Database of observed and calculated infrared peaks for hydrogen-related defects in natural diamond

This database contains observed (Table A1) and calculated (Tables B1-B3) infrared peak positions (absorption frequencies, cm-1) for different hydrogen-related defects observed in diamond. If known, the hydrogen-related defect type and C-H or N-H vibrational mode is included for each peak position. The majority of data was taken from the literature and the corresponding references are included for each peak position. Some new peak data was included from FTIR spectra analyzed by the authors. Additional data regarding the computation parameters for each simulated (calculated) FTIR spectra are included in the calculated database (Tables B1-B3). A more detailed description of how the database is structured can be found in the corresponding publication at https://doi.org/10.1016/j.diamond.2024.110866

Melting phase relations in the systems Mg2SiO4-H2O and MgSiO3-H2O and the formation of hydrous melts in the upper mantle

High-pressure and high-temperature melting experiments were conducted in the systems Mg2SiO4–H2O and MgSiO3–H2O at 6 and 13 GPa and between 1150 and 1900 °C in order to investigate the effect of H2O on melting relations of forsterite and enstatite. The liquidus curves in both binary systems were constrained and the experimental results were interpreted using a thermodynamic model based on the homogeneous melt speciation equilibrium, H2O + O2− = 2OH−, where water in the melt is present as both molecular H2O and OH− groups bonded to silicate polyhedra. The liquidus depression as a function of melt H2O concentration is predicted using a cryoscopic equation with the experimental data being reproduced by adjusting the water speciation equilibrium constant. Application of this model reveals that in hydrous MgSiO3 melts at 6 and 13 GPa and in hydrous Mg2SiO4 melts at 6 GPa, water mainly dissociates into OH− groups in the melt structure. A temperature dependent equilibrium constant is necessary to reproduce the data, however, implying that molecular H2O becomes more important in the melt with decreasing temperature. The data for hydrous forsterite melting at 13 GPa are inconclusive due to uncertainties in the anhydrous melting temperature at these conditions. When applied to results on natural peridotite melt systems at similar conditions, the same model infers the presence mainly of molecular H2O, implying a significant difference in physicochemical behaviour between simple and complex hydrous melt systems. As pressures increase along a typical adiabat towards the base of the upper mantle, both simple and complex melting results imply that a hydrous melt fraction would decrease, given a fixed mantle H2O content. Consequently, the effect of pressure on the depression of melting due to H2O could not cause an increase in the proportion, and hence seismic visibility, of melts towards the base of the upper mantle

The thermal expansion of gold: point defect concentrations and pre-melting in a face-centred cubic metal

On the basis of ab initio computer simulations, pre-melting phenomena have been suggested to occur in the elastic properties of hexagonal close-packed iron under the conditions of the Earth's inner core just before melting. The extent to which these pre-melting effects might also occur in the physical properties of face-centred cubic metals has been investigated here under more experimentally accessible conditions for gold, allowing for comparison with future computer simulations of this material. The thermal expansion of gold has been determined by X-ray powder diffraction from 40 K up to the melting point (1337 K). For the entire temperature range investigated, the unit-cell volume can be represented in the following way: a second-order Grüneisen approximation to the zero-pressure volumetric equation of state, with the internal energy calculated via a Debye model, is used to represent the thermal expansion of the `perfect crystal'. Gold shows a nonlinear increase in thermal expansion that departs from this Grüneisen–Debye model prior to melting, which is probably a result of the generation of point defects over a large range of temperatures, beginning at T/Tm > 0.75 (a similar homologous T to where softening has been observed in the elastic moduli of Au). Therefore, the thermodynamic theory of point defects was used to include the additional volume of the vacancies at high temperatures (`real crystal'), resulting in the following fitted parameters: Q = (V0K0)/γ = 4.04 (1) × 10−18 J, V0 = 67.1671 (3) Å3, b = (K0′ − 1)/2 = 3.84 (9), θD = 182 (2) K, (vf/Ω)exp(sf/kB) = 1.8 (23) and hf = 0.9 (2) eV, where V0 is the unit-cell volume at 0 K, K0 and K0′ are the isothermal incompressibility and its first derivative with respect to pressure (evaluated at zero pressure), γ is a Grüneisen parameter, θD is the Debye temperature, vf, hf and sf are the vacancy formation volume, enthalpy and entropy, respectively, Ω is the average volume per atom, and kB is Boltzmann's constant

High-pressure single-crystal structural analysis of AlSiO3OHAlSiO_{3}OH phase egg

We present the first equation of state and structure refinements at high pressure of single-crystal phase egg, AlSiO$_3$OH. Phase egg is a member of the Al$_2$O$_3$-SiO$_2$-H$_2$O system, which contains phases that may be stable along a typical mantle geotherm (Fukuyama et al. 2017) and are good candidates for water transport into Earth's deep mantle. Single-crystal synchrotron X-ray diffraction was performed up to 23 GPa. We observe the b axis to be the most compressible direction and the $β$ angle to decrease up to 16 GPa and then to remain constant at a value of ~97.8° up to the maximum experimental pressure reached. Structure refinements performed at low pressures reveal a distorted octahedron around the silicon atom due to one of the six Si-O bond lengths being significantly larger than the other five. The length of this specific Si-O$_4$ bond rapidly decreases with increasing pressure leading to a more regular octahedron at pressures above 16 GPa. We identified the shortening of the Si-O$_4$ bond and the contraction of the vacant space between octahedral units where the hydrogen atoms are assumed to lie as the major components of the compression mechanism of AlSiO$_3$OH phase egg

Hexagonal Na-0.41[Na0.125Mg0.79Al0.085](2)[Al0.79Si0.21](6)O-12 (NAL phase): Crystal structure refinement and elasticity

At lower mantle conditions, subducted mid oceanic ridge basalts (MORB) will crystallize more than 20 vol% of an aluminum-rich phase, which is referred to generally as the new aluminum (NAL) phase. Given that a significant proportion of the lower mantle may be comprised of subducted crust, the NAL phase may contribute to the bulk elastic properties of the lower mantle. In this study we report for the first time the structure, Raman spectrum and elasticity of single crystals of Na-0.41[Na0.125Mg0.79Al0.085](2)[Al0.79Si0.21]O-12 NAL phase, synthesized at 2260 degrees C and 20 GPa. The single-crystal structure refinement of NAL, which is consistent with the space group P6(3)/m, reveals dynamic disorder of Na atoms along channels within the structure. The elastic tensor was experimentally determined at ambient conditions by Brillouin scattering spectroscopy. The elastic modulii obtained from the VoigtReuss-Hill approximation using the elastic constants determined in this study are K-S = 206 GPa and mu = 129 GPa, whereas the isotropic compressional and shear sound velocities are nu(P) = 9.9 km/s and nu(S) = 5.8 km/s. The NAL phase is elastically anisotropic, displaying 13.9% compressional and shear wave anisotropy. Elastic constants as well as Raman active modes of NAL have also been calculated using density-functional theory and density-functional perturbation theory