Ordering phenomena in complex chalcogenides - The showcase of A ₂in₁₂Q₁₉ (A = K, Tl, NH₄; Q = Se, Te) and pseudobinary in₂Q₃

A group of solids with the general composition A₂In ₁₂Q₁₉ (A = K, Tl, NH₄; O = Se, Te) is characterized by combined Xray 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 K₂In ₁₂Se₁₉ 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 K₂In₁₂Se₁₉, which are apparently intrinsic. The formation of significantly larger domains is observed for K₂In₁₂Se_19-xTe_x and K_2-yTl_yIn₁₂Se₁₉ and results in a twofold superstructure that can be observed with X-ray diffraction also on a macroscopic scale. (NH₄)₂-In₁₂Se₁₉ 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 ₂In₁₂Se₁₉ but is not stable in the HRTEM. Instead, it disappears rapidly and is finally replaced by a twofold superstructure similar to K₂In₁₂Se_19-xTe _x and K_2-yTI_yIn₁₂Se₁₉. The reason was identified, as a combined process of domain broadening and NH₃ evaporation. As observed, for K₂In₁₂Se ₁₉ at T > 473 K, the superstructure reflections disappear. Surprisingly, the pseudobinary phase In₂Q₃ (Q: chalcogen) shows strong structural similarities to K₂In₁₂Se ₁₉ with respect to the internal structure of the nanodomains, Their three-dimensional arrangement, however, and the resulting superstructure are closer related to K₂In₁₂Se_19-xTe_x and K_2-yTl_yIn₁₂Se₁₉.