2D Homologous Series SrFMBiS (M = Pb, Ag_0.5Bi_0.5; n = 0, 1) and Commensurately Modulated Sr₂F₂Bi_2/3S₂

We report three new mixed-anion two-dimensional (2D) compounds: SrFPbBiS₃, SrFAg_0.5Bi_1.5S₃, and Sr₂F₂Bi_2/3S₂. Their structures as well as the parent compound SrFBiS₂ were refined using single-crystal X-ray diffraction data, with the sequence of SrFBiS₂, SrFPbBiS₃, and SrFAg_0.5Bi_1.5S₃ defining the new homologous series SrFMBiS (M = Pb, Ag_0.5Bi_0.5; n = 0, 1). Sr₂F₂Bi_2/3S₂ has a different structure, which is modulated with a q vector of 1/3b* and was refined in superspace group X2/m(0β0)00 as well as in the 1 × 3 × 1 superstructure with space group C2/m (with similar results). Sr₂F₂Bi_2/3S₂ features hexagonal layers of alternating [Sr₂F₂]²⁺ and [Bi_2/3S₂]^2-, and the modulated structure arises from the unique ordering pattern of Sr²⁺ cations. SrFPbBiS₃, SrFAg_0.5Bi_1.5S₃, and Sr₂F₂Bi_2/3S₂ are semiconductors with band gaps of 1.31, 1.21, and 1.85 eV, respectively. The latter compound exhibits room temperature red photoluminescence at ∼700 nm.