title: New type of charged defect in amorphous chalcogenides.
abstract: We report on density-functional-based tight-binding simulations of a series of amorphous arsenic sulfide models. In addition to the charged coordination defects previously proposed to exist in chalcogenide glasses, a novel defect pair, [As(4)](-)-[S(3)](+), consisting of a fourfold coordinated arsenic site in a seesaw configuration and a threefold coordinated sulfur site in a near-planar trigonal configuration, was found in several models. The valence-alternation pairs [S(3)](+)-S-1 are converted into [As(4)](-)-[S(3)](+) pairs under HOMO-to-LUMO electronic excitation. This structural transformation is accompanied by a decrease in the size of the HOMO-LUMO band gap, which suggests that such transformations could contribute to photodarkening in these materials.
authors.affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, United Kingdom. sis24@cam.ac.uk
authors.forename: S I
authors.initials: SI
authors.lastname: Simdyankin
authors.forename: T A
authors.initials: TA
authors.lastname: Niehaus
authors.forename: G
authors.initials: G
authors.lastname: Natarajan
authors.forename: Th
authors.initials: T
authors.lastname: Frauenheim
authors.forename: S R
authors.initials: SR
authors.lastname: Elliott