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The solvothermal synthetic exploration of the Bi–S–halogen phase space resulted in the synthesis of two bismuth sulfohalides with common structural motifs. Bi 13 S 18 I 2 was confirmed to have the previously reported composition and crystal structure. In contrast, the bromide analogue was shown to have a formula of neither Bi 19 S 27 Br 3 nor Bi 13 S 18 Br 2 , in contrast to the previous reports. The composition, refined from single crystal X-ray diffraction and confirmed by elemental analysis, high-resolution powder X-ray diffraction, and total scattering, is close to Bi 13 S 17 Br 3 due to the partial S/Br substitution in the framework. Bi 13 S 18 I 2 and Bi 13 S 17 Br 3 are n -type semiconductors with similar optical bandgaps of ∼0.9 eV but different charge and heat transport properties. Due to the framework S/Br disorder, Bi 13 S 17 Br 3 exhibits lower thermal and electrical conductivities than the iodine-containing analogue. The high Seebeck coefficients and ultralow thermal conductivities indicate that the reported bismuth sulfohalides are promising platforms to develop novel thermoelectric materials. 

The local atomic environment of Yb 3+ ions doped into Yb:Er:SrFCl, Yb:Er:SrFBr, and Yb:Er:BaFCl nanocrystals was probed using Yb L 2 edge EXAFS spectroscopy. A structural model derived from substitution of Yb 3+ for Sr 2+ or Ba 2+ in the fluorohalide lattice failed to provide a crystallochemically meaningful description of the first coordination shell of Yb 3+ . On this basis, the presence of Yb 3+ coordinated as YbF 4 Cl 5 or YbF 4 Br 5 capped square antiprisms of C 4 v symmetry was ruled out. Two alternative models were evaluated. The first model was inspired by YbF 9 capped square antiprisms that make up the crystal structure of orthorhombic YbF 3 . The second model was based on YbO 4 F 3 capped trigonal prisms encountered in monoclinic YbOF. Both models correctly reproduced radial structure functions and yielded chemically meaningful ytterbium–fluorine and ytterbium–oxygen distances. Results from EXAFS studies indicate that compositional and structural heterogeneities appear in the fluorohalide lattice upon aliovalent doping with Yb 3+ . From a compositional standpoint, extra fluoride anions and/or oxide anions appear to be incorporated in the vicinity of Yb 3+ dopants. From a structural standpoint, the local symmetry around Yb 3+ ( C s or C 1 ) is lower than that of the crystallographic sites occupied by alkaline-earth cations. These conclusions hold for three different fluorohalide host compositions and rare-earth doping levels spanning one order of magnitude.