From 1D Chain to 3D Network: Syntheses, Structures, and Properties of K₂MnSn₂Se₆, K₂MnSnSe₄, and K₂Ag₂SnSe₄

K 2 MnSn 2 Se 6 ( I ), K 2 MnSnSe 4 ( II ), and K 2 Ag 2 SnSe 4 ( 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 P 4/ ncc, a = 8.167(1), c = 19.724(4) Å, Z = 4, R 1/ wR 2 = 0.0435/0.0722 for 592 observed reflections and 29 variables; crystal data for II: space group P 4̄ n 2, a = 10.574(1), c = 8.301(2) Å, Z = 2, R 1/ wR 2 = 0.0339/0.0939 for 1208 observed reflections and 40 parameters; crystal data for III: space group P 2/ c, a = 7.575(2), b = 5.920(1), c = 12.148(2) Å, β = 113.56(3)°, Z = 2, R 1/ wR 2 = 0.0595/0.1135 for 1098 observed reflections and 45 parameters. The crystal structure of I consists of infinite [MnSn 2 Se 6 ] 2 - anionic chains containing [Sn 2 Se 6 4 - ] units linked by tetrahedrally coordinated Mn 2+ ions, with the K + cations situated between the chains. II contains a three-dimensional framework formed by corner- and edge-sharing SnSe 4 and MnSe 4 tetrahedra, with large channels hosting the K + cations. III is a layered compound containing [Ag 2 SnSe 4 ] 2 - anionic layers separated by K + counterions. The anionic layer is formed by parallel SiS 2 -type [AgSe 2 ] 3 - chains cross-linked by tetrahedral SnSe 4 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.