A Layered Tin Bismuth Selenide with Three Different Building Blocks that Account for an Extremely Large Lattice Parameter of 283 Å.

The layered compound Sn Bi Se exhibits an extraordinarily long-periodic 150R stacking sequence. The crystal structure contains three different building blocks, which form upon the addition of Sn to a Bi-rich bismuth selenide. Sn-doped Bi double ("2") layers similar to those in elemental bismuth, Sn Bi Se quintuple ("5") layers and Sn Bi Se septuple ("7") layers are arranged in a 7525757525|7525757525|7525757525 sequence, which corresponds to a structure with a=4.1819(4) and c=282.64(6) Å in space group R m. The structure of a microcrystal was determined using microfocused synchrotron radiation and refined as a formally commensurately modulated structure in (3+1)D superspace (superspace group R m(00γ)00), with a trivial basic structure that contains just one atom. The stacking sequence as well as the cation distribution are confirmed by aberration-corrected scanning transmission electron microscopy (STEM) in combination with chemical mapping by X-ray spectroscopy with atomic resolution. Stacking faults are not typical but have been observed occasionally.