A NON-DANGLING BOND MODEL OF PARAMAGNETIC DEFECTS IN GE-S GLASSES

The ESR spectra of the intrinsic defects in Ge-S glasses are studied at liquid nitrogen temperature in the whole glass-forming region. The Ge-related signal is found in glasses of all compositions and this signal is assigned to a two-atomic defect center (DCI) which is formed by the interaction of broken bonds of Ge with two-coordinated sulphur atoms forming (Ge-4...S-3)(0) centers, the subscript is for coordination, superscript for the charge, and the dots for a chemical bond of order 1/2. A multi-line spectrum is found not only in Ge-S glasses with excess of sulphur but also in glassy sulphur after excitation by Ar+ laser light (454 nm). This signal can be assigned to two defect centers: DCII (S-2... trans-S-3)(0) and DCIII (S-2... cis-S-3)(0) formed by interaction of a dangling bond of sulphur with a two-coordinated sulphur atom S in the S-n chains. Sulphur of these chains can be either in trans-coordination (DCII) or in the cis one (DCIII). The experimentally found compositional dependence of the DC densities is qualitatively explained. The proposed model is the first one which is able to explain the compositional dependence of the intrinsic paramagnetic defects in the whole glass-forming region. The model can also shed light on the absence of paramagnetic centers in other chalcogenide glasses.