Design in Solid State Synthesis Based on Phase Homologies: A1-xSn9-xBi11+xSe26(A=K, Rb, Cs)-A New Member of the Grand Homologous Series Am[M6Se8]m[M5+nSe9+n] with M=Sn and Bi

There is a strong interest in developing new chemical concepts in designing new materials with superior thermoelectric properties (1, 2). Our approach, which has been outlined in detail elsewhere (3), is focused on complex quaternary and ternary bismuth chalcogenides incorporating alkali metals. The examples of CsBi4Te6 (4), K2Bi8Se13 (5, 6), and K1.25Pb3.5Bi7.25Se15 (7) show that these exploratory investigations can lead to novel thermoelectric materials with promising properties. Our investigations of the quaternary system K/Sn/Bi/Se led to various compounds, e.g., K1+xSn3 2xBi7+xSe14 (8), K1+xSn4 2x Bi7+xSe15 (7), K1 xSn3 xBi11+xSe20 (8), K1 xSn4 xBi11+x Se21 (9), and K1 xSn5 xBi11+xSe22 (10), which exhibit a close structural and compositional relationship. This work helped us to identify a grand homologous series of phases with the general formula Am[M6Se8]m[M5+nSe9+n] (A= alkali metal, M=heavy group IV and V element). The quaternary selenides belong to this series which was recognized after the structural characterization of these compounds. Their structures are composed of [M5+nSe9+n] (NaCl-type) and [M6Se8]m (NaCl -type) units (11) of variable dimensions defined by n and m, which link to produce anionic frameworks with alkali metal (Am) filled