Cornucopia of Structures in the Pseudobinary System (SnSe) _xBi₂Se₃: A Crystal-Chemical Copycat

Pseudobinary phases (SnSe) _xBi₂Se₃ exhibit a very diverse structural chemistry characterized by different building blocks, all of which are cutouts of the NaCl type. For SnSe contents between x = 5 and x = 0.5, several new phases were discovered. Next to, for example, Sn₄Bi₂Se₇ ( x = 4) in the NaCl structure type and SnBi₄Se₇ ( x = 0.5) in the layered defect GeSb₂Te₄ structure type, there are at least four compounds (0.8 ≤ x ≤ 3) with lillianite-like structures built up from distorted NaCl-type slabs (L4,4-type Sn_2.22Bi_2.52Se₆, L4,5-type Sn_9.52Bi_10.96Se₂₆, L4,7-type Sn_11.49Bi_12.39Se₃₀, and L7,7-type Sn_3.6Bi_3.6Se₉). For two of them (L4,7 and L7,7), the cation distributions were determined by resonant X-ray scattering, which also confirmed the presence of significant amounts of cation vacancies. Thermoelectric figures of merit ZT range from 0.04 for Sn₄Bi₂Se₇ to 0.2 for layered SnBi₄Se₇; this is similar to that of the related compounds SnBi₂Te₄ or PbBi₂Te₄. Compounds of the lillianite series exhibit rather low thermal conductivities (∼0.75 W/mK for maximal ZT). More than other "sulfosalts", compounds in the pseudobinary system SnSe-Bi₂Se₃ adapt to changes in the cation-anion ratio by copying structure types of compounds containing lighter or heavier homologues of Sn, Bi, or Se and can incorporate significant amounts of vacancies. Thus, (SnSe) _xBi₂Se₃ is a multipurpose model system with vast possibilities for substitutional and structural modification aiming at the optimization of thermoelectric or other properties.