A single crystal study of α-CeIr2Si2, β-CeIr2Si2 and CeIr1.968(5)Ge2

The title compounds were synthesized by arc-melting the elements in an argon atmosphere and subsequent annealing. β-CeIr2Si2 and CeIr2Ge2 crystallize directly from the melt, while α-CeIr2Si2 was obtained after annealing at 870 K. The three iridium compounds were investigated by X-ray diffraction on both powders and single crystals: ThCr2Si2 type, I4/mmm, a=408.82(2), c=1016.96(6) pm, wR2=0.0546, 95 Fo2 values, 9 variables for α-CeIr2Si2; CaBe2Ge2 type, P4/nmm, a=414.60(5), c=985.5(2) pm, wR2=0.0530, 361 Fo2 values, 15 variables for β-CeIr2Si2 and a=423.95(6), c=1010.1(2) pm, wR2=0.0750, 331 Fo2 values, 16 variables for CeIr2Ge2. Refinement of the occupancy parameters indicated small defects on the Ir2 site of CeIr2Ge2 resulting in the composition CeIr1.968(5)Ge2 for the investigated crystal. Striking structural motifs in these silicides and germanides are condensed distorted IrSi4/4 and IrGe4/4 tetrahedra and additionally IrSi5/5 and IrGe5/5 square pyramids in β-CeIr2Si2 and CeIr2Ge2 forming three-dimensional infinite [Ir2Si2] and [Ir2Ge2] networks in which the cerium atoms fill cages of coordination number 18. The crystal chemistry and chemical bonding of these intermetallics are briefly discussed.