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2001[X]
1Author    Wolfgang Jeitschko, LudgerJ. Terbiichte, Ute Ch, RodewaldRequires cookie*
 Title    Ternary Rare Earth and Uranium Nickel Arsenides Lw2NiAs2 (Ln = Tb -Yb), LwNiAs {Ln = La -Nd, Sm, Gd, Dy -Yb), and UNiAs  
 Abstract    The new compounds TnbNiAsa 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 L«2NiAs2 (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 Tm 2N iAs2. These compounds are expected to undergo a displacive phase transition, with the centrosymmetric space group P6i/m m c 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)2N i(As_3)2 and Ln+3N iA s~3, thus suggesting semiconducting behavior. 
  Reference    Z. Naturforsch. 56b, 1281—1288 (2001); received August 12 2001 
  Published    2001 
  Keywords    Rare Earth Compounds, Arsenides, Displacive Phase Transition 
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 Identifier    ZNB-2001-56b-1281 
 Volume    56