Evidence of fivefold-coordinated Ge atoms in amorphous GeO${}_2$ under pressure using inelastic x-ray scattering

Elucidating pressure-induced structural transitions in glasses at a microscopic level is a formidable challenge. Here, the local environment of amorphous germania ($a$-GeO${}_2$) is investigated, from room pressure up to 18 GPa, using in situ inelastic x-ray-scattering measurements at the oxygen $K$-edge. The appearance of GeO${}_5$ units at intermediate pressures, a debated yet unsolved issue in the literature, is probed experimentally and their amount quantified. It is found that these units are minority species at all the pressures investigated, discarding the previously proposed fully pentacoordinated glassy state. The presence of GeO${}_5$ units, which occur as transient species as the glass undergoes a transition from the tetrahedral to octahedral Ge-coordination states, provides a microscopic basis for the understanding of glass behavior at intermediate pressures.

Figure 1

O $K$-edge IXS spectra for the (a) crystalline references in ambient conditions and (b) $a$-GeO${}_2$ for pressures ranging from 1 bar to 18 GPa. The energy range delimited by the two dashed lines at 531 and 533 eV highlights the region dominated by the GeO${}_5$ contributions.

Figure 2

Best fits of the O $K$-edge IXS spectra of $a$-GeO${}_2$ at 5 GPa using a linear combination of (a) α-quartz GeO${}_2$, rutile GeO${}_2$, and K${}_2$Ge${}_8$O${}_{17}$ crystals; and (b) α-quartz GeO${}_2$ and rutile GeO${}_2$ only. The spectra were fitted in the energy range 531–545 eV, shown by the two dashed lines. The difference in intensity between the amorphous (blue) and the crystalline samples (red) for energies above 545 eV results from the presence of the DAC environment in the former case.

Figure 3

(a) Experimental evolution of the relative proportion of ${}^{\left[4\right]}$Ge, ${}^{\left[5\right]}$Ge, and ${}^{\left[6\right]}$Ge as a function of pressure (bottom scale) and density (top scale). The plain colored areas show the maximum margin of error in the percentage determination. (b) Evolution of the average Ge-O coordination number as a function of pressure calculated from our IXS measurements and compared to data from the literature.