The structures of orthorhombic and vitreous arsenic

Abstract

The structure of the orthorhombic crystal modification of arsenic has been determined from an analysis of X-ray powder diffraction data. The structure is confirmed as being similar to that of black phosphorus, with space group Bmab and eight atoms per unit cell. Each atom is covalently bonded to three neighbours, with two bonds in an ab plane, of length 2·49 Å, making an angle of 94·1° and the third bond, of length 2·48 Å, at an angle of 98·5° with each of the other two. The structure differs significantly from that deduced from the black phosphorus structure by a simple scaling with bond lengths. The relationship of the glass structure with the structures of both orthorhombic and rhombohedral (metallic) arsenic has been considered. The structure of the vitreous phase is much closer to that of the orthorhombic crystal. A good description of the glass structure may be obtained using a quasi-crystalline model based on this structure with some expansion of the interlayer spacing and some additional disordering within the layers.