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 As_0.30Se_0.70, As_0.35Se_0.65, and As_0.40Se_0.60. 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 As_0.30Se_0.70 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 As_0.35Se_0.65 and As_0.40Se_0.60 is, however, broken by the appearance of As-As bonds, the fraction of which increases with the arsenic content of the glass. For the As_0.40Se_0.60 material, a substantial fraction of As-As and Se-Se defect pairs (∼11%) is frozen into the network structure on glass formation.