New intermetallic compounds Ln(2)Ni(2)Mg (Ln = Y, La-Nd, Sm, Gd-Tm) with Mo2FeB2 structure

The title compounds were synthesized by reacting the elements in sealed tan talum tubes in a high-frequency furnace. They crystallize with the Mo2FeB2 structure, a ternary ordered variant of the U3Si2 type, space group P4/mbm. All compounds were characterized through Guinier powder patterns and the l attice parameters were obtained from least-squares fits. Four structures we re refined from single crystal X-ray data: a = 740.5(1) pm, c = 372.5(1) pm , wR2 = 0.0430, 247F(2), values, 13 variables for Y2Ni1.90Mg, a = 764.5(1) pm, c = 394.39(9) pm, wR2 = 0.0371, 310 F-2 values, 12 variables for La2Ni2 Mg, a = 754.4(1) pm, c = 385.20(9) pm, wR2 = 0.0460,295 F-2 values, 12 vari ables for Pr2Ni2Mg, and a = 752.53(8) pm, c = 382.33(5) pm, wR2 = 0.0183, 2 91 F-o(2) values, and 12 variables for Nd2Ni2Mg. A refinement of the occupa ncy parameters indicated small defects on the nickel site of the yttrium co mpound, resulting in the composition Y2Ni1.90Mg for the investigated single crystal. The compounds with cerium, samarium, and gadolinium to thulium as rare earth component were characterized through their Guinier powder patte rns. The cell colume of Ce2Ni2Mg is smaller than that of Pr2Ni2Mg, indicati ng intermediate-valent cerium. The structures can be considered as an inter growth of distored AlB2 and CsCl related slabs of compositions LnNi(2) and LnMg. Chemical bonding in La2Ni2Mg and isotypic La2Ni2In is compared on the basis of extended Huckel calculations.