CaRu2Zn10, SrRu2Zn10 and EuRu2Zn10 – new superstructure variants of ThMn12

The zinc-rich intermetallic phases CaRu2Zn10, SrRu2Zn10 and EuRu2Zn10 were synthesized by induction-melting of the elements in sealed tantalum ampoules followed by annealing to increase the crystallinity. The samples were characterized by powder X-ray diffraction and the structures were refined from single crystal X-ray diffractometer data: new type, P42/nnm, a = 894.68(14), c = 518.44(9) pm, wR2 = 0.0830, 432 F 2 values, 22 variables for CaRu2Zn10, a = 907.01(10), c = 516.35(6), wR2 = 0.0469, 445 F 2 values, 22 variables for SrRu2Zn10 and a = 902.84(9), c = 515.91(5) pm, wR2 = 0.0469, 434 F 2 values, 22 variables for EuRu2Zn10. The three structures are new ordering variants of the aristotype ThMn12. They are discussed on the basis of a group-subgroup scheme and compared to the known superstructures CaCr2Al10, ErNi10Si2 and ScFe6Ga6. The calcium atoms within the Ca@Ru4Zn16 polyhedra have flattened tetrahedral ruthenium coordination, reducing the calcium site symmetry to 4 ‾ $\bar{4}$ 2m (instead of 4/mmm in the aristotype). Electronic structure calculations show a substantial charge transfer from calcium to ruthenium and an almost neutral zinc substructure.