Structural distortions of the metal dichalcogenide units in AMo₂S₄ (A = V, Cr, Fe, Co) and magnetic and electrical properties

Powder neutron diffraction data collected for AMo₂S₄ (A = V, Cr, Fe, Co) reveal that the materials crystallize in the space group Cc (a ˜ 11.8 Å, b ˜ 6.5 Å, c ˜ 13 Å, ß ˜ 114°). The structure consists of dichalcogenide (MS₂) layers of edge-sharing octahedra, separated by a layer in which 50% of cation sites are occupied in an ordered fashion. Cations in the MS₂ unit are distorted from an ideal hexagonal array. For the early transition series cations A = Cr, V, the distortion involves the formation of triangular clusters of cations, whereas for A = Fe, Co diamond-shape cation clusters are observed. The semiconducting properties of these materials are discussed in light of these results. Both FeMo₂S₄ and CoMo₂S₄ order antiferromagnetically with T_N = 110 and <300 K, respectively. Low-temperature powder neutron diffraction data demonstrate that both materials have a similar magnetic structure and that the ordered magnetic moments (µ(Fe) = 3.16(3)µ_Bµ(Co) = 2.14(3)µ_B) are confined to the A cations in the vacancy layer.