From 1D chain to 3D network:: Syntheses, structures, and properties of K2MnSn2Se6, K2MnSnSe4, and K2Ag2SnSe4

K2MnSn2Se6 (I), K2MnSnSe4 (II), and K2Ag2SnSe4 (III) were prepared by molten-salt (alkali-metal polyselenide flux) reactions at intermediate temperature (400-520 degrees 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) Angstrom, Z = 4, R1/wR2 = 0.0435/0.0722 for 592 observed reflections and 29 variables; crystal data for II: space group P (4) over bar n2, 10.574(1), c = 8.301(2) Angstrom, 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) Angstrom, beta = 113.56(3)degrees, Z = 2, R1/wR2 = 0.0595/0.1135 for 1098 observed reflections and 45 parameters. The crystal structure of I consists of infinite (1)(infinity)[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 (2)(infinity)[Ag2SnSe4](2-) anionic layers separated by K+. counterions. The anionic layer is formed by parallel SiS2-type (1)(infinity)[AgSe2](3-) chains crosslinked 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 degrees C, II decomposes at; about 243 degrees C, and III melts congruently at about 510 degrees C. I, II, and III are semiconductors with estimated band gaps of 2.0, 1.7, and 1.8 eV, respectively.