The structure of gallium phosphate glasses by high-energy X-ray diffraction

X-ray diffraction experiments are used to obtain short-range order information of gallium phosphate glasses of meta- and pyrophosphate compositions. Parameters of the first-neighbor peaks, such as coordination numbers and distances, are obtained. A strong decrease of the Ga-O coordination number from 6.0+/-0.2 to 4.6+/-0.2 upon Ga2O3 addition is found, which is accompanied by a shortening of the Ga-O distances. Only GaO6 Octahedra exist at the metaphosphate composition. Close to the pyrophosphate composition, the majority of Ga atoms occupies already tetrahedral sites. The Ga-O coordination number behaves equivalent with the ratio M-TO = n(OT)/n(Ga), thus, with the number n(O-T) of terminal oxygen atoms (O-T) in phosphoruS-O-T bonds which are available for the coordination of each Ga atom. Thus, P-O-T-Ga bridges are formed for all OT atoms. The GaOn polyhedra neither share O-T atoms nor form Ga-O-Ga bridges. With increasing fraction of GaO4 tetrahedra and decreasing lengths of the phosphate chains the network expands.