Structure–property relationships in ordered-defect sulphides

Ordered-defect phases of stoichiometry AB2S4 adopt a structure in which dichalcogenide slabs are separated by defective layers containing cations in 50% of the octahedral sites. The physical properties of such phases may be varied systematically by chemical substitution. This may be effected preferentially at sites in either the dichalcogenide or defective layers or at both sites simultaneously. The resulting cation distribution is related to the electronic properties of the constituent ions. In the series VxCr3-xS4 (0≤x≤1), there is an abrupt change in this distribution at x≥0.4, which drives the system into a semiconducting state, with a concomitant loss of long-range magnetic order. Substitution within the dichalcogenide slab in NiCr2-xVxS4 (0<x<2) induces a semiconductor to metal transition at x=0.5, accompanied by the formation of one-dimensional zigzag cation chains. More complex clustering schemes are also described and results presented for V1+xMo2-xS4 (0≤x≤2), which at 0.50≤x≤0.56, exhibits a change from triangular to zigzag clustering, with an accompanying semiconductor-to-metal transition.