Abstract
The rare earth metal (RE)-nickel-cadmium intermetallics RE2Ni2Cd (RE = La, Pr, Nd, Sm, Tb, Dy) were prepared from the elements in sealed niobium or tantalum tubes in a water-cooled sample chamber of a high-frequency furnace. They crystallize with a tetragonal Mo2FeB2 type low-temperature modification, space group P4/mbm, and an orthorhombic Mn2AlB2 type hightemperature modification, space group Cmmm. The cadmium compounds were characterized through their X-ray powder patterns. Five structures of the low-temperature modifications were refined from X-ray single crystal diffractometer data: α = 763.76(9), c = 387.26(8) pm, wR2 = 0.046, 205 F2 for La2Ni1.67(1)Cd; α = 752.93(7), c = 380.95(6) pm, wR2 = 0.061, 260 F2 for Pr2Ni2Cd; α = 750.88(9), c = 378.33(7) pm, wR2 = 0.051, 195 F2 for Nd2Ni2Cd; α = 743.6(1), c = 374.0(1) pm, wR2 = 0.036, 386 F2 for Sm2Ni1.93(1)Cd; α = 734.9(1), c = 366.1(2) pm, wR2 = 0.030, 252 F2 for Dy2Ni1.94(1)Cd, with 13(12) variables per refinement. The 4g nickel site is only fully occupied in the neodymium and the praseodymium compound. Both modifications can be considered as intergrowths of distorted AlB2 and CsCl related slabs. In both modification the nickel and cadmium atoms build up two-dimensional [Ni2Cd] networks. In the low-temperature modifications the nickel atoms form pairs, while nickel zig-zag chains occur in the high-temperature modifications. These nickel fragments are condensed via the cadmium atoms. The crystal chemistry and the chemical bonding in these intermetallics is discussed
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