The Ternary Lanthanoid Nickel Arsenides Tb12Ni30As21 and Dy12Ni30As21 with (La,Ce)12Rh30P21 Type Related Structure

The title compounds were synthesized from the elements by reactions at high temperature. They crystallize with a hexagonal structure which was determined from single‐crystal X‐ray data of Dy₁₂Ni₃₀As₂₁: P6₃/m, Z = 1, a = 1698.5(5) pm, c = 387.7(1) pm, R = 0.053 for 797 F values and 69 variable parameters. It belongs to a large family of hexagonal structures with a metal to metalloid ratio of 2 : 1. In these hexagonal structures an ordered distribution of occupied atomic positions around the hexagonal 6₃ axis is possible only for refinements in space groups of lower symmetry. This is discussed for the present case and the very closely related structures reported for (La, Ce)₁₂Rh₃₀P₂₁ and Nd₃Ni₇P₅.