Single crystals of Sm2S2-xSe1+x (0.1 ≤ x < 0.2) have been obtained for the first time through the oxidation of KSm2Cl5 with an excess of sulfur and selenium in equimolar amounts at 850 °C after seven days in evacuated silica tubes. They emerged as almost black, in thin layers deep red, lath-shaped needles (A and U type, respectively) as well as red, bead-shaped polyhedra (C type) of the gross chemical composition Sm2S2Se according to X-ray structure analyses. A much simpler method of synthesis is based on the direct fusion of the elements (samarium, sulfur and selenium) in appropriate molar ratios (2:2:1) in the presence of NaCl as a flux under otherwise analogous conditions (silica tubes, 7 d, 850 °C). A-Sm2S1.82Se1.18 crystallizes orthorhombically (a = 753.1(3), b = 401.9(1), c = 1565.8(6) pm, Z = 4) in the space group Pnma with the α-Gd2S3-type structure. Two crystallographically different Sm3+ cations are coordinated by eight (Sm1) and seven (Sm2) Ch2- anions (S2- and Se2-) as biand monocapped trigonal prisms, respectively. C-Sm2S1.90Se1.10 adopts the cubic γ-Ce2S3-type structure (I4̅3d; a = 858.7(2) pm, Z = 5.333) with trigon-dodecahedrally coordinated Sm3+ cations (CN = 8 ). Finally, U-Sm2S1.84Se1.16 with the orthorhombic U2S3-type structure (Pnma; a = 1105.3(6), b = 399.2(1), c = 1074.0(5) pm, Z = 4) exhibits two crystallographically different Sm3+ cations again, which are coordinated by seven (Sm1) and seven plus one (Sm2) Ch2- anions, respectively. The preferential occupation of S _ and Se2- anions at only one in the C-type, but three anionic sites each in the A- and U-type crystal structures of Sm2S2-xSe1+x (0 .1 ≤ x < 0.2 ) is discussed.
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