Ronald

Cohen

The thermal equation of states for fcc iridium (Ir) is obtained from first-principles molecular dynamics up to 3000 K and 540 GPa. The equation of state (EoS) is globally fitted to a simplified free-energy model and various parameters are derived. The theoretical principal Hugoniot is compared with shock wave experiments, where discrepancy suggests formation of new Ir phases. A few representative EoS parameters, such as bulk modulus KT, thermal expansivity alpha, Gruneisen parameter gamma, constant pressure capacity CP, and Debye temperature OD, are computed to compare with experimental data.

Subduction of oceanic lithosphere transports surface H2O into the mantle. Recent studies show that dense SiO2 in the form of stishovite, an abundant mineral in subducted oceanic crust at depths greater than -270 km, has the potential to host and transport a considerable amount of H2O into the lower mantle, but the H2O storage capacity of SiO2 phases at high pressure and temperature remains uncertain. We investigate the hydration of stishovite and its higher-pressure polymorphs, beta-stishovite and seifertite, with in situ X-ray diffraction experiments at high pressures and temperatures. The H2O contents in SiO2 phases are quantified based on observed increases in unit cell volume relative to the anhydrous SiO2 system. Density functional theory (DFT) computations permit calibration of water content as a function of volume change based on interstitial substitution of H2O. Regression of our experimental data indicates an H2O storage capacity in stishovite of -3.5 wt% in the transition zone and shallow lower mantle, decreasing to about 0.8 wt% at the base of the mantle. We find that SiO2-bearing subducted oceanic crust can accommodate all the H2O in slab lithosphere that survives sub-arc dehydration. Hydration of silica phases in subducted oceanic crust and their unparalleled capacity to host significant amounts of H2O even at high mantle temperatures provides a unique mechanism for transport and storage of water in the deepest mantle. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

Using density functional perturbation theory, we computed the phonon frequencies and Raman and IR activities of hafnia polymorphs (P4(2)nmc, Pca2(1), Pmn2(1), Pbca OI, brookite, and baddeleyite) for phase identification. We investigated the evolution of Raman and IR activities with respect to epitaxial strain and provide plots of frequency differences as a function of strain for experimental calibration and identification of the strain state of the sample. We found Raman signatures of different hafnia polymorphs: omega ( A(1g) ) = 300 cm(-1) for P4(2)nmc, omega (A( 1)) = 343 cm(-1) for Pca2(1), omega ( B-2) = 693 cm(-1) for Pmn2(1), omega (A( g)) = 513 cm(-1) for Pbca (OI), omega (A(g)) = 384 cm(-1) for brookite, and omega (A(g)) = 496 cm(-1) for baddeleyite. We also identified the Raman B-1g mode, an anti-phase vibration of dipole moments [omega (B-1g) = 758 cm(-1) for OI and omega ( B-1g ) = 784 cm(-1) for brookite], as the Raman signature of antipolar Pbca structures. We calculated a large splitting between the longitudinal optical and transverse optical modes [delta omega(LO) - TO ( A(1)(z)) = 255 cm(-1) in Pca2(1) and delta omega( LO) (- TO) ( A 1 ) = 263 cm(-1) in Pmn2(1)] to the same order as those observed in perovskite ferroelectrics and related them to the anomalously large Born effective charges of Hf atoms [ Z * ( Hf ) = 5.54]. Published under an exclusive license by AIP Publishing.

We examine the thermodynamic and dynamic stability (i.e., phonon dispersion) of mixed silicon-carbon clathrate frameworks using first-principles calculations as a function of pressure and composition. Silicon atoms were substituted on special framework Wyckoff positions in the Type-I and Type-II empty carbon clathrate structures over a broad compositional range, and the enthalpies of the mixed clathrates were compared to pure silicon and carbon clathrates, as well as the thermodynamic ground states. While all mixed clathrates examined were found to be metastable with respect to elemental formation components and/or silicon carbide, certain empty binary host lattices are found to be lower-energy phases than the pure-component clathrate endmembers at high pressure, in particular Type-I C22Si24 and Type-II C32Si104. This enhanced energetic stability is rationalized by a decrease of energy upon doping specific crystallographic positions. When occupied by small guest ions like Li+ and Na+, these mixed C-Si clathrate structures exhibit minima in their formation enthalpies under high-pressure conditions, providing insights into potential synthetic pathways.

This paper documents the seventeenth data release (DR17) from the Sloan Digital Sky Surveys; the fifth and final release from the fourth phase (SDSS-IV). DR17 contains the complete release of the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, which reached its goal of surveying over 10,000 nearby galaxies. The complete release of the MaNGA Stellar Library accompanies this data, providing observations of almost 30,000 stars through the MaNGA instrument during bright time. DR17 also contains the complete release of the Apache Point Observatory Galactic Evolution Experiment 2 survey that publicly releases infrared spectra of over 650,000 stars. The main sample from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), as well as the subsurvey Time Domain Spectroscopic Survey data were fully released in DR16. New single-fiber optical spectroscopy released in DR17 is from the SPectroscipic IDentification of ERosita Survey subsurvey and the eBOSS-RM program. Along with the primary data sets, DR17 includes 25 new or updated value-added catalogs. This paper concludes the release of SDSS-IV survey data. SDSS continues into its fifth phase with observations already underway for the Milky Way Mapper, Local Volume Mapper, and Black Hole Mapper surveys.