Structure of As-Se glasses by neutron diffraction with isotope substitution.

The method of neutron diffraction with selenium isotope substitution is used to measure the structure of glassy AsSe, AsSe, and AsSe. The method delivers three difference functions for each sample in which the As-As, As-Se, or Se-Se correlations are eliminated. The measured coordination numbers are consistent with the "8-N" rule and show that the AsSe network is chemically ordered, a composition near to which there is a minimum in the fragility index and a boundary to the intermediate phase. Chemical ordering in glassy AsSe and AsSe is, however, broken by the appearance of As-As bonds, the fraction of which increases with the arsenic content of the glass. For the AsSe material, a substantial fraction of As-As and Se-Se defect pairs (∼11%) is frozen into the network structure on glass formation.