Lithium hydroxide dihydrate: A new type of icy material at elevated pressure

We show that, in addition to the known monohydrate, LiOH forms a dihydrate at elevated pressure. The dihydrate involves a large number of H-bonds establishing chains along the direction. In addition, the energy surface exhibits a saddle point for proton locations along certain O interatomic distances, a feature characteristic for superprotonic conductors. However, MD simulations indicate that LiOH·2H_2O is not a superprotonic conductor and suggest the relevant interpolyhedral O–O distances being too large to allow for proton transfer between neighboring Li-coordinated polyhedra at least on the time scale of the MD-simulations

Thermoelastic Properties of Ringwoodite [Fe_x,Mg_(1-x)]_2SiO_4: Its Relationship to the 520 km Seismic Discontinuity

We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model vibrational density of states (VDoS) to calculate elastic moduli and sound velocities of gamma-[Fe_x,Mg_(1-x)]_2SiO_4 (ringwoodite), the most abundant mineral of the lower Earth's transition zone (TZ). Comparison with experimental values at room-temperature and high pressure or ambient-pressure and high temperature shows good agreement with our first-principles findings. Then, we investigate the contrasts associated with the beta-to-gamma-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and temperatures relevant to the TZ. This information offers clearly defined reference values to advance the understanding of the nature of the 520 km seismic discontinuity.Comment: 29 pages, 6 figures, 2 tables. Under Revie

Ahrensite, γ-Fe_2SiO_4, a new shock-metamorphic mineral from the Tissint meteorite: Implications for the Tissint shock event on Mars

Ahrensite (IMA 2013-028), γ-Fe_2SiO_4, is the natural Fe-analog of the silicate-spinel ringwoodite (γ-Mg_2SiO_4). It occurs in the Tissint Martian meteorite, where it forms through the transformation of the fayalite-rich rims of olivine megacrysts or Fe-rich microphenocrysts in contact with shock melt pockets. The typical sequence of phase assemblages traversing across a Tissint melt pocket into olivine is: quenched melt or fayalite-pigeonite intergrowth ⇒ bridgmanite + wüstite ⇒ ahrensite and/or ringwoodite ⇒ highly-deformed olivine + nanocrystalline ringwoodite ⇒ deformed olivine. We report the first comprehensive set of crystallographic, spectroscopic, and quantitative chemical analysis of type ahrensite, and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible. We also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets. The ahrensite and ringwoodite crystals show no preferred orientation, consistent with random nucleation and incoherent growth within a highly strained matrix of olivine. Grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration (1–10 ms)

Controlled formation of metastable germanium polymorphs

The nucleation of metastable germanium polymorphs on decompression is studied using in situ synchrotron x-ray diffraction. We show that the transition pathway is critically dependent on the hydrostaticity. Quasihydrostatic conditions result in the nucleation of the rhombohedral r8 phase, followed by the cubic bc8 and hexagonal diamond phases. In contrast, the presence of shear yields the tetragonal st12 phase. Thus, targeted nucleation of a metastable polymorph is now possible. This observation has implications for the technological exploitation of Ge, but also for other tetrahedral systems

First Principles Calculation of Elastic Properties of Solid Argon at High Pressures

The density and the elastic stiffness coefficients of fcc solid argon at high pressures from 1 GPa up to 80 GPa are computed by first-principles pseudopotential method with plane-wave basis set and the generalized gradient approximation (GGA). The result is in good agreement with the experimental result recently obtained with the Brillouin spectroscopy by Shimizu et al. [Phys. Rev. Lett. 86, 4568 (2001)]. The Cauchy condition was found to be strongly violated as in the experimental result, indicating large contribution from non-central many-body force. The present result has made it clear that the standard density functional method with periodic boundary conditions can be successfully applied for calculating elastic properties of rare gas solids at high pressures in contrast to those at low pressures where dispersion forces are important.Comment: 4 pages, 5 figures, submitted to PR