AsS layered-structure compound: new kind of covalent crystals

Abstract

The molecular phase As₄S₄ transforms under high pressures (>4 GPa) and temperatures (>400 °C) into a new covalent modification with a layered structure. This modification is metastable at normal pressure up to 260 °C and represents a chemically stable narrow gap (∼0.8 eV) semiconductor. The AsS high pressure phase has an orthorhombic structure (Pbc2₁ space group, a = 7.4537(7) Å, b = 10.270(2) Å, c = 18.258(1) Å) with 32 As and 32 S atoms per elementary cell. The cost for the formation of a layered covalent structure at a non-stoichiometric composition is the presence in the structure of covalent bonds between the same type of atoms (As–As) and of anomalously weak covalent bonds between dissimilar atoms (As–S) with lengths 2.35–2.5 Å atypically long for these bonds. One can expect that many of the known pseudo-molecular compounds, for example, As₄S₃, As₄Se₄, P₄Se₄ and others, under high pressure should transform to new covalent layered-structure semiconducting modifications as well.