No abstract

Structural transformations at high pressure in NaAlGe 3 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 Ge-O and R Ge...Ge ) were extracted assuming contributions of two scattering paths (Ge-O and Ge …Ge). We observed a significant extension of the Ge-O distance from 1.73 to 1.82 Å between 3 and ∼26 GPa, accompanied by an increase of the fitted number of nearest neighbors from ∼4 to ∼6. These observations can be attributed to the change from tetrahedral to octahedral Ge coordination. Second shell bond distances Ge …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 Å 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 NaAlGe 3 O 8 beyond 34 GPa is considerably higher than the one reported for amorphous GeO 2 from x-ray diffraction analysis but it is similar to the one reported for the Ge octahedron in crystalline rutile-type GeO 2 . We attribute the high compressibility of the Ge-O bond in NaAlGe 3 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 NaAlGe 3 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.

No abstract