The Crystal Structure of Maucherite (Ni₁₁As₈)

X-ray diffraction patterns of maucherite (Ni₁₁As₈) indicate the presence of a tetragonal supercell with a = 6.8724(4) Å, c = 21.821(1) Å, space group P4₁2₁2, Z = 4; the subcell is also tetragonal with á = a/2, ć = c, space group I4₁/amd, Z = 1. The crystal structure has been determined using X-ray intensity data collected from a synthetic maucherite crystal on a four-circle diffractometer and refined to an R value of 0.063. The structure contains six non-equivalent Ni atoms per unit cell, five in square pyramidal coordination with As in equipoint position 8b and one in a stretched octahedral coordination in equipoint position 4a. The As polyhedra form single chains of trigonal prisms arranged so that the prism faces contribute to the square pyramidal Ni sites. Alternate prisms share a face with a prism from an adjacent chain; the site which includes the shared face is vacant to account for the unusual stoichiometry of maucherite. Each Ni is coordinated to four or five neighboring Ni atoms with Ni-Ni distances of 2.48 Å to 2.87 Å. The shorter distances arise through shared pyramidal faces and shared As polyhedral prisms; the longer ones arise through shared pyramidal-octahedral faces and shared pyramidal edges. It is suggested that 3d electron σ bonding between these related Ni atoms supplements the Ni-As σ bonds. The principal features of the maucherite structure have counterparts in the crystal structures of α-Ni₇S₆ and millerite (NiS).