From the Ternary Eu(Au/In)2 and EuAu4(Au/In)2 with Remarkable Au/In Distributions to a New Structure Type: The Gold-Rich Eu5Au16(Au/In)6 Structure

The ternary Eu(Au/In)(2), (EuAu0.46In1.54(2)) (I), EuAu4(Au/In)(2) (EuAu4+xIn2-x with x = 0.75(2) (II), 0.93(2), and 1.03(2)), and Eu5Au16(Au/In)(6) (Eu5Au17.29In4.71(3)) (III) have been synthesized, and their structures were characterized by single-crystal X-ray diffraction. I and II crystallize with the CeCu2-type (Pearson Symbol oI12; Imma; Z = 4; a = 4.9018(4) angstrom; b = 7.8237(5) angstrom; c = 8.4457(5) angstrom) and the YbAl4Mo2-type(tI14; I4/mmm; Z = 2; a = 7.1612(7) angstrom; c = 5.5268(7) angstrom) and exhibit significant Au/In disorder. I is composed of an Au/In-mixed diamond-related host lattice encapsulating Eu atoms, while the structure of II features ribbons of distorted, squared Au-8 prisms enclosing Eu, Au, and In atoms. Combination of these structural motifs leads to a new structure type as observed for Eu5Au16(Au/In)6 (Eu5Au17.29In4.71(3)) (oS108; Cmcm; Z = 4; a = 7.2283(4) angstrom; b = 9.0499(6) angstrom; c = 34.619(2) angstrom), which formally represents a one-dimensional intergrowth of the series EuAu2-"EuAu4In2". The site preferences of the disordered Au/In positions in II were investigated for different hypothetical "EuAu4(Au/In)(2)" models using the projector-augmented wave method and indicate that these structures attempt to optimize the frequencies of the heteroatomic Au In contacts. A chemical bonding analysis on two "EuAu5In" and "EuAu4In2" models employed the TB-LMTO-ASA method and reveals that the subtle interplay between the local atomic environments and the bond energies determines the structural and site preferences for these systems.