Post-perovskite Phase Research Articles

Searching for post-perovskite transition in CaSnO3 at high pressure: an ultrasonic velocity study to 18 GPa

Compressional (P) and shear (S) wave velocities of CaSnO3 perovskite were measured at room temperature using ultrasonic interferometry up to 18 GPa. Discontinuities of the magnitude suggested in previous theoretical calculations are not observed in derived velocities over the entire experimental pressure range explored here. A least squares fitting of the P and S wave velocities to third-order finite strain equations yielded zero pressure adiabatic bulk and shear moduli and their first pressure derivatives, K=166(2) GPa, K′=5.4(2), G=88(1) GPa and G′=1.2(1). These results are in agreement with previous experiments, within mutual uncertainties. Examinations of the current data as well as comparison with previous measurements at lower pressures conclude the presence of a post-perovskite phase transition in CaSnO3 is not visible in the elasticity at ambient temperature within 0–18 GPa range.

Recent advances in the study of mantle phase transitions

We review recent advances in the study of phase transitions of minerals in the Earth's mantle, including unsolved problems regarding these processes. The phase boundaries of (Mg,Fe) 2SiO 4 in the mantle transition zone are modified under the wet conditions by changes in thermodynamic properties of its polymorphs because of differences in the water solubilities of these minerals. The shift of phase boundaries has significant implications for the topography of the 410 km and 660 km seismic discontinuities. The post-perovskite phase with CaIrO 3 structure exists at pressures above 110 GPa and at high temperature. The effects of Al and Fe on phase transition are under debate. Several post-stishovite phases have been reported but equilibrium boundaries and existence in the real lower mantle are also still debatable. Spin transition occurs in magnesiowustite, perovskite, and post-perovskite phases at high pressures of the lower mantle and core. The pressure interval in the spin transition can be large at high temperature and so the transition might not cause a discrete change in physical properties under real Earth conditions.

Elastic properties of the post-perovskite phase of Fe 2O 3 and implications for ultra-low velocity zones

The properties of the post-perovskite phase of Fe 2O 3 have been investigated using both the generalized gradient approximation (GGA) and GGA + U methods. The low-spin post-perovskite phase is found to have monoclinic symmetry. While GGA and GGA + U predict very different spin transition pressures, bulk elastic properties are found to be similar. Linear mixing of the elastic properties of the post-perovskite phase of Fe 2O 3 with those previously calculated for the MgSiO 3 phase shows that the properties of ferric-iron-enriched post-perovskite are compatible with seismic constraints for ultra-low velocity zones. Agreement with observations for ultra-low velocity zones is further increased if they are presumed to be hotter than normal mantle.

Rietveld structure refinement of MgGeO3 post-perovskite phase to 1 Mbar

Using the CaIrO{sub 3}-type structure model (space group Cmcm), lattice parameters and atomic positions of the MgGeO{sub 3} post-perovskite (pPv) phase were determined based on Rietveld refinements at 78--109 GPa and first-principles calculations based on density functional theory. The reproducibility of structural parameters obtained for different samples, consistency with theoretical calculations, and good agreement with expected bond lengths based on structurally similar materials all provide evidence for both validity of CaIrO{sub 3}-type structure model for the pPv phase in MgGeO{sub 3} exceeding 1 Mbar and reliability of structural parameters obtained by Rietveld refinements approaching 1 Mbar. The MgGeO{sub 3} pPv phase exhibits strong anisotropy in axial compressibility, with the b-axis being most compressible. The polyhedral bulk modulus for the GeO{sub 6} octahedron is 1.9x larger than that for the MgO{sub 8} hendecahedron. Examination of neighboring O-O distances shows that the O-O distance aligned along the a direction is one of the longest and that aligned along c is one of the shortest, and these may be related to the lower compressibility along c compared with a. Comparison of structural features of MgGeO{sub 3} pPv with those for MgSiO{sub 3}, NaMgF{sub 3}, and CaIrO{sub 3} pPv show that MgSiO{sub 3} pPvmore » has more similarity with NaMgF{sub 3} and MgGeO{sub 3} pPv than with CaIrO{sub 3} pPv in such parameters as degree of octahedral distortion, implying that both NaMgF{sub 3} and MgGeO{sub 3} pPv are better analogs to MgSiO{sub 3} pPv than CaIrO{sub 3} pPv.« less

Pressure‐volume‐temperature relations in MgO: An ultrahigh pressure‐temperature scale for planetary sciences applications

In situ crystallography based on diamond anvil cells have been extended to the multimegabar regime. Temperatures in these experiments have crossed the 2500 K mark. Yet, current high pressure‐temperature (PT) standards of calibration produce uncertainties that inhibit clear conclusions about phenomena of importance to planetary processes, e.g., the postperovskite transition in Earth's mantle. We introduce a new thermal equation of state (EOS) of MgO which appears to be predictive up to the multimegabar and thousands of kelvin range. It is obtained by combining first principles local density approximation quasi‐harmonic (QHA) calculations with experimental low‐pressure data. This EOS agrees exceptionally well with shock compression data. The postspinel and postperovskite phase boundaries recalculated using our EOS match seismic observations. The latter, in particular, supports the idea that postperovskite transforms back to perovskite before the core‐mantle boundary. The recalculated experimental Clapeyron slope of the postperovskite transition is also more consistent with those obtained by first principles calculations.

Lateral variations in CMB heat flux and deep mantle seismic velocity caused by a thermal–chemical-phase boundary layer in 3D spherical convection

Numerical simulations of thermo-chemical, multi-phase, compressible mantle convection in a three-dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear-wave velocity V s above the core–mantle boundary (CMB) and lateral variations in heat flux across the CMB ( q CMB), when compositional variations and the post-perovskite phase transition are included. For simple thermal convection, the V s– q CMB relationship is reasonably but not perfectly linear. The post-perovskite transition introduces a non-linearity that amplifies fast V s anomalies in cold regions, but there is still a unique mapping between δ V s and q CMB. Lateral variations in composition such as piles of dense material introduce another non-linearity that affects hot upwelling regions, and introduces a non-uniqueness in δ V s– q CMB if the dense material (e.g., MORB) is seismically fast compared to the surrounding material. In this case, dense piles are ringed by sharp, low- V s anomalies. If the CMB is covered by a global dense layer than variations in δ V s and q CMB are reduced but so is the mean value of q CMB. In all cases, the peak-to-peak lateral variation in q CMB is similar to or larger than twice the mean value, which might create problems for generating a dynamo according to existing numerical dynamo simulations. Analytical scalings are developed to explain the observed trends.

Numerical modelling of dislocations and deformation mechanisms in CaIrO 3 and MgGeO 3 post-perovskites—Comparison with MgSiO 3 post-perovskite

In this study, we propose a theoretical approach to test the validity of the isomechanical analogues for post-perovskite structures. Intrinsic plastic properties are evaluated for three materials exhibiting a post-perovskite phase: MgSiO 3, MgGeO 3 and CaIrO 3. Dislocation properties of each structure are determined using the Peierls–Nabarro model based on first-principles calculations of generalised stacking fault and the plastic properties are extended to crystal-preferred orientations using a visco-plastic self-consistent method. This study provides intrinsic parameters of plastic deformation such as dislocation structures and Peierls stresses that can be directly compared between the three materials. It appears that it is very difficult to draw any simple conclusions on polycrystalline deformation simply by comparing single crystal properties. In particular, contrasting single crystal properties of MgSiO 3 and CaIrO 3 lead to similar crystal-preferred orientation of the polycrystal aggregates.

Possible structural polymorphism in Al-bearing magnesiumsilicate post-perovskite

aB s T rac T In the present study, we summarize indications for the existence of kinked post-perovskite structures in the MAS system. X-ray diffraction data and Raman spectra of aluminous magnesium metasilicate post-perovskite are inconsistent with the CaIrO3 structure. Instead the observations are consistent with structures intermediate between the perovskite and the CaIrO3 structure. Ab initio calculations show that the enthalpies of the kinked structures are slightly higher than the CaIrO3 structure at 0 K. Finite temperature, minor element chemistry, kinetics of phase transformation, and actual stress regime are plausible reasons for the observed differences between the present and the previously reported postperovskite phases.

Deformation and texture development in CaIrO 3 post-perovskite phase up to 6 GPa and 1300 K

At near ambient conditions, CaIrO 3 is isostructural with the high-pressure polymorph MgSiO 3 “post-perovskite” (pPv). MgSiO 3 pPv is thought to be a major phase in the earth's lowermost mantle. CaIrO 3 can thus serve as an analog for studying deformation of the pPv phase under conditions achievable with a multi-anvil deformation apparatus. Here we study the rheologic behavior of CaIrO 3 pPv at a variety of pressure and temperature conditions from 2 GPa to 6 GPa and 300 K to 1300 K and various strain rates. Sintered, polycrystalline CaIrO 3 pPv, cylindrical in shape, was deformed in the D-DIA multi-anvil press in several shortening cycles up to 20% axial strain at each temperature and pressure. Shortening cycles were followed by lengthening back to 0% strain. Quantitative texture information was obtained using in-situ synchrotron X-ray diffraction and the Rietveld method to analyze images. In all cases we find that (010) lattice planes align perpendicular to the compression direction upon shortening, and that there is little change in texture with temperature or pressure. This texture pattern is consistent with slip on (010)[100]. The texture observed here is different from that produced in room temperature diamond anvil cell (DAC) experiments on MgGeO 3 and MgSiO 3 pPv which both display textures of (100) and {110} lattice planes at high angles to the compression direction. This implies that CaIrO 3 pPv may not be a good analog for the plastic behavior of MgSiO 3 pPv.

Element partitioning between magnesium silicate perovskite and ferropericlase: New insights into bulk lower-mantle geochemistry

In this study, we investigated iron–magnesium exchange and transition-metal trace-element partitioning between magnesium silicate perovskite (Mg,Fe)SiO 3 and ferropericlase (Mg,Fe)O synthetised under lower-mantle conditions (up to 115 GPa and 2200 K) in a laser-heated diamond anvil cell. Recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy (ATEM) and nanometer-scale secondary ion mass spectroscopy (nanoSIMS). Iron concentrations in both phases were obtained from X-ray energy dispersive spectroscopy measurements and nanoSIMS. Our results are the first to show that recently reported spin-state and phase transitions in the lower mantle directly affect the evolution of Fe–Mg exchange between both phases. Mg-perovskite becomes increasingly iron-depleted above 70–80 GPa possibly due to the high spin–low spin transition of iron in ferropericlase. Conversely, the perovskite to post-perovskite transition is accompanied by a strong iron enrichment of the silicate phase, ferropericlase remaining in the Fe-rich phase though. Nanoparticles of metallic iron were observed in the perovskite-bearing runs, suggesting the disproportionation of ferrous iron oxide, but were not observed when the post-perovskite phase was present. Implications on the oxidation state of the Earth and core segregation will be discussed. Transition trace-element (Ni, Mn) concentrations (determined with the nanoSIMS) show similar trends and could thus be used to trace the origin of diamonds generated at depth. This study provides new results likely to improve the geochemical and geophysical models of the Earth's deep interiors.

焼結ダイヤモンドアンビルを用いたゲルマニウム酸塩のポストペロブスカイト転移の観察

Transformation from perovskite to post-perovskite was observed in MnGeO3 and MgGeO3 in the Kawai-type apparatus equipped with sintered diamond anvils by means of in situ X-ray diffraction method. In MnGeO3, post-perovskite phase was observed at 63.5 GPa and 1233 K. The transition pressure was ∼ 6 GPa higher than previous report by the laser heated diamond anvil cell experiment. In MgGeO3, we observed the transformation from perovskite to post-perovskite and the reversal transformation under the conditions of 61-64 GPa and 1100-1700 K. The phase boundary was determined to be P (GPa) = 5.2×10-3 T (K)+55.2, where P is pressure and T is absolute temperature, which is almost consistent with previous result by diamond anvil experiments. However, reliable pressure standard is needed to determine the boundary precisely.

Local structure and electronic–spin transition of Fe-bearing MgSiO 3 perovskite under conditions of the Earth's lower mantle

We report first-principles electronic structure calculations on the structural and electronic–spin behaviours of Fe-bearing MgSiO 3 crystals up to the pressure of Earth's mantle. The transition pressure of the Fe-bearing MgSiO 3 from the orthorhombic perovskite (OPv) to the orthorhombic post-perovskite (OPPv) phase decreases with increasing Fe concentration. The lattice distortion has impacts on the electronic–spin behaviour of the Fe ions in the PVs. The spin-polarizations of the Fe ions in the (Fe,Mg)SiO 3 OPvs and OPPvs keep unchanged up to the pressures in the lower mantle. Meanwhile, the Fe-bearing MgSiO 3 OPv containing Fe Mg Fe Si pairs exhibits multiple-magnetic moments co-existing in a large pressure range (from about 78 to 110 GPa), and finally becomes non-magnetic at pressure higher than 110 GPa. These results provide a mechanism to understand the recent experimental results about Fe valence states and the electronic transitions of the Fe-bearing MgSiO 3 under high pressure.

Phase transitions in pyrolite and MORB at lowermost mantle conditions: Implications for a MORB-rich pile above the core–mantle boundary

Subduction of mid-oceanic ridge basalt (MORB) gives rise to strong chemical heterogeneities in the Earth's mantle, possibly extending down to the core–mantle boundary. Phase relations in both pyrolite and MORB compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions. The results demonstrate that the post-perovskite phase transition occurs in pyrolite between 116 and 121 GPa at 2500 K, while post-perovskite and SiO2 phase transitions occur in MORB at ∼4 GPa lower pressure at the same temperature. Theory predicts that these phase changes in pyrolite and MORB cause shear wave velocity increase and decrease, respectively. Near the northern margin of the large low shear velocity province in the lowermost mantle beneath the Pacific, reflections from a negative shear velocity jump near 2520-km depth are followed by reflections from a positive velocity jump 135 to 155-km deeper. These negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of MORB and pyrolitic material. This may be a direct demonstration of the presence of accumulations of subducted MORB crust in the deep mantle.

Structure constraints and instability leading to the post-perovskite phase transition of MgSiO 3

Recent experience with Rietveld refinement of structural analogues and literature surveys, suggests anion–anion repulsion limits the stability of the perovskite phase, including in the MgSiO 3 perovskite to post-perovskite transition. Assuming rigid octahedral coordination, still to be tested experimentally, the critical point where intra- and inter-octahedral anion–anion distances are equal provides a useful metric for predicting the pressure of the perovskite/post-perovskite transition and the Clapeyron slope of the phase boundary, once pressure and temperature derivatives of relevant structure parameters are known. The inter-octahedral anion–anion distances and the polyhedral volume ratio are rigorously formulated as a function of octahedral rotation in this work, assuming the orthorhombic ( Pbnm) perovskite structure, where regular octahedra share each corner and conform to the in- and anti-phase rotation schemes designated by space group symmetry. These mathematical expressions are consistent with structure data from 70 perovskite-structured materials surveyed in the literature at ambient as well as extreme conditions and define structure constraints, such as the minimum polyhedral volume ratio, which must be reached before the phase transition to the post-perovskite structure-type can proceed. The formalism we present is general for perovskite ( Pbnm) and dependent on the accuracy with which structures can be determined from, sometimes compromised, high pressure diffraction data.

Post-perovskite phase in selected sesquioxides from density-functional calculations

We search for the existence of the post-perovskite structure in several nonmagnetic ${M}_{2}{\mathrm{O}}_{3}$ sesquioxides using density-functional calculations. For each material we consider the corundum, ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}$-type II, perovskite, and post-perovskite structures. The perovskite structure is unstable with respect to at least one of the other structures at all pressures for all materials. The post-perovskite structure is stable above $120\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, above $344\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}$, above $136\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$, and above $47\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ in ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$.

High-pressure structure and bonding in CaIrO3: The structure model of MgSiO3 post-perovskite investigated with time-of-flight neutron powder diffraction

The structure of CaIrO3 (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc21 is inconclusive. As CaIrO3 (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca2+-centered polyhedra are an order of magnitude more compressible than Ir4+-centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSiO3 post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra.

Equations of state of CaIrO3 perovskite and post-perovskite phases

Unit-cell lattice parameters have been measured to ~8 GPa in a diamond anvil cell for two single crystals of CaIrO3: one with the perovskite (Pbnm) and the other with the post-perovskite (Cmcm) structure. The CaIrO3 post-perovskite structure is more compressible than the perovskite. A third-order Birch Murnaghan equation of state has been used to fit the measured P-V data with the following refined parameters: V0 = 229.463(8) Å3, K0 = 198(3) GPa, K’ = 1.2(8); and V0 = 226.38(1) Å3, K0 = 181(3) GPa, K’ = 2.3(8) for CaIrO3 perovskite and post-perovskite, respectively. The compressibility of the unit-cell axes of the perovskite structure is highly anisotropic with βa >> βc >> βb. In contrast, the b axis is the most compressible in the post-perovskite structure, whereas the a and c axes have similar compressibilities (with c slightly less compressible than a) and are much stiffer. A comparison between the compressibility of CaIrO3 perovskite and post-perovskite with the isostructural MgSiO3 phases, reveals a similar general behavior, although in detail CaIrO3 perovskite is more and the postperovskite less anisotropic than the corresponding MgSiO3 compounds.

Instabilities in a fluid layer with phase changes

The onset of convection in a bottom heated horizontal fluid layer containing two different phases is investigated with emphasis on the effect of the location of the phase change interface. A general analysis, applicable when the phase boundary is at any location in the layer, is used to determine how both exothermic and endothermic phase changes influence layer instability. A perturbation analysis is presented for phase change boundaries close to the bottom of the fluid layer. Stability of the layer depends on three dimensionless parameters: R α , a latent heat parameter; S, a phase boundary distortion parameter; and ξ , a phase boundary location parameter. Both exothermic and endothermic phase changes can be either stabilizing or destabilizing depending on S and R α . Application is made to the exothermic olivine–spinel (in Earth and Mars) and perovskite–post-perovskite (in Earth) phase transitions and the endothermic spinel–perovskite phase change (in Earth). The perturbation analysis provides a quantitative analytic criterion to predict whether a phase change near the lower boundary is stabilizing or destabilizing. When the exothermic phase change is stabilizing (destabilizing), the layer is most stable (unstable) when the phase change is in the middle.

A bound on heat flow below a double crossing of the perovskite‐postperovskite phase transition

A double crossing of the perovskite‐postperovskite phase transition in (Mg,Fe)SiO3 has been proposed to explain seismic reflections from structures near the base of the mantle. Estimates of temperature inferred from the phase transitions can be extrapolated to the core‐mantle boundary (CMB) using a simple model for the thermal boundary layer. However, a complication arises when flow is driven by the (negative) buoyancy of the postperovskite phase. Latent heat release and advective transport increase the temperature gradient and heat flow below the region of postperovskite. A minimum bound on the temperature gradient at the CMB is obtained by noting that the temperature gradient at the base of the postperovskite region must be at least as steep as the transition temperature with depth. Both the temperature and heat flow at the CMB depend on the vertical velocity at the phase transition. A representative velocity of 1 mm/yr gives a local heat flux of 160 m W m−2. Increasing the velocity by a factor of two yields a substantially higher heat flow and an unreasonably hot temperature at the top of the core.

Lattice preferred orientation in CaIrO3 perovskite and post-perovskite formed by plastic deformation under pressure

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.