High-energy x-ray diffraction study of amorphous (Si_0.71Ge _0.29)O₂

Synthetic amorphous (Si_0.71Ge_0.29)O₂ has been investigated by high-energy synchrotron x-ray diffraction (E = 130.1 keV), in order to achieve a better understanding of amorphous structures built from the two network-forming oxides SiO₂ and GeO₂. The interatomic distances and bond angles that were determined are predominantly the same as in pure a-SiO₂ and a-GeO₂, respectively. According to the first coordination shell, the total coordination number C N (T O) = 3.4(5) (T = Si, Ge) indicates the existence of a considerable amount of cations that are connected to less than four oxygen ions. This result is clearly different with respect to the coordinations found in the structures of the respective pure amorphous oxides. If the second coordination shell is also taken into account, a second average (Ge-O) interatomic distance at r = 2.23(1) Å can be determined and C N (T O) increases to 4.2(1.1), indicating that all cations are actually connected to four anions. However, regular [GeO ₄] tetrahedra are no longer supposed to be the only possible germanium-oxygen coordination polyhedra in the structure investigated. Based on these findings, we infer that SiO₂ and GeO₂ predominantly form a homogeneous but not completely continuous random network.