GPa, respectively. For the unquenchable CaSiO3 perovskite, elasticity theory and systematics were used to predict K 0=212(7) GPa and G 0=112(5) GPa; this shear modulus is 37% less than that for (Mg,Fe)SiO3 perovskite, suggesting that CaSiO3 perovskite cannot be ignored in modeling the composition of the Earth’s lower mantle. Received: 27 June 1997 / Revised, accepted: 25 November 1997  相似文献   

15.
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   

16.
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   

17.
Far-infrared absorbance spectra were collected from CaGeO3 with a metastable orthorhombic perovskite structure from 0 to 24.4 GPa. The absorbance data are compatible with a reflectance spectrum which was collected at ambient conditions from a polished, densely compacted polycrystal. The reflectance spectrum shows 18 IR modes from 155 to 786 cm?1. A detailed model for the density of states constructed from these new data results in accurate calculation of heat capacity and new data on entropy. Peak positions increase linearly with pressure. Mode Grüneisen parameters (ranging from 0.72–1.56) decrease almost linearly with increasing mode frequency which is consistent with deformations of the oxygen sublattice dominating the lattice vibrations. Neither discontinuous changes in the number of modes nor in these frequencies nor in band widths are observed at pressures up to 24.4 GPa. Thus, conversion to the tetragonal phase at ～12 GPa is not indicated.  相似文献   

18.
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   

19.
In order to confirm the possible existence of FeGeO3 perovskite, we have performed in situ X-ray diffraction measurements of FeGeO3 clinopyroxene at pressures up to 40 GPa at room temperature. The transition of FeGeO3 clinopyroxene into orthorhombic perovskite is observed at about 33GPa. The cell parameters of FeGeO3 perovskite are a=4.93(2) Å, b=5.06(6) Å, c=6.66(3) Å and V=166(3) Å3 at 40 GPa. On release of pressure, the perovskite phase transformed into lithium niobate structure. The previously reported decomposition process of clino-pyroxene into Fe2GeO4 (spinel)+GeO2 (rutile) or FeO (wüstite) +GeO2 (rutile) was not observed. This shows that the transition of pyroxene to perovskite is kinetically accessible compared to the decomposition processes under low-temperature pressurization.  相似文献   

20.
High pressure perovskites in the system CaTiO3?CaSiO3 in the composition range from 0 to 50% CaSiO3 have been re-examined using powder x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. In runs performed at 9 GPa and 1200 °C, a solid solution is obtained with compositions ranging from CaTiO3 to near Ca(Ti0.65Si0.35)O3, and with the orthorhombic distortion in the CaTiO3 end-member diminishing to pseudocubic for Ca(Ti0.65Si0.35)O3. Raman spectra show intensity changes and band broadening which are attributed to the reduction of orthorhombic character, but still indicate lower than cubic symmetry for the entire solid solution range. An ordered intermediate, Ca2TiSiO6, has been recovered from a run at 14 GPa at 1200 °C. The space group is Fm3m, with a=7.410(2)?Å, and the compound has the double perovskite structure. In this structure, Si-rich octahedral sites and Ti-rich octahedral sites alternate along the three principal axes. Transmission electron microscopy confirms the presence of the Fm3m ordered structure, and also documents the presence of lesser amounts of at least three other ordered perovskite structures which are not resolved in the x-ray data. These may account for extra features observed in the Raman spectrum which are inconsistent with mode assignments for the Fm3m phase. Non face-centered ordering schemes for these regions are suggested based on modeling of the HRTEM patterns.  相似文献   

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