Cation-Poor Complex Metallic Alloys in Ba(Eu)–Au–Al(Ga) Systems: Identifying the Keys that Control Structural Arrangements and Atom Distributions at the Atomic Level

Four complex intermetallic compounds
BaAu6±xGa6±y (x = 1, y = 0.9) (I), BaAu6±xAl6±y (x = 0.9, y =
0.6) (II), EuAu6.2Ga5.8 (III), and EuAu6.1Al5.9 (IV) have been synthesized, and their structures and homogeneity ranges
have been determined by single crystal and powder X-ray diffraction.
Whereas I and II originate from the NaZn13-type structure (cF104–112, Fm3̅c), III (tP52, P4/nbm) is derived
from the tetragonal Ce2Ni17Si9-type,
and IV (oP104, Pbcm) crystallizes in a new orthorhombic structure type. Both I and II feature formally anionic networks with completely
mixed site occupation by Au and triel (Tr = Al,
Ga) atoms, while a successive decrease of local symmetry from the
parental structures of I and II to III and, ultimately, to IV correlates with increasing
separation of Au and Tr on individual crystallographic sites. Density
functional theory-based calculations were employed to determine the
crystallographic site preferences of Au and the respective triel element
to elucidate reasons for the atom distribution (“coloring scheme”).
Chemical bonding analyses for two different “EuAu6Tr6” models reveal maximization of the number of
heteroatomic Au–Tr bonds as the driving force for atom organization.
The Fermi levels fall in broad pseudogaps for both models allowing
some electronic flexibility. Spin-polarized band structure calculations
on the “EuAu6Tr6” models hint
to singlet ground states for europium and long-range magnetic coupling
for both EuAu6.2Ga5.8 (III) and
EuAu6.1Al5.9 (IV). This is substantiated
by experimental evidence because both compounds show nearly identical
magnetic behavior with ferromagnetic transitions at TC = 6 K and net magnetic moments of 7.35 μB/f.u. at 2 K. The effective moments of 8.3 μB/f.u.,
determined from Curie–Weiss fits, point to divalent oxidation
states for europium in both III and IV.