K2MnSn2Se6 (I), K2MnSnSe4 (II), and K2Ag2SnSe4 (III) were prepared by molten-salt (alkali-metal polyselenide flux) reactions at intermediate temperature (400−520 °C). Their crystal structures were determined by single-crystal X-ray diffraction techniques. Crystal data for I: space group P4/ncc, a = 8.167(1), c = 19.724(4) Å, Z = 4, R1/wR2 = 0.0435/0.0722 for 592 observed reflections and 29 variables; crystal data for II: space group P4̄n2, a = 10.574(1), c = 8.301(2) Å, Z = 2, R1/wR2 = 0.0339/0.0939 for 1208 observed reflections and 40 parameters; crystal data for III: space group P2/c, a = 7.575(2), b = 5.920(1), c = 12.148(2) Å, β = 113.56(3)°, Z = 2, R1/wR2 = 0.0595/0.1135 for 1098 observed reflections and 45 parameters. The crystal structure of I consists of infinite [MnSn2Se6]2- anionic chains containing [Sn2Se64-] units linked by tetrahedrally coordinated Mn2+ ions, with the K+ cations situated between the chains. II contains a three-dimensional framework formed by corner- and edge-sharing SnSe4 and MnSe4 tetrahedra, with large channels hosting the K+ cations. III is a layered compound containing [Ag2SnSe4]2- anionic layers separated by K+ counterions. The anionic layer is formed by parallel SiS2-type [AgSe2]3- chains cross-linked by tetrahedral SnSe4 via bridging selenium atoms of the chains. The results from differential scanning calorimetry (DSC) measurements show that I is stable up to 590 °C, II decomposes at about 243 °C, and III melts congruently at about 510 °C. I, II, and III are semiconductors with estimated band gaps of 2.0, 1.7, and 1.8 eV, respectively.
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Chen et al. (2000) studied this question.