PREPARATION, CRYSTAL-STRUCTURE, AND PHYSICAL-PROPERTIES OF THE URANIUM NICKEL PHOSPHIDE U3NI3.34P6

The new ternary phosphide U3Ni3.34P6 was prepared by reaction of the e lemental components in a tin flux and its crystal structure was determ ined 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 U3Ni4P6; however, the nickel sit e was found to be occupied to only 83.6(5)%. The formation of these de fects 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. U3Ni3.34P 6 contains two different uranium sites, which are assigned the oxidati on numbers +3 and +4 on the basis of their differing U-P bond lengths. The structure is related to the structures of several tetragonal tran sition metal phosphides and arsenides, e.g., UNi1.51P2 with ThCr2Si2 t ype structure and U2Cu4As5. Magnetic susceptibility measurements sugge st ferrimagnetism with the Curie temperature T-C = 139(2) K, the Weiss constant Theta = 107(3) K, and a magnetic moment of mu(exp) = 2.1(1)m u(B) per average uranium atom. Four-probe electrical conductivity meas urements indicate semimetallic behavior. (C) 1995 Academic Press, Inc.