Ternary Plumbides Atpb(2) (A = Ca, Sr, Ba, Eu; T= Rh, Pd, Pt) With Distorted, Lonsdaleite-Related Substructures Of Tetrahedrally Connected Lead Atoms

The plumbides CaTPb2 (T = Rh, Pd), EuTPb2 (T = Rh, Pd, Pt), SrTPb2 (T = Rh, Pd, Pt) and BaTPb2 (T = Pd, Pt) were obtained by direct reactions of the elements in sealed tantalum tubes in an induction furnace. The moisture sensitive polycrystalline samples were characterized by X-ray powder diffraction. They crystallize with the orthorhombic MgCuAl2-type structure, space group Cmcm. The structures of CaRhPb2 (a = 433.78(3), b = 1102.06(8), c = 798.43(6) pm, wR = 0.0285, 432 F-2 values and 16 variables) and EuPdPb2 (a = 457.24(5), b = 1158.27(13), c = 775.73(8), wR = 0.0464, 464 F-2 values and 16 variables) were refined from single crystal X-ray diffractometer data. The characteristic structural motif is the distorted tetrahedral substructure built up by the lead atoms with Pb-Pb distances of 326-327 pm in CaRhPb2 and of 315-345 pm in EuPdPb2. With increasing size of the alkaline earth (Eu) cation, the lead substructure becomes more anisotropic with a shift of the [TPb2] polyanions from three- to two-dimensional, leading to significantly increased moisture sensitivity. Temperature dependent magnetic susceptibility studies reveal Pauli paramagnetism for SrRhPb2, SrPtPb2, BaPdPb2 and BaPtPb2. EuRhPb2 and EuPdPb2 are Curie-Weiss paramagnets with stable divalent europium as is also evident from Eu-151 Mossbauer spectra. EuRhPb2 is a ferromagnet with T-C = 17.7(2) K, while EuPdPb2 orders antiferromagnetically at T-N = 15.9 K. This is in agreement with the full magnetic hyperfine field splitting of the Eu-151 Mossbauer spectra at T = 6 K.