New Magnesium Compounds / fcE 2 Cu 2 Mg ( RE = Y , La - Nd , Sm , Gd - Tm , Lu ) with Mo 2 FeB 2 Type Structure

The title compounds were synthesized by reaction of the elements in sealed tantalum tubes in a water-cooled sample chamber in a high-frequency furnace. These magnesium intermetallics crystallize with the tetragonal Mo2FeB2 type structure, space group PMmbm. The lattice pa­ rameters of all compounds were refined from X-ray powder data. Single crystal X-ray data yielded a = 792.09(6), c = 396.31(8) pm, wR2 = 0.0396, 315 F 2 values for La2Cu2Mg, a = 778.30(5), c = 384.04(5) pm, wR2 = 0.0954, 214 F 2 values for Nd2Cu2Mg, and a = 762.65(5), c = 374.09(3) pm, wR2 = 0.0566, 186 F 2 values for Y2Cu2Mg with 12 variable parameters for each refinement. The /?£,2Cu2Mg structures can be described as an intergrowth of distorted A1B2 and CsCl related slabs of compositions RECui and REMg. Chemical bonding in La2Cu2Mg was investigated on the basis of extended Hückel calculations and compared to isotypic La2Cu2ln. This structure was also refined from single crystal X-ray data: PMmbm, a = 780.8(2), c = 400.1(2) pm, wR2 = 0.0351, 211 F 2 values and 12 variable parameters.