Structure and properties of the stannide Eu2Au2Sn5, and its relationship with the family of BaAl4-related structures

The stannide Eu2Au2Sn5 was prepared by high-frequency melting of the elemen ts in a sealed tantalum tube. The structure of Eu2Au2Sn5 was refined from s ingle crystal X-ray data: P2(1)/m, a = 928.6(2), b = 465.8(2), c = 1042.9(3 ) pm, beta = 92.28(2)degrees, wR2 = 0.0653, 1220 F-2 values and 56 variable s. The structure of Eu2Au2Sn5 is of a new type. It can be considered as an ordered defect variant of the BaAl4 type. Due to the ordered defects, the c oordination number (CN) of the two cry stallographically different europium sites is reduced from CN 16 to CN 14. The gold and tin atoms in Eu2Au2Sn5 form a complex three-dimensional [Au2Sn5] polyanion in which the europium a toms are embedded. Within the polyanion short Au-Sn and Sn-Sn distances are indicative of strongly bonding Au-Sn and Sn-Sn interactions. A detailed gr oup-subgroup scheme for various ordered and defect variants of the BaAl4 fa mily is presented. Eu2Au2Sn5 shows Curie-Weiss behavior above 50 K with an experimental magnetic moment of 7.90(5) mu(B)/Eu, indicating divalent europ ium. Antiferromagnetic ordering is detected at 5.8(5) K at low fields and a metamagnetic transition occurs at a critical field of 1.4(2) T. Eu2Au2Sn5 is a metal with a specific resistivity of 150+/-20 mu Omega cm at room temp erature. The results of Eu-151 and Sn-119 Mossbauer spectroscopic experimen ts are compatible with divalent europium and show complex hyperfine field s plitting with a transferred magnetic hyperfine field at the tin nuclei at l ow temperature.