The Arsenides LnPd 3As 2 ( Ln = La-Nd, Sm, Gd) and Structure Refinement of CePd 2- xAs 2 with the ThCr 2Si 2 Structure

Abstract

The title compounds were prepared in well-crystallized form by annealing the corresponding binary arsenides in a NaCl/KCl flux. The compounds Ln Pd 3As 2 crystallize with a new monoclinic structure type, which was determined from single-crystal X-ray data of GdPd 3As 2: C2/ m, a = 1656.3(6) pm, b = 404.6(2) pm, c = 993.7(4) pm, β = 107.85(2)°, Z = 6, R = 0.025 for 1728 structure factors and 58 variable parameters. These arsenides belong to a large structural family with a metal to metalloid ratio of 2:1. Somewhat unusual features in the structure of GdPd 3As 2 are the (distorted) octahedral coordination of one gadolinium site and the square-planar coordination of arsenic atoms around two palladium sites. All of these, however, are also observed for the corresponding atoms in the previously reported, closely related structure of Th 5Fe 19P 12. CePd 2- x As 2 has the tetragonal ThCr 2Si 2 structure ( a = 425.1(2)pm, c = 1026.1(6)pm, R = 0.023 for 244 F values and 11 variables) with an As-As distance of 247.1(1) pm. The refinement of the occupancy parameter of the palladium position resulted in a value of 87.9(2)% corresponding to the formula CePd 1.758(4)As 2. It is argued that the formation of these defects reduces antibonding (destabilizing) Pd-Pd interactions.