New Intermetallic Compounds Ln 2 Ni 2 Mg ( Ln = Y, La–Nd, Sm, Gd–Tm) with Mo 2 FeB 2 Structure

The title compounds were synthesized by reacting the elements in sealed tantalum tubes in a high‐frequency furnace. They crystallize with the Mo 2 FeB 2 structure, a ternary ordered variant of the U 3 Si 2 type, space group P 4/ mbm . All compounds were characterized through Guinier powder patterns and the lattice parameters were obtained from least‐squares fits. Four structures were refined from single crystal X‐ray data: a  = 740.5(1) pm, c  = 372.5(1) pm, w R 2 = 0.0430, 247  F $_o^2$ values, 13 variables for Y 2 Ni 1.90 Mg, a  = 764.5(1) pm, c  = 394.39(9) pm, w R 2 = 0.0371, 310  F $_o^2$ values, 12 variables for La 2 Ni 2 Mg, a  = 754.4(1) pm, c  = 385.20(9) pm, w R 2 = 0.0460, 295  F $_o^2$ values, 12 variables for Pr 2 Ni 2 Mg, and a  = 752.53(8) pm, c  = 382.33(5) pm, w R 2 = 0.0183, 291  F $_o^2$ values, and 12 variables for Nd 2 Ni 2 Mg. A refinement of the occupancy parameters indicated small defects on the nickel site of the yttrium compound, resulting in the composition Y 2 Ni 1.90 Mg for the investigated single crystal. The compounds with cerium, samarium, and gadolinium to thulium as rare earth component were characterized through their Guinier powder patterns. The cell colume of Ce 2 Ni 2 Mg is smaller than that of Pr 2 Ni 2 Mg, indicating intermediate‐valent cerium. The structures can be considered as an intergrowth of distored AlB 2 and CsCl related slabs of compositions LnNi 2 and LnMg . Chemical bonding in La 2 Ni 2 Mg and isotypic La 2 Ni 2 In is compared on the basis of extended Hückel calculations.