Crystal Structure and Magnetic Ordering Transitions in CeNiIn₄, EuNiIn₄ and EuCuIn₄

Polycrystalline CeNiIn4 was prepared by arc-melting of the elements and subsequent annealing at 970 K in vacuum. EuNiIn4 and EuCuIn4 were synthesized from the elements by reactions in sealed tantalum tubes. These indium-rich compounds crystallize with the YNiAl4-type structure which was refined for EuCuIn4 from single-crystal X-ray diffraction data: Cmcm, a = 450.04(9), b = 1698.7(4), c = 740.2(2) pm, wR2 = 0.0606, 495 F-2 values, 24 variables. The EuCuIn4 structure is built up from a complex three-dimensional [Cuing] polyanion (265-279 pm Cu-In and 296-331 pm In-In) in which the europium atoms occupy distorted hexagonal channels. The Eu-Eu distances within these channels (450 pm) are significantly shorter than the distances between Eu atoms in neighboring channels (552 pm). The magnetic properties and the specific heats of the europium compounds have been investigated. Both europium compounds show the magnetism of divalent Eu ions and antiferromagnetic ordering at low temperatures. EuCuIn4 is magnetically ordered via a surprisingly complex sequence of three transitions