An EXAFS structural approach of the lanthanum-gallium-sulfur glasses

Three La: Ga:S glassy compositions spanning the range of the vitreous region, have been investigated by EXAFS (gallium K-edge, lanthanum LIII edge). As a first result, the gallium is found tetracoordinated for any composition and the GaS4 polyhedra are the glassy former units (average GaS distances = 2.27 Å). The lanthanum modifying cation has the same mean surrounding as in the crystalline state, but with more dispersed LaS distances (average LaS surrounding = seven first neighbours, situated from 2.91 to 2.93 Å). However disordered, this environment is well defined. A structural model is then proposed as a covalent network of GaS4 tetrahedra, intercalated by the essentially ionic LaS channels. This model is close to the modified random network usually presented for the oxide glasses.