Asymmetries in local bonding sites in amorphous semiconductors:: very high field NMR of 75As

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 yield asymmetries in the electric field gradient at the arsenic sites that disagree with those inferred from previous smaller field studies. These results are also inconsistent with the predictions of a common model (raft model) for the structure of glassy As2S3 and other chalcogenide glasses. Specifically, the As sites in crystalline and glassy As2S3 are nearly axially symmetric tall three chalcogen-arsenic-chalcogen bond angles are very nearly equal because the electric field gradients are nearly axially symmetric). The crystalline results are in agreement with the X-ray structure determinations, but in the glasses there is no evidence for the asymmetric sites suggested to exist at the edges of the 'outrigger rafts' in the raft model. (C) 1998 Elsevier Science B.V. All rights reserved.