Laves phases forming in the system ScCo${}_2$-"InCo${}_2$"-TaCo${}_2$

The binary Laves phases ScCo${}_2$ (MgCu${}_2$ type) and TaCo${}_2$ (MgNi${}_2$ type) show solid solutions with indium; synthesis of Sc${}_{1-x}$In${}_x$Co${}_2$ samples in sealed tantalum tubes results in tantalum uptake from the container material. Ternary and quaternary samples of these Laves phases were synthesized by direct reactions of the elements followed by different annealing sequences in induction or muffle furnaces. The following structures were refined from single-crystal X-ray diffractometer data: Sc${}_{0.08}$In${}_{0.19}$Ta${}_{0.73}$Co${}_2$, Sc${}_{0.26}$In${}_{0.5}$Ta${}_{0.24}$Co${}_2$, Sc${}_{0.25}$In${}_{0.34}$Ta${}_{0.41}$Co${}_2$ (all MgCu${}_4$Sn type, F$\overline{4}$3m); Sc${}_{0.53}$Ta${}_{0.47}$Co${}_2$ (MgCu${}_2$ type, Fd$\overline{3}$m); Sc${}_{0.5}$In${}_{0.5}$Co${}_2$, Sc${}_{0.51}$In${}_{0.49}$Co${}_2$, Sc${}_{0.63}$In${}_{0.37}$Co${}_2$ and Sc${}_{0.48}$Ta${}_{0.52}$Co${}_2$ (Sc${}_{0.5}$In${}_{0.5}$Co${}_2$ type, P6${}_3$mc); Sc${}_{0.63}$In${}_{0.15}$Ta${}_{0.22}$Co${}_2$ and Sc${}_{0.49}$In${}_{0.28}$Ta${}_{0.23}$Co${}_2$ (new type, P6${}_3$mc , non-centrosymmetric ordering variant of MgNi${}_{0.9}$Cu${}_{1.1}$). The superstructure formation of the MgNi${}_{0.9}$Cu${}_{1.1}$ variant is discussed on the basis of a group-subgroup scheme along with crystal chemical details on Laves phases.