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Persistent Octahedral Coordination in Amorphous GeO2 Up to 100 GPa by Kβ X-Ray Emission Spectroscopy

G. Spiekermann, M. Harder, K. Gilmore, P. Zalden, Ch. J. Sahle, S. Petitgirard, M. Wilke, N. Biedermann, C. Weis, W. Morgenroth, J. S. Tse, E. Kulik, N. Nishiyama, H. Yavaş, and C. Sternemann
Phys. Rev. X 9, 011025 – Published 6 February 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.

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  • Received 30 September 2017
  • Revised 23 November 2018

DOI:https://doi.org/10.1103/PhysRevX.9.011025

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

G. Spiekermann1,2,3,*, M. Harder2, K. Gilmore4, P. Zalden5, Ch. J. Sahle4, S. Petitgirard6, M. Wilke1, N. Biedermann1,5, C. Weis7, W. Morgenroth8, J. S. Tse9, E. Kulik2, N. Nishiyama2,10, H. Yavaş2,11, and C. Sternemann7

  • 1Institut für Geowissenschaften, Universität Potsdam, 14476 Potsdam, Germany
  • 2Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany
  • 3Deutsches GeoForschungsZentrum GFZ, 14473 Potsdam, Germany
  • 4European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38043 Grenoble, France
  • 5European XFEL, 22869 Schenefeld, Germany
  • 6Universität Bayreuth, Bayerisches Geoinstitut BGI, 95447 Bayreuth, Germany
  • 7Fakultät Physik/DELTA, Technische Universität Dortmund, 44221 Dortmund, Germany
  • 8Institut für Geowissenschaften, Universität Frankfurt, 60438 Frankfurt am Main, Germany
  • 9Department of Physics, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada
  • 10Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
  • 11Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA

  • *geospiek@uni-potsdam.de

Popular Summary

The structure of amorphous germanium dioxide (GeO2) at high pressure is of great interest to geophysicists because it is a structural analog to silicon dioxide (SiO2), the main component of molten minerals in Earth’s mantle. Researchers debate whether the compaction mechanism in the amorphous oxides significantly deviates from that of their crystalline forms. Can the average coordination number—a parameter that describes the number of nearest oxygen neighbors to germanium—be higher in amorphous GeO2 than in its crystalline polymorphs? Here, we present x-ray emission spectra of high-pressure noncrystalline GeO2 that help us to answer to this question.

We compress glassy GeO2 stepwise up to a pressure of 100 GPa in diamond anvil cells. At each pressure step, we record germanium valence-to-core x-ray emission spectra. The Kβ′′ emission line, which is part of the spectra, shows distinct changes in energy and intensity. This allows us to deduce the average germanium–oxygen bond distance as well as the average coordination number. We find that the coordination number remains predominantly 6, as in all known high-pressure crystalline GeO2 polymorphs. This finding supports the concept that compositional differences, rather than differences in relative coordination numbers, are responsible for the high densities of melts in Earth’s lower mantle.

The Kβ′′ emission line, present in the x-ray emission spectra of many compounds, is highly sensitive to physical structure. Although known for many years, its potential had not been fully developed. Therefore, our approach for interpreting this line will be useful in a broad range of material science, where an x-ray probe is required to investigate the structure of amorphous material.

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Vol. 9, Iss. 1 — January - March 2019

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