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(O_T)/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 O_T atoms. The GaO_n 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.