Triangular Zn₃ and Ga₃ units in Sr₂Au₆Zn₃, Eu₂Au₆Zn₃, Sr₂Au₆Ga₃, and Eu₂Au₆Ga₃. Structure, magnetism, ¹⁵¹Eu Moessbauer and ^69;71Ga solid state NMR spectroscopy

The gold-rich intermetallic compounds Sr₂Au₆Zn₃, Eu₂Au₆Zn₃, Sr₂Au₆Ga₃, and Eu₂Au₆Ga₃ were synthesized from the elements in sealed tantalum ampoules in induction or muffle furnaces. The europium compounds are reported for the first time and their structures were refined from single crystal X-ray diffractometer data: Sr₂Au₆Zn₃ type, R anti 3c, a = 837.7(1), c = 2184.5(4) pm, wR2 = 0.0293, 572 F² values for Eu₂Au_6.04Zn_2.96 and a = 838.1(2), c = 2191.7(5) pm, wR2 = 0.0443, 513 F² values for Eu₂Au_6.07Ga_2.93 with 20 variables per refinement. The structures consist of a three-dimensional gold network with a 6R stacking sequence, similar to the respective diamond polytype. The cavities of the network are filled in a ratio of 2:1 by strontium (europium) atoms and Ga₃ (Zn₃) triangles in an ordered manner. Sr₂Au₆Zn₃ and Sr₂Au₆Ga₃ are diamagnetic with room temperature susceptibilities of -3.5 x 10^-4 emu mol^-1. Temperature dependent susceptibility and ¹⁵¹Eu Moessbauer spectroscopic measurements show a stable divalent ground state for both europium compounds. Eu₂Au₆Zn₃ and Eu₂Au₆Ga₃ order antiferromagnetically below Neel temperatures of 16.3 and 12.1 K, respectively. Anisotropic electrical conductivity of Sr₂Au₆Ga₃ is proven by an alignment of the crystallites in the magnetic field. Orientation-dependent ^69;71Ga NMR experiments combined with quantum mechanical calculations (QM) give evidence for a highly anisotropic charge distribution of the Ga atoms.