Network vs Molecular Structural Characteristics of Ge-Doped Arsenic Sulfide Glasses: A Combined Neutron/X-ray Diffraction, Extended X-ray Absorption Fine Structure, and Raman Spectroscopic Study

Combined neutron/X-ray diffraction, Ge and As K-edge extended X-ray absorption fine structure analysis, and Raman spectroscopy are employed to study the compositional dependence of the short- and intermediate-range structures of As-rich GexAsyS100-x-y glasses with a constant Ge:As atomic ratio of 1:17.3. The structures of glasses with compositions near stoichiometry (35 <= x + y <= 43) are dominated by the presence of a predominantly heteropolar-bonded As2S3 network. However, an increasing metal content (x + y = 55) results in a novel glass consisting predominantly of As4S3 molecules, held together by van der Waals forces. The formation of this molecular glass implies an apparently anomalous situation of near-zero connectivity and dimensionality with increasing average coordination number. A further increase in metal content (60 <= x + y <= 65) results in the formation of As-As homopolar-bonded structural regions that coexist with As4S3 molecules. Such unusual evolution of intermediate-range order is shown to be closely related to the compositional variation of thermophysical properties and density fluctuation in these glasses.