Letter to the Editor Evidence for valence alternation, and a new structural model of amorphous selenium Xiaodong Zhang, D.A. Drabold * Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA Received 2 June 1998; received in revised form 11 August 1998 Abstract A molecular-dynamics simulation has been carried out for amorphous selenium. The simulation used 64 atoms in a constant volume simple cubic cell. The pair correlation function, gr, and structure factor, S Q, were computed and compared with experimental and previous theoretical studies. The average coordination number is exactly 2. Only one defect, an intimate valence alternation pair type defect was produced. The electronic density of states and the local- ization of the electronic state also manifest some properties of valence alternation pair defect type. The results give new evidence for valence alternation in a-Se. Ó 1998 Published by Elsevier Science B.V. All rights reserved. PACS: 61.43.Dq; 71.23.-k; 71.23.Cq 1. Introduction The photoconducting and semiconducting properties of amorphous Se [1,2] are connected with the properties and concentration of defects in the structure. In spite of the accumulation of much experimental data, the atomic structure of a-Se remains controversial. Among the numerous studies on a-Se, the models by Street and Mott [3], Kastner et al. [4] have received considerable at- tention. They postulated that for amorphous se- lenium defects with the smallest energy were charged pairs consisting of a positively charged triply coordinated atom C  3 and a negatively charged singly coordinated atom C  1 , `valence al- ternation pairs' (VAP). Here C denotes a chalco- gen atom, and subscripts and superscripts represent coordination number and charge state, respectively. Most of the C  3 and C  1 were conjec- tured to be bound together to form `intimate' VAP (IVAP). This model is successful in explaining the majority of experimental facts and is widely used. There have been many experimental results to support the VAP model [5]. First-principles molecular-dynamics simulation using Car±Parrinello(CP) method [6] by Hohl and Jones [7] suggested that single threefold coordi- nated atoms C 0 3 are the most numerous defect type. We noticed that the concentration of defects atoms is 23% in Hohl's calculation. As to our know- ledge, all previous theoretical models have con- tained well over 10% coordination defects, so that the defects cannot be easily analyzed as isolated entities. Such samples bear only limited relevance to real a-Se, in part because this arti®cial defect± defect interaction causes a reduction in the local- Journal of Non-Crystalline Solids 241 (1998) 195±199 * Corresponding author. Tel.: +1-740 593 1715; fax: +1-740 593 0433; e-mail: drabold@ohiou.edu 0022-3093/98/$ ± see front matter Ó 1998 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 8 0 7 - 2