A new high-pressure structure of SiO2directly converted fromα-quartz under nonhydrostatic compression

High-pressure behavior of SiO₂is one of the prototypical subjects in several research areas including condensed matter physics, inorganic chemistry, mineralogy, materials science, and crystallography. Therefore, numerous studies have been performed on the structure evolution of SiO₂under pressure. Here, we show a new structure directly converted fromα-quartz under uniaxial compression. Ourab initiocalculations elucidate a simple transition pathway fromα-quartz to the Fe₂P-type phase, and an intermediate state with the Li₂ZrF₆-type structure appears in this structure conversion. Some interesting properties are found on this intermediate state. (1) The Li₂ZrF₆-type phase is metastable probably due to a volumetric unbalance between the Li and Zr sites but becomes more energetically stable thanα-quartz over ∼12 GPa. (2) It is vibrationally stable at 0 GPa, suggesting that this phase can be recovered down to ambient condition once synthesized. (3) The crystal structures of Li₂ZrF₆-type SiO₂and phase D, one of dense magnesium hydrous silicates, are found identical, suggesting the stabilization of their solid solution under high-P,Tcondition.

Experimental and theoretical thermal equations of state of MgSiO3 post-perovskite at multi-megabar pressures

The MgSiO₃ post-perovskite phase is the most abundant silicate phase in a super-Earth’s mantle, although it only exists within the Earth’s lowermost mantle. In this study, we established the thermal equation of state (EoS) of the MgSiO₃ post-perovskite phase, which were determined by using both laser-heated diamond anvil cell and density-functional theoretical techniques, within a multi-megabar pressure range, corresponding to the conditions of a super-Earth’s mantle. The Keane and AP2 EoS models were adopted for the first time to extract meaningful physical properties. The experimentally determined Grüneisen parameter, which is one of the thermal EoS parameters, and its volume dependence were found to be consistent with their theoretically obtained values. This reduced the previously reported discrepancy observed between experiment and theory. Both the experimental and theoretical EoS were also found to be in very good agreement for volumes at pressures and temperatures of up to 300 GPa and 5000 K, respectively. Our newly developed EoS should be applicable to a super-Earth’s mantle, as well as the Earth’s core-mantle boundary region.

Ab initio lattice thermal conductivity of MgSiO3 perovskite as found in Earth's lower mantle

The lattice thermal conductivity (κ(lat)) of MgSiO3 perovskite (Mg-Pv) under high-pressure and high-temperature conditions was computed based on the ab initio anharmonic lattice dynamics method with the density functional perturbation theory. κ(lat) of Mg-Pv is found to increase with increasing pressure from 9.8 (at 23.5 GPa) to 43.6  W m(-1)  K(-1) (at 136 GPa) at 300 K, while decreasing with increasing temperature from 28.1 (at 300 K) to 2.3  W m(-1)   K(-1) (at 4000 K) at 100 GPa. A multiphase composite average yielded a mantle Rayleigh number adequate to promote the vigorous thermal convection of the mantle that is expected geophysically.

Thermoelastic properties and crystal structure of CaPtO3post-perovskite from 0 to 9 GPa and from 2 to 973 K

ABX3post-perovskite (PPV) phases that are stable (or strongly metastable) at ambient pressure are important as analogues of PPV-MgSiO3, a deep-Earth phase stable only at very high pressure. The thermoelastic and structural properties of orthorhombic PPV-structured CaPtO3have been determined to 9.27 GPa at ambient temperature and from 2 to 973 K at ambient pressure by time-of-flight neutron powder diffraction. The equation-of-state from this high-pressure study is consistent with that found by Lindsay-Scott, Wood, Dobson, Vočadlo, Brodholt, Crichton, Hanfland & Taniguchi [(2010).Phys. Earth Planet. Inter.182, 113–118] using X-ray powder diffraction to 40 GPa. However, the neutron data have also enabled the determination of the crystal structure. Thebaxis is the most compressible and thecaxis the least, with theaandcaxes shortening under pressure by a similar amount. Above 300 K, the volumetric coefficient of thermal expansion, α(T), of CaPtO3can be represented by α(T) =a0+a1(T), witha0= 2.37 (3) × 10−5 K−1anda1= 5.1 (5) × 10−9 K−2. Over the full range of temperature investigated, the unit-cell volume of CaPtO3can be described by a second-order Grüneisen approximation to the zero-pressure equation of state, with the internal energy calculatedviaa Debye model and parameters θD(Debye temperature) = 615 (8) K,V0(unit-cell colume at 0 K) = 227.186 (3) Å3,K′0(first derivative with respect to pressure of the isothermal incompressibilityK0) = 7.9 (8) and (V0K0/γ′) = 3.16 (3) × 10−17 J, where γ′ is a Grüneisen parameter. Combining the present measurements with heat-capacity data gives a thermodynamic Grüneisen parameter γ = 1.16 (1) at 291 K. PPV-CaPtO3, PPV-MgSiO3and PPV-CaIrO3have the same axial incompressibility sequence, κc > κa > κb. However, when heated, CaPtO3shows axial expansion in the form αc > αb > αa, a sequence which is not simply the inverse of the axial incompressibilities. In this respect, CaPtO3differs from both MgSiO3(where the sequence αb > αa > αcis the same as 1/κi) and CaIrO3(where αb > αc > αa). Thus, PPV-CaPtO3and PPV-CaIrO3are better analogues for PPV-MgSiO3in compression than on heating. The behaviour of the unit-cell axes of all three compounds was analysed using a model based on nearest-neighbourB—XandA—Xdistances and angles specifying the geometry and orientation of theBX6octahedra. Under pressure, all contract mainly by reduction in theB—XandA—Xdistances. On heating, MgSiO3expands (at high pressure) mainly by lengthening of the Si—O and Mg—O bonds. In contrast, the expansion of CaPtO3(and possibly also CaIrO3), at atmospheric pressure, arises more from changes in angles than from increased bond distances.

Prediction of a hexagonal SiO2 phase affecting stabilities of MgSiO3 and CaSiO3 at multimegabar pressures

Ultrahigh-pressure phase relationship of SiO(2) silica in multimegabar pressure condition is still quite unclear. Here, we report a theoretical prediction on a previously uncharacterized stable structure of silica with an unexpected hexagonal Fe(2)P-type form. This phase, more stable than the cotunnite-type structure, a previously postulated postpyrite phase, was discovered to stabilize at 640 GPa through a careful structure search by means of ab initio density functional computations over various structure models. This is the first evidential result of the pressure-induced phase transition to the Fe(2)P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO(9) tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO(2) on the stability of MgSiO(3) and CaSiO(3) at multimegabar pressures. A postperovskite phase of MgSiO(3) breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe(2)P-type SiO(2) and CsCl (B2)-type MgO. CaSiO(3) perovskite, on the other hand, directly dissociates into SiO(2) and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO(2), MgSiO(3), and CaSiO(3) indicate that the Fe(2)P-type SiO(2) could be one of the dominant components in the deep mantles of terrestrial exoplanets and the cores of gas giants.

Postperovskite phase equilibria in the MgSiO3-Al2O3 system

We investigate high-P,T phase equilibria of the MgSiO(3)-Al(2)O(3) system by means of the density functional ab initio computation methods with multiconfiguration sampling. Being different from earlier studies based on the static substitution properties with no consideration of Rh(2)O(3)(II) phase, present calculations demonstrate that (i) dissolving Al(2)O(3) tends to decrease the postperovskite transition pressure of MgSiO(3) but the effect is not significant ( approximately -0.2 GPa/mol% Al(2)O(3)); (ii) Al(2)O(3) produces the narrow perovskite+postperovskite coexisting P,T area (approximately 1 GPa) for the pyrolitic concentration (x(Al2O3) approximately 6 mol%), which is sufficiently responsible to the deep-mantle D'' seismic discontinuity; (iii) the transition would be smeared (approximately 4 GPa) for the basaltic Al-rich composition (x(Al2O3) approximately 20 mol%), which is still seismically visible unless iron has significant effects; and last (iv) the perovskite structure spontaneously changes to the Rh(2)O(3)(II) with increasing the Al concentration involving small displacements of the Mg-site cations.

Equation of state of the postperovskite phase synthesized from a natural (Mg,Fe)SiO3 orthopyroxene

Using the laser-heated diamond anvil cell, we investigate the stability and equation of state of the postperovskite (ppv, CaIrO(3)-type) phase synthesized from a natural pyroxene composition with 9 mol.% FeSiO(3). Our measured pressure-volume data from 12-106 GPa for the ppv phase yield a bulk modulus of 219(5) GPa and a zero-pressure volume of 164.9(6) A(3) when K'(0) = 4. The bulk modulus of ppv is 575(15) GPa at a pressure of 100 GPa. The transition pressure is lowered by the presence of Fe. Our x-ray diffraction data indicate the ppv phase can be formed at P > 109(4) GPa and 2,400(400) K, corresponding to approximately 400-550 km above the core-mantle boundary. Direct comparison of volumes of coexisting perovskite and CaIrO(3)-type phases at 80-106 GPa demonstrates that the ppv phase has a smaller volume than perovskite by 1.1(2)%. Using measured volumes together with the bulk modulus calculated from equation of state fits, we find that the bulk sound velocity decreases by 2.3(2.1)% across this transition at 120 GPa. Upon decompression without further heating, it was found that the ppv phase could still be observed at pressures as low at 12 GPa, and evidence for at least partial persistence to ambient conditions is also reported.

Carbon under extreme conditions: phase boundaries and electronic properties from first-principles theory

At high pressure and temperature, the phase diagram of elemental carbon is poorly known. We present predictions of diamond and BC8 melting lines and their phase boundary in the solid phase, as obtained from first-principles calculations. Maxima are found in both melting lines, with a triple point located at approximately 850 GPa and approximately 7,400 K. Our results show that hot, compressed diamond is a semiconductor that undergoes metalization upon melting. In contrast, in the stability range of BC8, an insulator to metal transition is likely to occur in the solid phase. Close to the diamond/liquid and BC8/liquid boundaries, molten carbon is a low-coordinated metal retaining some covalent character in its bonding up to extreme pressures. Our results provide constraints on the carbon equation of state, which is of critical importance for devising models of Neptune, Uranus, and white dwarf stars, as well as of extrasolar carbon-rich planets.

MgSiO3 postperovskite at D'' conditions

The postperovskite transition in MgSiO(3) at conditions similar to those expected at the D'' discontinuity of Earth's lower mantle offers a paradigm for interpreting the properties of this region. Despite consistent experimental and theoretical predictions of this phase transformation, the complexity of the D'' region raises questions about its geophysical significance. Here we report the thermoelastic properties of Cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle. These are (i) the jumps in shear and longitudinal velocities similar to those observed in certain places of the D'' discontinuity and (ii) the anticorrelation between shear and bulk velocity anomalies as detected within the D'' region. In addition, the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle.