@article{WeisSpiekermannSternemannetal.2018,
  author    = {Weis, Christopher and Spiekermann, Georg and Sternemann, Christian and Harder, Manuel and Vanko, Gyorgy and Cerantola, Valerio and Sahle, Christoph J. and Forov, Yury and Sakrowski, Robin and Kupenko, Ilya and Petitgirard, Sylvain and Yavas, Hasan and Bressler, Christian and Gawelda, Wojciech and Tolan, Metin and Wilke, Max},
  title     = {Combining X-ray K beta(1,3), valence-to-core, and X-ray Raman spectroscopy for studying Earth materials at high pressure and temperature},
  series = {Journal of analytical atomic spectrometry},
  volume    = {34},
  journal   = {Journal of analytical atomic spectrometry},
  number    = {2},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {0267-9477},
  doi       = {10.1039/c8ja00247a},
  pages     = {384 -- 393},
  year      = {2018},
  abstract  = {X-ray emission and X-ray Raman scattering spectroscopy are powerful tools to investigate the local electronic and atomic structure of high and low Z elements in situ. Notably, these methods can be applied for in situ spectroscopy at high pressure and high temperature using resistively or laser-heated diamond anvil cells in order to achieve thermodynamic conditions which appear in the Earth's interior. We present a setup for combined X-ray emission and X-ray Raman scattering studies at beamline P01 of PETRA III using a portable wavelength-dispersive von Hamos spectrometer together with the permanently installed multiple-analyzer Johann-type spectrometer. The capabilities of this setup are exemplified by investigating the iron spin crossover of siderite FeCO3 up to 49.3 GPa by measuring the Fe M2,3-edge and the Fe Kβ1,3 emission line simultaneously. With this setup, the Fe valence-to-core emission can be detected together with the Kβ1,3 emission line providing complementary information on the sample's electronic structure. By implementing a laser-heating device, we demonstrate the strength of using a von Hamos type spectrometer for spin state mapping at extreme conditions. Finally, we give different examples of low Z elements' absorption edges relevant for application in geoscience that are accessible with the Johann-type XRS spectrometer. With this setup new insights into the spin transition and compression mechanisms of Earth's mantle materials can be obtained of importance for comprehension of the macroscopic physical and chemical properties of the Earth's interior.},
  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}
}
@misc{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 = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam Mathematisch-Naturwissenschaftliche Reihe},
  number    = {699},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-42775},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-427755},
  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{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}
}