Persistent Octahedral Coordination in Amorphous GeO2 Up to 100 GPa by Kβ′′

Abstract

We measure valence-to-core x-ray emission spectra of compressed crystalline GeO 2 up to 56 GPa and of amorphous GeO 2 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 00 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 GeO 2 high-pressure polymorphs.

Density of amorphous GeO2 to 133 GPa with possible pyritelike structure and stiffness at high pressure

“…The change in compressibility in a-GeO 2 around 80 GPa is of special interest, as two contradictory coordination evolution paths have been suggested in this pressure range: A change to sevenfold coordination [41] or a distortion of sixfold octahedron [18]. Indeed, a distortion, or rotation, of the GeO 2 octahedrons can bring oxygen atoms closer to the Ge atoms (bond shortening), without necessarily increasing the CN above 6, as is the case in the crystalline structure evolution from rutile to CaCl 2 to α-PbO 2 to the pyrite structure [51].…”

“…Higher coordination, with CN > 6, in the compressed glass has been recently claimed [16,17]; however, a consensus has not yet been found whether the glass could be denser than the crystalline structures, when reaching such higher coordination. In this context, quantitative density measurements shed light on how to link the atomic structure arrangement and the macroscopic behavior of glasses and melts [12,[18][19][20][21]. We emphasize that density measurements are of prime importance for the discussion of macroscopic properties of such materials with application to planetary interiors and solid state physics.…”

“…The density increase in compressed amorphous oxides, determined by tracking volumetric changes, goes along with changes of their atomic scale structure (cation-anion distances and average coordination), which can be probed by several x-ray techniques, such as x-ray total scattering data (x-ray diffraction, XRD) [17,19,27,28] and x-ray absorption spectroscopy (XAS) [10] or x-ray Raman spectroscopy (XRS) [12,14], x-ray emission spectroscopy through the valenceto-core emission (vtc-XES) [18], and indirectly by optical spectroscopy techniques [29][30][31][32]. A general consensus has been reached on the pressure range of the transition from a tetrahedral coordination with CN = 4 to a closest-packed octahedral structure with CN = 6.…”

“…In SiO 2 this transformation to CN ∼ 6 is completed at about 40-60 GPa [12,13,27,33]. For GeO 2 , the transition from CN = 4 to CN = 6 takes place between 0 and ∼12 GPa, documented by a variety of techniques like XAS [10,11,34], XRS [14], XES [18], x-ray total scattering (XRD), and neutron diffraction [16,[35][36][37][38][39].…”

“…On one hand, recent x-ray total scattering data (XRD) proposed a sharp increase in coordination of Ge beyond 6, with CN = 6.5 at 50 GPa and CN > 7 at 90 GPa [16]. On the other hand, a recent x-ray emission spectroscopy (XES) [18] study observed a plateau of CN = 6 up to 100 GPa, while ab initio molecular dynamic (AIMD) calculations [41] show a gradual increase of the CN above 6 only starting at 80 GPa, reaching CN = 6.5 for pressure beyond 120 GPa. Finally, the only existing density study for amorphous GeO 2 (a-GeO 2 ) up to 55 GPa suggests a possible density crossover at 50 GPa [11], but this remains elusive because of a lack of data at high pressure to conclude on such crossover.…”