A single crystal study of alpha-CeIr2Si2, beta-CeIr2Si2 and CeIr1.968((5))Ge-2

The title compounds were synthesized by are-melting the elements in an argo n atmosphere and subsequent annealing. beta -CeIr2Si2 and CeIr2Ge2 crystall ize directly from the melt, while alpha -CeIr2Si2 was obtained after anneal ing at 870 K. The three iridium compounds were investigated by X-ray diffra ction on both powders and single crystals: ThCr2Si2 type, I4/mmm, a=408.82( 2), c = 10167.96(6) pm. wX2 = 0.0546, 95 F-0(2) values, 9 variables for alp ha -CeIr2Si2; CaBe2Ge2 type, P4/nmm, a= 414.60(5), c= 985.5(2) pm, wR2 = 0. 0530, 361 F-0(2) values, 15 variables for beta -CeIr2Si2 and a = 423.95(6), c = 1010.1(2) pm, wR2 = 0.0750, 331 F-0(2) values, 16 variables for CeIr2G e2. Refinement of the occupancy parameters indicated small defects on the I r2 site of CeIr2Ge2 resulting in the composition CeIr1.968(5)Ge2 for the in vestigated crystal. Striking structural motifs in these silicides and germa nides are condensed distorted IrSi4/4 and IrGe4/4 tetrahedra and additional ly IrSi5/5 and IrGe5/5 square pyramids in beta -CeIr2Si2 and CeIr2Ge2 formi ng three-dimensional infinite [Ir2Si2] and [Ir2Ge2] networks in which the c erium atoms fill cages of coordination number 18. The crystal chemistry and chemical bonding of these intermetallics are briefly discussed. (C) 2001 E lsevier Science Ltd. All rights reserved.