Ordering Phenomena in Complex Chalcogenides – the Showcase of A 2 In 12 Q 19 ( A = K, Tl, NH 4 ; Q = Se, Te) and Pseudobinary In 2 Q 3

A group of solids with the general composition A 2 In 12 Q 19 ( A = K, Tl, NH 4 ; Q = Se, Te) is characterized by combined X‐ray single‐crystal and high‐resolution transmission‐electron microscopy (HRTEM). Similar nanosize domains with variable sizes and complex internal structures are common to all three compounds. Although a partial ordering of domains for the bulk of K 2 In 12 Se 19 is dominating, the observed ordering patterns in microdomains range from total random orientation to a pattern with a ninefold superstucture (rare precursor phase not stable under HRTEM conditions). In spite of testing various synthesis conditions it was not possible to avoid these unusual structural features for K 2 In 12 Se 19 , which are apparently intrinsic. The formation of significantly larger domains is observed for K 2 In 12 Se 19– x Te x and K 2– y Tl y In 12 Se 19 and results in a twofold superstructure that can be observed with X‐ray diffraction also on a macroscopic scale. (NH 4 ) 2 In 12 Se 19 is a special case where an initial weak ordering is observed that is characterized by ring X‐ray reflections forming hexagons around certain reciprocal lattice positions. This pattern has apparent similarities to K 2 In 12 Se 19 but is not stable in the HRTEM. Instead, it disappears rapidly and is finally replaced by a twofold superstructure similar to K 2 In 12 Se 19– x Te x and K 2– y Tl y In 12 Se 19 . The reason was identified as a combined process of domain broadening and NH 3 evaporation. As observed for K 2 In 12 Se 19 at T > 473 K, the superstructure reflections disappear. Surprisingly, the pseudobinary phase In 2 Q 3 ( Q : chalcogen) shows strong structural similarities to K 2 In 12 Se 19 with respect to the internal structure of the nanodomains. Their three‐dimensional arrangement, however, and the resulting superstructure are closer related to K 2 In 12 Se 19– x Te x and K 2– y Tl y In 12 Se 19 .