Architecture of new {Ca, Eu, Yb}4CuMg complex intermetallics based on polyicosahedral clusters

The crystal structures of R4CuMg (R––Ca, Eu, Yb) complex intermetallics, solved by single crystal X-ray diffraction, were found to represent a new trigonal prototype (hR144-h7c2ba, space group R 3 m), being the first examples of 4:1:1 rare earth-rich phases not crystallizing as cF96-Gd4RhIn, adopted by more than 200 compounds known so far. The divalent character of the R component, together with Cu atomic size could contribute to explain the observed structural peculiarity. Indeed, the occurrence of fragments typical of compositionally similar compounds, namely Cu-centered trigonal prisms and Mg-centered core-shell polyicosahedral clusters with R at vertices, prompted a search for significant structural relationships. In this work, a description of the rhombohedral R4CuMg crystal structure is proposed as a linear intergrowth along the c-direction of the two types of slabs R9.5CuMg3.5 (parent type: hP28-kh2ca, SG 194) and R13Cu6Mg (parent type:hR60-b6a2, SG 160). The ratio of these slabs in the studied structure is 2:1, corresponding to the simple equation 6×R9.5CuMg3.5+3×R13Cu6Mg = 24×R4CuMg,normalized to the unit cell content. According to the proposed description, symmetry properties of the slabs (depicted by the p3m1 layer group) were used for screening of possible space groups for other potential members of the same/similar intergrowth families.