ASYMMETRIES IN LOCAL BONDING SITES IN AMORPHOUS-SEMICONDUCTORS - VERYHIGH-FIELD NMR OF AS-75

Nuclear magnetic resonance of As-75 in crystalline and glassy samples of As2S3 has been studied at magnetic fields up to 22 T. The results y ield asymmetries in the electric field gradient at the arsenic sites t hat disagree with those inferred from previous smaller field studies. These results are also inconsistent with the predictions of a common m odel (raft model) for the structure of glassy As2S3 and other chalcoge nide glasses. Specifically, the As sites in crystalline and glassy As2 S3 are nearly axially symmetric tall three chalcogen-arsenic-chalcogen bond angles are very nearly equal because the electric field gradient s are nearly axially symmetric). The crystalline results are in agreem ent with the X-ray structure determinations, but in the glasses there is no evidence for the asymmetric sites suggested to exist at the edge s of the 'outrigger rafts' in the raft model. (C) 1998 Elsevier Scienc e B.V. All rights reserved.