FORMATION OF AS-8 DIMERS IN MOLECULAR SOLID-STATE ARSENIC

Molecular yellow arsenic (y-As) consists of tetrahedral As-4 molecules that may be packed in various ways. All y-As modifications, both diso rdered and crystalline, are metastable and undergo irreversible transi tions (polymerization) under a action of heat and light, which cause a change in the nature of bonding in the molecules. Polymerization of y -As leads to the formation of amorphous arsenic (a-As) possessing a co ntinuous random network structure. DTA studies show that polymerizatio n is an activated exothermic process. The value of its enthalpy agrees satisfactorily with an estimate of the excess energy of strained 'ban ana-shaped' bonds in an As-4 molecule. Quantum chemical calculations, applying the semiempirical CNDO/BW approach, show that at the initial polymerization stage we have a formation of As-8 dimers due to cleavag e of one bond in the tetrahedral AS(4) molecule. Simulation of this pr ocess shows that formation of a stable As-8 cluster, possessing either D-2h or O-h symmetry, may take place if the dimerization reaction pat h possesses D-2d symmetry. In this case a pair of approaching molecule s is positioned in staggered 'face-to-face' configuration, which may b e considered as a conformation with a six-membered chair-shaped ring d ominating in the structure of polymerized a-As. The most favourable is found to be a molecular As-8 dimer with eclipsed 'edge-to-edge' confi guration (D-2h symmetry). (C) 1997 Elsevier Science B.V.