Cluster simulations of structural transformations in yellow arsenic

Abstract

Yellow arsenic (y-As) consists of tetrahedral As 4 molecules that may be packed in some amorphous and crystalline structures. Like many other arsenic structures, y-As is metastable and undergoes irreversible transitions (polymerization) under irradiation. The process of y-As polymerization, which is observed experimentally, usually leads to the formation of amorphous arsenic (a-As) possessing a continuous random network structure. Our previous quantum chemical simulation for an eight-atom cluster model performed using semi-empirical CNDO/BW approach, combined with optimization technique of cyclic coordinate descent, have shown a formation of molecular dimers due to breaking of one bond in each tetrahedral As 4 molecule accompanied with bond switching over to chair-like structure. In that case a pair of approaching As 4 molecules is positioned in a staggered “face-to-face” configuration ( D 2 d symmetry), which may be considered as a conformation with a six-membered chair-shaped ring dominating in the structure of a polymerized a-As. Two energetically preferable configurations of an As 8 cluster have been found here after careful semi-empirical optimization of the face-to-face structure: they possess either cubane configuration ( O h symmetry) or eclipsed “edge-to-edge” configuration ( D 2 h symmetry). For the first time, a two-dimensional energy surface E( z, θ) has been calculated in order to analyze possible paths of structural transformations in y-As. Analogous qualitative results have been just obtained by us when using ab initio Hartree–Fock method combined with the electron correlation corrections for eight-atom cluster model of molecular arsenic. A comparison of both quantum chemical simulations, together with data from previous experimental studies allows us to describe a possible mechanism of the initial stage of the polymerization of y-As.