The solidus for the mantle of Mars is an important geophysical parameter in modeling the thermal history and evolution of the planet. This study provides solidus data for a simplified model Martian mantle composition from the midmantle (8 GPa) to the core-mantle boundary (25 GPa) using multianvil experiments. Combining this work with previous studies, the solidus for the entire mantle of Mars is constrained to within 70 degrees C. The major mineral assemblages and phase transitions observed are consistent with those predicted for an iron-rich Martian mantle. The solidus for the Martian composition falls above the solidus of MORB and below that of terrestrial peridotite, providing direct melting data for geophysical modeling of the Martian interior. Our solidus also predicts that a Martian mantle plume should produce melt at depths over a range of 200-350 km in the mantle.

Crystalline polar metallocenes are potentially useful active materials as piezoelectrics, ferroelectrics, and multiferroics. Within density functional theory (DFT), we computed structural properties, energy differences for various phases, molecular configurations, and magnetic states, computed polarizations for different polar crystal structures, and computed dipole moments for the constituent molecules with a Wannier function analysis. Of the systems studied, Mn-2(C9H9N)(2) is the most promising as a multiferroic material, since the ground state is both polar and ferromagnetic. We found that the predicted crystalline polarizations are 30-40% higher than the values that would be obtained from the dipole moments of the isolated constituent molecules, due to the local effects of the self-consistent internal electric field, indicating high polarizabilities.

Hydrothermal experiments aiming at the crystal growth of stishovite near ambient pressure and temperature were performed in conventional autoclave systems using 1 M (molar) NaOH, 0.8 M Na2CO3, and pure water as a mineralizing agent. It was found that the hydrothermal metastability of stishovite and coesite is very different from the thermal metastability in all mineralizing agents and that because of this fact crystals could not be grown. While stishovite and coesite are thermally metastable up to 500 and >1000 degrees C, respectively, their hydrothermal metastability is below 150 and 200 degrees C, respectively. The thermally induced conversion of stishovite and coesite leads to amorphous products, whereas the hydrothermally induced conversion leads to crystalline quartz. Both stishovite and coesite are minerals occurring in nature where they can be exposed to hydrothermal conditions. The low hydrothermal stability of these phases may be an important factor to explain the rarity of these minerals in nature.

Na4Si24 is the precursor to Si-24, a recently discovered allotrope of silicon. With a quasidirect band gap near 1.3 eV, Si-24 has potential to transform silicon-based optoelectronics including solar energy conversion. However, the lack of large, pure crystals has prevented the characterization of intrinsic properties and has delayed deposition based metastable growth efforts. Here, we report an optimized synthesis methodology for single-crystalline Na4Si24 with crystals approaching the millimeter-size scale with conditions near 9 GPa and 1123 K. Single-crystal diffraction was used to confirm the open-framework structure, and Na atoms remain highly mobile within the framework channels, as determined by electrical conductivity and electron energy loss spectroscopy measurements. An epitaxial relationship between Na4Si24 and diamond cubic silicon (DC-Si), observed through high-resolution transmission electron microscopy, is proposed to facilitate the growth of high-quality Na4Si24 crystals from DC-Si wafers mixed with metallic Na and could provide a viable path forward for scaling efforts of Na4Si24 and Si-24.

Using dynamic compression technique, the equation of state for Fe-8.6 wt% Si was measured up to 240 GPa and 4,670 K. A least squares fit to the experimental data yields the Hugoniot parameters C-0 = 4.6030.101 km/s and lambda = 1.5050.037 with initial density rho(0)=7.3860.021 g/cm(3). Based on the Hugoniot data, the calculated isothermal equation of state is consistent with static compression data when the lattice Gruneisen parameter gamma(l) =1.65(7.578/rho) and electronic Gruneisen parameter gamma(e)=1.83. The calculated pressure-density data at 300 K were fitted to a third-order Birch-Murnaghan equation of state with zero pressure the parameters K-0=192.16.3 GPa, K0'=4.710.27 with fixed rho(0 epsilon) =7.578 +/- 0.050 g/cm(3). Under the conditions of Earth's core, the densities of Fe-8.6 +/- 2.0 wt% Si and Fe-3.8 +/- 2.9 wt% Si agree with preliminary reference Earth mode (PREM) data of the outer and the inner core, respectively. These are the upper limits for Si in the core assuming Si is the only light element. Simultaneously considering the geophysical and geochemical constraints for a Si-S-bearing core, the outer core may contain 3.8 +/- 2.9 wt% Si and 5.6 +/- 3.0 wt% S.

We study the aging and Mn doping effect on third generation lead based relaxor single crystals. We measured the polarization (PE) and strain with applied field on two perpendicular orientations of the rhombohedral pseudocubic [001] poled crystal. To understand these effects along the average dipoles/defect dipoles direction, we adopt a simple model with direction cosine and sine of polarization and strain. We found that when the PE measurement is perpendicular to average defect dipoles, a double loop is observed, and when it is parallel an asymmetric response is observed. We propose that the varied response found in PE measurements depend on the relative direction of average dipoles/defect dipoles to the measurement direction.

Calcium carbonate (CaCO3) significantly affects the properties of upper mantle and plays a key role in deep carbon recycling. However, its phase relations above 3 GPa and 1000 K are controversial. Here we report a reversible temperature-induced aragonite-amorphization transition in CaCO3 at 3.9-7.5 GPa and temperature above 1000 K. Amorphous CaCO3 shares a similar structure as liquid CaCO3 but with much larger C-O and Ca-Ca bond lengths, indicating a lower density and a mechanism of lattice collapse for the temperature-induced amorphous phase. The less dense amorphous phase compared with the liquid provides an explanation for the observed CaCO3 melting curve overturn at about 6 GPa. Amorphous CaCO3 is stable at subduction zone conditions and could aid the recycling of carbon to the surface.

Pressure-induced phase transitions in single-crystal PbZr0.54Ti0.46O3 are investigated with high-pressure Raman scattering and x-ray single crystal and powder diffraction. The appearance of a Raman peak near 380cm(-1) indicates a structural transition at 3 GPa. A second transition, driven by an soft optical phonon, occurs at 9 GPa. A third transition occurs above 27 GPa, accompanied by a large changes in the Raman spectrum and splitting of the (pseudo-cubic) (111) and (220) diffraction lines. We identify the transitions as a monoclinic (Cm) to rhombohedral (R3m) transition at 3 GPa, followed by a rhombohedral (R3m) to rhombohedral (R-3c) transition at 9 GPa, and a further symmetry-lowering transition at 27 GPa.

We report the discovery of a long-sought-after phase of titanium nitride with stoichiometry Ti3N4 using diamond anvil cell experiments combined with in situ high-resolution x-ray diffraction and Raman spectroscopy techniques, supported by ab initio calculations. Ti3N4 crystallizes in the cubic Th3P4 structure [space group I (4) over bar 3d (220)] from a mixture of TiN and N-2 above approximate to 75 GPa and approximate to 2400 K. The density (approximate to 5.22 g/cc) and bulk modulus (K-0 = 290 GPa) of cubic-Ti3N4 (c-Ti3N4) at 1 atm, estimated from the pressure-volume equation of state, are comparable to rocksalt TiN. Ab initio calculations based on the GW approximation and using hybrid functionals indicate that c-Ti3N4 is a semiconductor with a direct band gap between 0.8 and 0.9 eV, which is larger than the previously predicted values. The c-Ti3N4 phase is not recoverable to ambient pressure due to dynamic instabilities, but recovery of Ti3N4 in the defect rocksalt (or related) structure may be feasible.