Taku Tsuchiya

Systematics of elasticity: Ab initio study in B1-type alkaline earth oxides

The elastic properties and their pressure dependence of four B1-type alkaline earth oxides, MgO, CaO, SrO, and BaO, are calculated using the ab initio full-potential linear muffin-tin-orbital (FP-LMTO) generalized gradient approximated (GGA) method to elucidate their systematics. The calculated results agree quite well with the comparable experimental data. The large pressure dependence of c11 and c44 of MgO observed over 25 GPa is not predicted as well as the previous local-density approximation (LDA) calculations. It is inferred that the high-pressure measurement of elastic constant is quite sensitive to nonhydrostaticity. The deviation from the Cauchy relation and the elastic anisotropy are investigated. It is found that the interatomic interaction in SrO is nearest to the two-body force and the many-body contribution is largest in MgO. The elastic anisotropy in SrO and BaO are almost the same and only MgO has a large positive anisotropy under low pressure. The normalized elastic constants cij′ are int...

First-principles study of hydrogen bond symmetrization of phase D under high pressure

Abstract We have investigated the physical and structural properties of phase D (MgSi2O6H2) at high pressure by means of a first-principles calculation method. Phase D is important as a dense hydrous magnesium silicate with high stability under pressure, and as one of the most likely candidates for a water reservoir in the Earth’s lower mantle. The calculated compression behavior of phase D is in very good agreement with experimental results. We found a distinct but continuous change from asymmetric to symmetric hydrogen bonding in phase D at 40 GPa. This pressure-induced hydrogen bond symmetrization has a significant effect on the compression behavior of phase D. The bulk modulus increases by about 20% with this structural change. This behavior of pressure-induced hydrogen bond symmetrization is very similar to that previously reported by us for δ-AlOOH. The transition is reversible and second-order, and thus the high-pressure state is probably unquenchable.

Temperature profile in the lowermost mantle from seismological and mineral physics joint modeling

The internal structure of the core-mantle boundary (CMB) region of the Earth plays a crucial role in controlling the dynamics and evolution of our planet. We have developed a comprehensive model based on the radial variations of shear velocity in the D″ layer (the base of the lower mantle) and the high-P,T elastic properties of major candidate minerals, including the effects of post-perovskite phase changes. This modeling shows a temperature profile in the lowermost mantle with a CMB temperature of 3,800 ± 200 K, which suggests that lateral temperature variations of 200–300 K can explain much of the large velocity heterogeneity observed in D″. A single-crossing phase transition model was found to be more favorable in reproducing the observed seismic wave velocity structure than a double-crossing phase transition model.

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

Ultrahigh-pressure phase relationship of SiO2 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 Fe2P-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 Fe2P-type structure among all dioxide compounds. The crystal structure consists of closely packed, fairly regular SiO9 tricapped trigonal prisms with a significantly compact lattice. Additional investigation further elucidates large effects of this phase change in SiO2 on the stability of MgSiO3 and CaSiO3 at multimegabar pressures. A postperovskite phase of MgSiO3 breaks down at 1.04 TPa along an assumed adiabat of super-Earths and yields Fe2P-type SiO2 and CsCl (B2)-type MgO. CaSiO3 perovskite, on the other hand, directly dissociates into SiO2 and metallic CaO, skipping a postperovskite polymorph. Predicted ultrahigh-pressure and temperature phase diagrams of SiO2, MgSiO3, and CaSiO3 indicate that the Fe2P-type SiO2 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 MgSiO3–Al2O3 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 Rh2O3(II) phase, present calculations demonstrate that (i) dissolving Al2O3 tends to decrease the postperovskite transition pressure of MgSiO3 but the effect is not significant (≈-0.2 GPa/mol% Al2O3); (ii) Al2O3 produces the narrow perovskite+postperovskite coexisting P,T area (≈1 GPa) for the pyrolitic concentration (xAl2O3 ≈6 mol%), which is sufficiently responsible to the deep-mantle D″ seismic discontinuity; (iii) the transition would be smeared (≈4 GPa) for the basaltic Al-rich composition (xAl2O3 ≈20 mol%), which is still seismically visible unless iron has significant effects; and last (iv) the perovskite structure spontaneously changes to the Rh2O3(II) with increasing the Al concentration involving small displacements of the Mg-site cations.

Molecular dynamics simulation for evaluating melting point of wurtzite-type GaN crystal

A two-phase molecular dynamics simulation of coexisting solid and liquid has been carried out to investigate the melting point of wurtzite-type GaN crystals. The melting point is determined by examining the movement of the interface between the solid and liquid during the simulation. The potential is a two-body interatomic one composed of the long-range Coulomb interaction, the Gilbert-type short-range repulsion, the covalent bonding and covalent repulsion of the modified Morse type, and the van der Waals interaction. The melting point and the interface morphology depend on the crystallization direction. The melting point Tm(K) increases with pressure P(GPa), but there appears a discontinuity in the vicinity of 8–9GPa. This is due to the solid-electrolyte-like behavior of Ga atoms with a partial charge in the high-pressure region. The discontinuity has not yet been confirmed by experiment. The least-squares fitted result is Tm=2538+177P−4.62P2 at pressures lower than 8GPa and Tm=2825+210P−5P2 at pressures...

First principles investigation of the structural and elastic properties of hydrous wadsleyite under pressure

[1] In order to clarify the effect of protonation of wadsleyite under high-pressure conditions, we determined defect structures of Mg 1.875 SiO 4 H 0.25 (1Mg 2+ ↔ V 2- Mg + 2H + , 1.65 wt % H 2 O), Mg 1.75 SiO 4 H 0.5 (2Mg 2+ ↔ 2V 2- Mg + 4H - , 3.3 wt % H 2 O) hydrous wadsleyite and their elastic properties by means of the density functional first principles method. Structural optimization calculations indicate that the most stable structures have monoclinic symmetry with magnesium M3 site vacancies. Protons are found to bond to the O1 site to align the OH dipoles along the edges of M3 vacancies. Calculated elastic constants, bulk and shear moduli, are found to decrease almost linearly with increasing water content but to increase linearly with increasing pressure. At 15 GPa and static 0 K condition, incorporation of 3.3 wt % H 2 O into wadsleyite, which corresponds to the maximum solubility of hydrous wadsleyite, reduces Vp and V s by about 3.9 and 4.8%, respectively. This indicates that 1 wt % H 2 O hydration of wadsleyite corresponds to the temperature effects on bulk and shear moduli about 430 K (0 GPa) to 340 K (20 GPa) and 350 K (0 GPa) to 290 K (20 GPa), respectively. The transversely isotropic aggregates demonstrate largest positive polarization anisotropy V SH - V SV when the c axis aligns vertically in both dry and wet cases.

The P‐V‐T equation of state and thermodynamic properties of liquid iron

The equation of state (EoS) and thermodynamic properties of non-magnetic liquid iron were investigated from energy (E)-pressure (P)-volume (V)-temperature (T) relationships calculated by means of ab initio molecular dynamics simulations at 60–420 GPa and 4000–7000 K. Its internally consistent thermodynamic and elastic properties, in particular, density, adiabatic bulk modulus, and P wave velocity, were then analyzed. Compared to the seismological data of the Earths outer core, pure liquid iron is found to have an 8–10% larger density and 3–10% larger bulk modulus than the Earths values. Results also show that the P wave velocity of liquid iron has marginal temperature dependence as the bulk sound velocity of solid iron. The new EoS model and thermodynamic properties of liquid iron may serve as fundamental data for the thermochemical modeling of the Earths core.

Vibrational properties of δ-AlOOH under pressure

Abstract We have performed first-principles calculations to investigate the behavior of the hydrogen bond in δ-AlOOH under pressure. The highest OH-stretching A1 and B2 mode frequencies decrease under pressure leading to hydrogen bond symmetrization. After hydrogen bond symmetrization, the corresponding frequencies gradually increase. This softening and subsequent hardening of the OH bonds is a good spectroscopic indicator of hydrogen bond symmetrization and is observed in our GGA static calculations at ~30 GPa without considering tunneling effects. We have also carried out calculations of Raman peak intensities in several supercells with various hydrogen orderings to investigate the potential effect of H-disorder on the Raman spectrum of δ-AlOOH. Our results suggest that the four broad Raman bands observed experimentally in the range of OH-stretching mode frequencies could originate in H-disorder in this phase.

Phase stability and elastic properties of the NAL and CF phases in the NaMg2Al5SiO12 system from first principles

Abstract New hexagonal aluminous (NAL) phase and orthorhombic calcium-ferrite (CF) type phase are considered to be major mantle components of the mid-ocean ridge basalt (MORB) at pressure and temperature conditions in the lower mantle, which can potentially host alkali elements with large ionic radii. The high-pressure stability and elastic properties of both NAL and CF phases are therefore of fundamental importance for understanding the fate of subducted MORB. Here we report those properties of the NaMg2Al5SiO12 system studied by means of the first-principles computation method. NAL was found to transform to the CF phase at 39.6 GPa, accompanied by discontinuities in density (+1.8%), as well as compressional wave (-0.2%), shear wave (+0.9%), and bulk sound (-1.0%) velocities. The property of subducted MORB was evaluated using these results, and the velocity contrast between pyrolite and MORB of ~ -5% was found to be quite comparable to the shear velocity anomaly observed for seismic scatterers at depths from 1100 to 1800 km. However, since the transformation of the NAL to CF phase within MORB produces insignificant increases in the seismic velocities, it would be seismologically undetectable. On the other hand, the anisotropy change associated with the phase transition is significant and could be seismically detectable using observations such as shear wave splitting measurements since the CF phase is considerably more anisotropic compared to the NAL phase.