Contribution of Disulfide S22– Anions to the Crystal and Electronic Structures in Ternary Sulfides, Ba12In4S19, Ba4M2S8 (M = Ga, In)

Three semiconducting ternary sulfides have been synthesized from the mixture of elements with about 20% excess of sulfur (to establish oxidant rich conditions) by solid-state reactions at high temperature. Ba12In4S19 ≡ (Ba2+)12(In3+)4(S2–)17(S2)2–, 1, crystallizes in the trigonal space group R3̅ with a = 9.6182(5) Å, b = 9.6182(5) Å, c = 75.393(7) Å, and Z = 6, with a unique long period-stacking structure of a combination of monometallic InS4 tetrahedra, linear dimeric In2S7 tetrahedra, disulfide S22– anions, and isolated sulfide S2– anions that is further enveloped by Ba2+ cations. Ba4In2S8 ≡ (Ba2+)4(In3+)2(S2–)6(S2)2–, 2, crystallizes in the triclinic space group P1̅ with a = 6.236(2) Å, b = 10.014(4) Å, c = 13.033(5) Å, α = 104.236(6)°, β = 90.412(4)°, γ = 91.052(6)°, and Z = 2. Ba4Ga2S8 ≡ (Ba2+)4(Ga3+)2(S2–)6(S2)2–, 3, crystallizes in the monoclinic P21/c with a = 12.739(5) Å, b = 6.201(2) Å, c = 19.830(8) Å, β = 104.254(6)° and Z = 4. Compounds 2 and 3 represent the first one-dimensional (1D) chain structure in ternary Ba/M/S (M = In, Ga) systems. The optical band gaps of 1 and 3 are measured to be around 2.55 eV, which agrees with their yellow color and the calculation results. The CASTEP calculations also reveal that the disulfide S22– anions in 1–3 contribute mainly to the bottom of the conduction bands and the top of valence bands, and thus determine the band gaps.