@article{SublettSendulaLamadridetal.2019,
  author    = {Sublett, David Matthew and Sendula, Eszter and Lamadrid, Hector and Steele-MacInnis, Matthew and Spiekermann, Georg and Burruss, Robert C. and Bodnar, Robert J.},
  title     = {Shift in the Raman symmetric stretching band of N-2, CO2, and CH4 as a function of temperature, pressure, and density},
  series = {Journal of Raman spectroscopy : JRS},
  volume    = {51},
  journal   = {Journal of Raman spectroscopy : JRS},
  number    = {3},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {0377-0486},
  doi       = {10.1002/jrs.5805},
  pages     = {555 -- 568},
  year      = {2019},
  abstract  = {The Raman spectra of pure N-2, CO2, and CH4 were analyzed over the range 10 to 500 bars and from -160 degrees C to 200 degrees C (N-2), 22 degrees C to 350 degrees C (CO2), and -100 degrees C to 450 degrees C (CH4). At constant temperature, Raman peak position, including the more intense CO2 peak (nu+), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature (PT) range studied. At constant pressure, the peak position for CO2 and CH4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N-2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N-2 is located between 0 degrees C and 50 degrees C at pressures above similar to 50 bars and is pressure dependent. Below similar to 50 bars, the inflection temperature was observed as low as -120 degrees C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N-2, CO2, and CH4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature-dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N-2-CO2-CH4 gas systems in a future study.},
  language  = {en}
}
@article{SpiekermannHarderGilmoreetal.2019,
  author    = {Spiekermann, Georg and Harder, M. and Gilmore, Keith and Zalden, Peter and Sahle, Christoph J. and Petitgirard, Sylvain and Wilke, Max and Biedermann, Nicole and Weis, Thomas and Morgenroth, Wolfgang and Tse, John S. and Kulik, E. and Nishiyama, Norimasa and Yava{\c{s}}, Hasan and Sternemann, Christian},
  title     = {Persistent Octahedral Coordination in Amorphous GeO₂ Up to 100 GPa by Kβ'' X-Ray Emission Spectroscopy},
  series = {Physical Review X},
  volume    = {9},
  journal   = {Physical Review X},
  number    = {1},
  publisher = {American Physical Society by the American Institute of Physics},
  address   = {Melville, NY},
  issn      = {2469-9926},
  doi       = {10.1103/PhysRevX.9.011025},
  pages     = {10},
  year      = {2019},
  abstract  = {We measure valence-to-core x-ray emission spectra of compressed crystalline GeO₂ up to 56 GPa and of amorphous GeO₂ up to 100 GPa. In a novel approach, we extract the Ge coordination number and mean Ge-O distances from the emission energy and the intensity of the Kβ'' emission line. The spectra of high-pressure polymorphs are calculated using the Bethe-Salpeter equation. Trends observed in the experimental and calculated spectra are found to match only when utilizing an octahedral model. The results reveal persistent octahedral Ge coordination with increasing distortion, similar to the compaction mechanism in the sequence of octahedrally coordinated crystalline GeO₂ high-pressure polymorphs.},
  language  = {en}
}
@article{PetitgirardSahleWeisetal.2019,
  author    = {Petitgirard, Sylvian and Sahle, C. J. and Weis, C. and Gilmore, K. and Spiekermann, Georg and Tse, J. S. and Wilke, Max and Cavallari, C. and Cerantola, V and Sternemann, Christian},
  title     = {Magma properties at deep Earth's conditions from electronic structure of silica},
  series = {Geochemical perspectives letters},
  volume    = {9},
  journal   = {Geochemical perspectives letters},
  publisher = {Association of Geochemistry},
  address   = {Paris},
  issn      = {2410-339X},
  doi       = {10.7185/geochemlet.1902},
  pages     = {32 -- 37},
  year      = {2019},
  abstract  = {SiO(2 )is the main component of silicate melts and thus controls their network structure and physical properties. The compressibility and viscosities of melts at depth are governed by their short range atomic and electronic structure. We measured the O K-edge and the Si L-2,L-3-edge in silica up to 110 GPa using X-ray Raman scattering spectroscopy, and found a striking match to calculated spectra based on structures from molecular dynamic simulations. Between 20 and 27 GPa, Si-[4] species are converted into a mixture of Si-[5] and Si-[6] species and between 60 and 70 GPa, Si-[6] becomes dominant at the expense of Si-[5] with no further increase up to at least 110 GPa. Coordination higher than 6 is only reached beyond 140 GPa, corroborating results from Brillouin scattering. Network modifying elements in silicate melts may shift this change in coordination to lower pressures and thus magmas could be denser than residual solids at the depth of the core-mantle boundary.},
  language  = {en}
}
@article{PetitgirardSpiekermannGlazyrinetal.2019,
  author    = {Petitgirard, Sylvain and Spiekermann, Georg and Glazyrin, Konstantin and Garrevoet, Jan and Murakami, Motohiko},
  title     = {Density of amorphous GeO2 to 133 GPa with possible pyritelike structure and stiffness at high pressure},
  series = {Physical review : B, Condensed matter and materials physics},
  volume    = {100},
  journal   = {Physical review : B, Condensed matter and materials physics},
  number    = {21},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {2469-9950},
  doi       = {10.1103/PhysRevB.100.214104},
  pages     = {8},
  year      = {2019},
  abstract  = {Germanium oxide is a prototype network-forming oxide with pressure-induced structural changes similar to those found in crystals and amorphous silicate oxides at high pressure. Studying density and coordination changes in amorphous GeO2 allows for insight into structural changes in silicate oxides at very high pressure, with implications for the properties of planetary magmas. Here, we report the density of germanium oxide glass up to 133 GPa using the x-ray absorption technique, with very good agreement with previous experimental data at pressure below 40 GPa and recent calculation up to 140 GPa. Our data highlight four distinct compressibility domains, corresponding to changes of the local structure of GeO2. Above 80 GPa, our density data show a compressibility and bulk modulus similar to the counterpart crystal phase, and we propose that a compact distorted sixfold coordination, similar to the structural motif of the pyritelike crystalline GeO2 polymorph, is likely to be stable in that pressure range. Our density data point to a smooth continuous evolution of the average coordination for pressure above 20 GPa with persistent sixfold coordination, without sharp density or density slope discontinuities. These observations are in very good agreement with theoretical calculations and spectroscopic measurements, and our results indicate that glasses and melts may behave similarly to their high-pressure solid counterparts with comparable densities, compressibility, and possibly average coordination.},
  language  = {en}
}