Tropochemical Cell-Twinning in the New Quaternary Bismuth Selenides KxSn6-2xBi2+xSe9 and KSn5Bi5Se13

The quaternary KxSn6-2xBi2+xSe9 and KSn5Bi5Se13 were discovered from reactions involving K2Se, Bi2Se3, Sn, and Se. The single crystal structures reveal that KxSn6-2xBi2+xSe9 is isostructural to the mineral heyrovskyite, Pb6Bi2S9, crystallizing in the space group Cmcm with a = 4.2096(4) (ANGSTROM), b = 14.006(1) (ANGSTROM), and c = 32.451(3) (ANGSTROM) while KSn5Bi5Se13 adopts a novel monoclinic structure type (C2/m, a = 13.879(4) (ANGSTROM), b = 4.205(1) (ANGSTROM), c = 23.363(6) (ANGSTROM), (beta)= 99.012(4) °). These compounds formally belong to the lillianite homologous series xPbS·Bi2S3, whose characteristic is derivation of the structure by tropochemical cell-twinning on the (311) plane of the NaCl-type lattice with a mirror as twin operation. The structures of KxSn6-2xBi2+xSe9 and KSn5Bi5Se13 differ in the width of the NaCl-type slabs that form the three-dimensional arrangement. While cell -twinning of 7 octahedra wide slabs results in the heyrovskyite structure, 4 and 5 octahedra wide slabs alternate in the structure of KSn5Bi5Se13. In both structures, the Bi and Sn atoms are extensively disordered over the metal sites. Some physicochemical properties of KxSn6-2xBi2+xSe9 and KSn5Bi5Se13 are reported.