Preparation, Crystal Structure, and Physical Properties of the Uranium Nickel Phosphide U 3Ni 3.34P 6

Abstract

The new ternary phosphide U 3Ni 3.34P 6 was prepared by reaction of the elemental components in a tin flux and its crystal structure was determined from single-crystal X-ray data: P4/ mmm, a = 381.8(1) pm, c = 1350.1(4) pm, Z = 1, and R = 0.018 for 200 structure factors and 18 variable parameters. The ideal composition is U 3Ni 4P 6; however, the nickel site was found to be occupied to only 83.6(5)%. The formation of these defects is rationalized from bonding considerations. One phosphorus site had to be refined with a split position and models suggesting various kinds of short-range order for this position are discussed. U 3Ni 3.34P 6 contains two different uranium sites, which are assigned the oxidation numbers +3 and +4 on the basis of their differing U-P bond lengths. The structure is related to the structures of several tetragonal transition metal phosphides and arsenides, e.g., UNi 1.51P 2 with ThCr 2Si 2 type structure and U 2Cu 4As 5. Magnetic susceptibility measurements suggest ferrimagnetism with the Curie temperature T c = 139(2) K, the Weiss constant Θ = 107(3) K, and a magnetic moment of μ exp = 2.1(1)μ B per average uranium atom. Four-probe electrical conductivity measurements indicate semimetallic behavior.