Planar versus Puckered Nets in the Polar Intermetallic Series EuGaTt (Tt = Si, Ge, Sn)

The ternary polar intermetallic compounds EuGaTt (Tt = Si, Ge, Sn) have been synthesized and characterized
experimentally, as well as theoretically. EuGaSi crystallizes in the hexagonal AlB2-type structure (space group
P6/mmm, Z = 1, Pearson symbol hP3) with randomly distributed Ga and Si atoms on the graphite-type planes: 
a = 4.1687(6) Å, c = 4.5543(9) Å. On the other hand, EuGaGe and EuGaSn adopt the hexagonal YPtAs-type
structure (space group P63/mmc, Z = 4, Pearson symbol hP12):  a = 4.2646(6) Å and c = 18.041(5) Å for
EuGaGe; a = 4.5243(5) Å and c = 18.067(3) Å for EuGaSn. The three crystal structures contain formally [GaTt]2-
polyanionic 3-bonded, hexagonal networks, which change from planar to puckered and exhibit a significant decrease
in interlayer Ga−Ga distances as the size of Tt increases. Magnetic susceptibility measurements of this series of
compounds show Curie−Weiss behavior above 86(5), 95(5), and 116(5) K with magnetic moments of 7.93, 7.97,
and 7.99 μB for EuGaSi, EuGaGe, and EuGaSn, respectively, indicating a 4f7 electronic configuration (Eu2+) for Eu
atoms. X-ray absorption spectra (XAS) are also consistent with these magnetic properties. Electronic structure
calculations supplemented by a crystal orbital Hamilton population (COHP) analysis identifies the synergy between
atomic sizes, from both Eu and Tt atoms, and the orbital contributions from Eu toward influencing the structural
features of EuGaTt. A multicentered interaction between planes of Eu atoms and the [GaTt]2- layers rather than
through-space Ga−Ga bonding is seen in ELF distributions.