Since several thousands of years, metallic elements and especially their alloys and intermetallic compounds constitute one of the material foundations responsible for the development of humankind. The large variety of known intermetallic compounds comprise beneficial properties, including ductility, and hardness, and feature structural patterns ranging from simple close-packed structures to complex quasicrystals. Especially complex structures often feature unique atomic arrangements with both localized and delocalized bonding interactions, making their characterization and the understanding of structure-bonding-property relations and formation conditions a fascinating quest in the search for overarching schemes in the class of intermetallics. In the following contribution, two examples of structural complexity in intermetallic compounds, with an emphasis on atomic arrangements and formation conditions, will be discussed.

Au₄Si. Gold and silicon do not form stable binary phases. The binary system is of scientific and technological interest because the eutectic mixture Au₈₀Si₂₀ shows a remarkably low melting point of 363(3)°C. The application of fast cooling techniques led to a metastable binary phase 'Au₄Si' close to the eutectic composition, but due to the structural complexity, and sensitivity of the compound, the characterization has remained a challenge. By testing different preparation methods with variable cooling rates, good-quality single crystals of Au₄Si were obtained recently [1]. Au₄Si crystallizes in a complex √18×√2×1 superstructure of the PtHg₄ type, based on the distortion of vertex-sharing Si@Au₈ cubes into bisdisphenoids. Au₄Si decomposes upon heating and at room temperature even in high vacuum, highlighting its metastability. Electronic structure analysis reveals a pseudogap at the Fermi energy, which is enhanced by the superstructure through the relief of Au-Au antibonding interactions. The pseudogap is associated with a Zintl-type bonding scheme, which can be extended to the locally ordered liquid. 

Nd-Ru. Despite the interesting magnetic and superconducting properties of the phases in the binary systems RE-Ru (RE = La, Pr, Nd), the compounds at about 35-38 at.% Ru have remained uncharacterized due to their complex crystal structure [2,3]. High-resolution diffraction experiments (beamline Cristal, synchrotron Soleil, France) on single crystals of the Nd compound revealed a composite structure comprising observable satellites up to the 29^th order. The partial overlap required manual indexing and processing of the satellite reflections which will be explained in detail in this contribution. 

The resulting 3D+1 structure was solved in space group X4/nbm(00g)00ss with a = 15.6130(8) Å, c = 6.3258(4) Å, q ≈ 2/23. In [001] direction, Nd atoms form chimneys, hosting linear Ru-Ru chains. This structural arrangement is closely related to that of Y₄₄Ru₂₅ [4] and in turn to that of Nd₅1r₃ (Pu₅Rh₃-type, [5]).