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The new compounds Tm 2 NiAs 2  and Yb 2 NiAs 2  crystallize with a hexagonal structure similar to that reported for Zr 2 NiAs 2 . It was refined for Tm 2 NiAs 2  from single-crystal X-ray data in the polar space group P6 3 mc\ a = 408.4(1), c = 1374.2(3) pm, R = 0.044 for 176 structure factors and 16 variable parameters. The compounds Ln 2 NiAs 2  (Ln = Tb -Er) are confirmed to crystallize with an analogous structure from X-ray powder data. In contrast to the earlier structure refinement of Zr 2 NiAs 2  in the higher-symmetric space group P 6 3 /mmc, where the honeycomb layers of nickel and arsenic atoms are described as planar, these layers are puckered in Tm 2 NiAs 2 . These compounds are expected to undergo a displacive phase transition, with the centrosymmetric space group P6 3 /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 AIB 2  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 ) 2 Ni(As -3 ) 2  and Ln +3 NiAs -3 , thus suggesting semiconducting behavior.

Ternary Rare Earth and Uranium Nickel Arsenides Ln 2 NiAs 2 (Ln = Tb -Yb), LnNiAs (Ln = La -Nd, Sm, Gd, Dy -Yb), and UNiAs