Cu₅In₃-Cu₃In₂ Revisited

The η-phase field of the Cu–In system is unusually rich and shows wealth of phases that are all related to the B8 (NiAs/Ni2In) type. Previous electron diffraction work has revealed extensive super structure ordering; in this study, we report single crystal diffraction experiments on high-quality samples of two of the phases in the system, ht1-Cu₅In₃ and ht2-Cu₅In₃. Both these phases constitute super structures, caused by the ordering of interstitials. The structure ht2-Cu₅In₃ is a relatively simple structure with a bidimensional modulation but where only first order satellites are visible, indicating incomplete order whereas ht1-Cu₅In₃ displays many high order satellites and the refined structure exhibit nearly perfect order with step-function-like occupancy domains. The refinement of the structure of ht1-Cu₅In₃ is challenging for several reasons. It is difficult to integrate the data because of the unusual combination of very closely spaced satellites caused by q-vectors close to (1/3 1/3 0) and a paucity of reflections in total, caused by the small substructure. A second challenge is the refinement of what amounts to a two-dimensional step function. In effect, the shape of a two-dimensional occupation domain is defined by a Fourier series and the problem of defining that shape is non-trivial.