Determination of the superstructures for the stannides ZrIrSn, HfCoSn, and HfRhSn

Abstract

The title compounds are already known in the literature, however, they have only been characterized by their subcells on the basis of X-ray powder data. Well crystallized samples of ZrIrSn, HfCoSn, and HfRhSn were prepared from the elements by arc-melting and subsequent annealing at 970 K. ZrIrSn, HfCoSn, and HfRhSn crystallize with a pronounced subcell of space group P[unk]2m (ZrNiAl type). Additional very weak reflections required a doubling of the c axis. The three structures (all space group P([unk]2c) were refined from single-crystal X-ray data: a = 732.1(2) pm, c = 732.2(2) pm, wR2 = 0.0367, 539 F² values, 18 variables for ZrIrSn, a = 713.1(3) pm, c = 705.4(3) pm, wR2 = 0.0930, 342 F² values, 16 variables for HfCoSn, and a = 732.0(3) pm, c = 714.8(2) pm, wR2 = 0.0259. 528 F² values, 18 variables for HfRhSn. The three stannides crystallize with a substitution derivative of the Mg₂Ga structure. The Zr(Hf) and tin atoms are ordered on the magnesium positions and the Co(Rh,Ir) atoms on the gallium positions of the Mg₂Ga type. The distortions in the structures of ZrIrSn, HfCoSn, and HfRhSn are most likely due to packing reasons (size of the zirconium and hafnium atoms). The structures of ZrIrSn, HfCoSn, and HfRhSn are described considering a group-subgroup scheme, also including the FePdP, Lu₃Co₂In₄, Mg₉Sn₅, Ti₄Ni₂Ga₃, and TiFeSi type structures.