Ternary Rare Earth and Uranium Nickel Arsenides Ln2NiAs2 (Ln = Tb -Yb), LnNiAs (Ln = La -Nd, Sm, Gd, Dy -Yb), and UNiAs

Abstract

Abstract The new compounds Tm2NiAs2 and Yb2NiAs2 crystallize with a hexagonal structure similar to that reported for Zr2NiAs2. It was refined for Tm2NiAs2 from single-crystal X-ray data in the polar space group P63mc\ a = 408.4(1), c = 1374.2(3) pm, R = 0.044 for 176 structure factors and 16 variable parameters. The compounds Ln2NiAs2 (Ln = Tb -Er) are confirmed to crystallize with an analogous structure from X-ray powder data. In contrast to the earlier structure refinement of Zr2NiAs2 in the higher-symmetric space group P 63/mmc, where the honeycomb layers of nickel and arsenic atoms are described as planar, these layers are puckered in Tm2NiAs2. These compounds are expected to undergo a displacive phase transition, with the centrosymmetric space group P63/mmc to be correct at high temperature. Since the room temperature structure is polar, these compounds might be classified as ferroelectric from a symmetry point of view. The equiatomic title compounds LnNiAs and UNiAs crystallize with a related hexagonal structure corresponding to a superstructure of the AIB2 type with lattice constants varying between a = 416.2(1), c = 1636.1(4) pm for LaNiAs and a = 401.9(2), c = 1488.5(5) pm for YbNiAs. The crystal chemistry of these hexagonal structures is briefly discussed. Using oxidation numbers chemical bonding in the lanthanoid containing compounds may be rationalized with the formulas (Ln+3)2Ni(As-3)2 and Ln+3NiAs-3, thus suggesting semiconducting behavior.