Atomic models of amorphous selenium

Abstract

A critical review of the literature has been carried out on the structure of amorphous selenium. Two molecular models were then constructed to follow the conclusions derived from the survey and to also fit newly obtained radial distribution functions for bulk quenched amorphous selenium. These computer models followed four criteria: (1) the molecules were disordered chains; (2) the chains were packed to maximize Van der Waals attractions; (3) the close-packing was achieved by letting the magnitude of the dihedral angle vary slightly and by letting the phase of the dihedral angle vary randomly; (4) closed rings were assumed to be few in number and thus negligible in fitting the RDF. It was found that the percentage of cis and trans configurations in the molecules could vary widely and still be consistent with both the radial distribution function and with physical criteria. Thus the cis/trans ratio, or the phase of the dihedral angle, is not unique for amorphous selenium. The structure of a-Se may be described as consisting of coiled disordered chains which are packed in such a way as to minimize the Van der Waals bond lengths in a minimum cluster which has a radius of 11 Å.