Ternäre Palladiumpnictide der Erdalkali- und Seltenen Erdmetalle mit ThCr₂Si₂- und CaBe₂Ge₂-Struktur

Abstract

Fifteen compounds of compositionMPd₂ Pn ₂ (M = alkaline earth or rare earth metal,Pn = As, Sb, Bi) were prepared. TheirGuinier powder patterns show that the arsenides crystallize with the ThCr₂Si₂ type structure, the bismuthides with the closely related CaBe₂Ge₂ type structure. The antimonides most likely also have the CaBe₂Ge₂ structure as is demonstrated by a structure refinement of EuPd₂Sb₂ from single crystal X ray data (R = 0.039 for 366 independent structure factors and 15 variable parameters). The structure of SrPd₂As₂ (ThCr₂Si₂ type) was refined to a residual ofR = 0.020 for 182F values and 9 variables. EuPd₂Sb₂ is paramagnetic and a metallic conductor. A comparison of the cell volumes suggests intermediate valency for Eu in EuPd₂As₂.

Chemical bonding and especially the reasons for the adoption of the ThCr₂Si₂ or CaBe₂Ge₂ type structures by these compounds are discussed. It is suggested that in going from the phosphides to the bismuthides the ThCr₂Si₂ structure is loosing and the CaBe₂Ge₂ structure is gaining stability due to decreasing Pd-Pd bonding and increasing Pd-pnictogen bonding. This trend is caused by the increasing size of the pnictogen component.