New Intermetallic Compounds Nd 4 Co 2 Mg 3 and Sm 4 Co 2 Mg 3 – an Intergrowth of AlB 2 and CsCl Related Slabs

The new magnesium based intermetallic compounds Nd 4 Co 2 Mg 3 and Sm 4 Co 2 Mg 3 were synthesized by induction melting of the elements in sealed tantalum tubes in a water‐cooled sample chamber. The compounds were studied by X‐ray powder and single crystal diffractometer data: P 2/ m , a = 765.42(14), b = 380.53(5), c = 832.47(16) pm, β = 109.79(1)°, wR2 = 0.0489, 1026 F 2 values for Nd 4 Co 2 Mg 3 , a = 760.12(15), b = 377.11(6), c = 826.84(16) pm, β = 109.68(1)°, wR2 = 0.0518, 731 F 2 values for Sm 4 Co 2 Mg 3 with 30 variable parameters for each refinement. These ternary magnesium compounds crystallize with a new structure type. They are simple 1:3 intergrowth variants of distorted AlB 2 and CsCl related slabs of compositions RE Co 2 and RE Mg. The Co–Co distance within the AlB 2 related slab is 237 pm in Nd 4 Co 2 Mg 3 and 235 pm in Sm 4 Co 2 Mg 3 . The crystal chemistry of this new structure type is discussed in line with the related structures of Mn 2 AlB 2 and Mo 2 FeB 2 which are 1:1 intergrowth variants of the two slabs.