Solitonic Generation of Valence Alternation Pairs in As₂S₃

Abstract

The crystal structure of As₂S₃ has degeneracies of two kinds as shown in a previous paper. By inserting locally other possible degenerate structures to a ground state, valence alternation pairs (VAP) can be generated as soliton like defects. The degeneracies concerning the arrangement of lone pairs and the directions of interhelix covalent bonds lead, respectively, to the generations of the VAPs of C₁–C₃ and P₂–P₄ types. We calculate the structures and formation energies of the VAP’s by using a valence force field model. The formation energy of a VAP is large in multilayered crystal but much reduced in a single layer suggesting its easy production in amorphous regions. Production of VAP’s induces changes of the layer structure in a surrounding region.