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.

We conducted shock wave experiments on iron carbide Fe3C up to a Hugoniot pressure of 245 GPa. The correlation between the particle velocity (u(p)) and shock wave velocity (u(s)) can be fitted into a linear relationship, u(s) = 4.627(+/- 0.073) + 1.614(+/- 0.028) u(p). The density-pressure relationship is consistent with a single-phase compression without decomposition. The inference is further supported by the comparison of the observed Hugoniot density with the calculated Hugoniot curves of possible decomposition products. The new Hugoniot data combined with the reported 300-K isothermal compression data yielded a Gruneisen parameter of gamma = 2.23(7.982/rho)(0.29). The thermal equation of state of Fe3C is further used to calculate the density profile of Fe3C along the Earth's adiabatic geotherm. The density of Fe3C was found to be too low (by similar to 5%) to match the observed density in the Earth's inner core, and Fe3C is unlikely a dominant component of the inner core.

Single crystals of a complex Zintl compound with the composition Na4Ge13 were synthesized for the first time using a high-pressure/high-temperature approach. Single-crystal diffraction of synchrotron radiation revealed a hexagonal crystal structure with P6/m space group symmetry that is composed of a three-dimensional sp(3) Ge framework punctuated by small and large channels along the crystallographic c axis. Na atoms are inside hexagonal prism-based Ge cages along the small channels, while the larger channels are occupied by layers of disordered sixfold Na rings, which are in turn filled by disordered [Ge-4](4-) tetrahedra. This compound is the same as "Na1-xGe3+z" reported previously, but the availability of single crystals allowed for more complete structural determination with a formula unit best described as Na4Ge12(Ge-4)0(.25). The compound is the first known example of a guest-host structure where discrete Zintl polyanions are confined inside the channels of a three- dimensional covalent framework. These features give rise to temperature-dependent disorder, as confirmed by first-principles calculations and physical properties measurements. The availability of single-crystal specimens allowed for measurement of the intrinsic low-temperature transport properties of this material and revealed its semiconductor behavior, which was corroborated by theoretical calculations.

Varying the substrate temperature changes structural and magnetic properties of spinel ferrite Ni0.65Zn0.35Fe2O4 (NZFO) thin films. X-ray diffraction of films grown at different temperature display only (004) reflections, without any secondary peaks, showing growth orientation along the c-axis. We find an increase in crystalline quality of these thin films with the rise of substrate temperature. The surface topography of thin films grown at different growth temperatures reveal that these films are smooth with low roughness; however, the thin films grown at 800 degrees C exhibit lowest average and root mean square (rms) roughness among all thin films. We find iron and nickel to be more oxidized (greater Fe3+ and Ni3+ content) in films grown and annealed at 700 degrees C and 800 degrees C, compared to those films grown at lower temperatures. The magnetic moment is observed to increase with an increase of substrate temperature and all thin films possess high saturation magnetization and low coercive field at room temperature. Films grown at 800 degrees C exhibit a ferrimagneticeparamagnetic phase transition well above room temperature. The observed large magnetizations with soft magnetic behavior in NZFO thin films above room temperature suggest potential applications in memory, spintronics, and multifunctional devices. (c) 2018 Elsevier B.V. All rights reserved.