Ordering Phenomena in Complex Chalcogenides – the Showcase of A2In12Q19 (A = K, Tl, NH4; Q = Se, Te) and Pseudobinary In2Q3

Abstract

A group of solids with the general composition A2In12Q19 (A = K, Tl, NH4; Q = 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 K2In12Se19 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 K2In12Se19–xTex and K2–yTlyIn12Se19 and results in a twofold superstructure that can be observed with X-ray diffraction also on a macroscopic scale. (NH4)- 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 K2In12Se19 but is not stable in the HRTEM. Instead, it disappears rapidly and is finally replaced by a twofold superstructure similar to K2In12Se19–xTex and K2–yTlyIn12Se19. The reason was identified as a combined process of domain broadening and NH3 evaporation. As observed for K2In12Se19 at T 473 K, the superstructure reflections disappear. Surprisingly, the pseudobinary phase In2Q3 (Q: chalcogen) shows strong structural similarities to K2In12Se19 with respect to the internal structure of the nanodomains. Their three-dimensional arrangement, however, and the resulting superstructure are closer related to K2In12Se19–xTex and K2–yTlyIn12Se19.