Ge coordination in NaAlGe3O8 glass upon compression to 131 GPa

Structural transformations at high pressure in $\mathrm{NaAlG}{\mathrm{e}}_{3}{\mathrm{O}}_{8}$ glass were investigated by means of x-ray absorption spectroscopy at the Ge $K$ edge in combination with a diamond anvil cell. The obtained results provide a detailed picture of the local structural behavior of Ge in a chemically complex glass under compression. First and second shell bond distances (${R}_{\text{Ge-O}}$ and ${R}_{\mathrm{Ge}...\mathrm{Ge}}$) were extracted assuming contributions of two scattering paths (Ge-O and Ge\dots{}Ge). We observed a significant extension of the Ge-O distance from 1.73 to 1.82 \AA{} between 3 and $\ensuremath{\sim}26\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, accompanied by an increase of the fitted number of nearest neighbors from $\ensuremath{\sim}4$ to $\ensuremath{\sim}6$. These observations can be attributed to the change from tetrahedral to octahedral Ge coordination. Second shell bond distances Ge\dots{}Ge are also consistent with this structural transformation. Between 34 and 131 GPa, the evolution of the fitted Ge-O distance implies a gradual volume reduction of the Ge octahedra. At the highest probed pressure of 131 GPa a Ge-O distance of 1.73 \AA{} was found, which is similar to the one obtained at ambient conditions for Ge in fourfold coordination. The compressibility of the Ge-O octahedron in $\mathrm{NaAlG}{\mathrm{e}}_{3}{\mathrm{O}}_{8}$ beyond 34 GPa is considerably higher than the one reported for amorphous $\mathrm{Ge}{\mathrm{O}}_{2}$ from x-ray diffraction analysis but it is similar to the one reported for the Ge octahedron in crystalline rutile-type $\mathrm{Ge}{\mathrm{O}}_{2}$. We attribute the high compressibility of the Ge-O bond in $\mathrm{NaAlG}{\mathrm{e}}_{3}{\mathrm{O}}_{8}$ glass to the presence of Al and Na that increase the system's complexity and therefore its degrees of freedom. Beyond 110 GPa the data on $\mathrm{NaAlG}{\mathrm{e}}_{3}{\mathrm{O}}_{8}$ glass indicate the onset of polyhedral distortion. The performed study provides insights into the structural changes of complex and polymerized germanate glasses or melts at extreme pressure conditions.