Dimorphic EuCu<sub>5</sub>Cd

Two modifications of EuCu₅Cd were synthesized from the elements in sealed tantalum ampoules. The high-temperature (HT) modification was obtained from an induction-melted sample that was finally annealed at 873 K, while the low-temperature (LT) phase is formed in a muffle furnace by annealing a pre-melted sample at 673 K. DSC measurements gave a transition temperature of 823 ±2 K. Both structures were refined on the basis of single-crystal X-ray diffractometer data: R3̅m, a = 511.58(7), c = 3059.6(6) pm, wR2 = 0.0289, 316 F² values, 22 variables for HT-EuCu_4.85(1)Cd_1.15(1) and Pnma, a = 2545.6(5), b = 511.60(1), c = 1060.3(2) pm, wR2 = 0.0386, 2044 F² values, 121 variables for LT-EuCu_4.92(1)Cd_1.08(1). A small degree of Cu/Cd mixing was observed for three sites. Both structures adopt new types. The europium atoms have high coordination numbers of 18, 19 and 20 in LT- and 18 and 20 in HT-EuCu₅Cd. These three types of polyhedra are the basic building units also in the closely related structures of CeCu₆, CeCu₅Au, CeCu_4.38In_1.62, and CeNi₅Sn, a family of stacking variants. Temperature-dependent magnetic susceptibility measurements of a HT-EuCu₅Cd sample showed an experimental magnetic moment of 7.56(1) μ_B per Eu atom in the paramagnetic regime, compatible with divalent europium. HT-EuCu₅Cd orders antiferromagnetically at 17.7(5) K.