Ge coordination in NaAlGe3O8 glass upon compression to 131 GPa

Article

关键词: POLYCRYSTALLINE DIAMOND;    RUTILE-TYPE;    SILICATE;    SI;    CRYSTALLINE;    MELTS;   

DOI  :  10.1103/PhysRevB.101.214103

来源: SCIE

【 摘 要 】

Structural transformations at high pressure in NaAlGe3O8 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 (RGe-O and R-Ge center dot center dot center dot Ge) were extracted assuming contributions of two scattering paths (Ge-O and Ge center dot center dot center dot Ge). We observed a significant extension of the Ge-O distance from 1.73 to 1.82 angstrom between 3 and similar to 26 GPa, accompanied by an increase of the fitted number of nearest neighbors from similar to 4 to similar 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 angstrom 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 NaAlGe3O8 beyond 34 GPa is considerably higher than the one reported for amorphous GeO2 from x-ray diffraction analysis but it is similar to the one reported for the Ge octahedron in crystalline rutile-type GeO2. We attribute the high compressibility of the Ge-O bond in NaAlGe3O8 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 NaAlGe3O8 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.

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