Zinc-lead ordering in equiatomic rare earth plumbides REZnPb (RE=La-Nd and Sm-Tb)

The plumbides REZnPb (RE=rare earth metals La-Nd and Sm-Tb) were synthesized by induction melting of the elements in sealed niobium ampoules. The samples were characterized by X-ray powder diffraction. The structures of the praseodymium and neodymium compound were refined from single-crystal X-ray diffractometer data: YPtAs type, P6₃/mmc, a=461.32(8), c=1658.00(3) pm, wR2=0.0588, 267 F² values, 12 variables for PrZnPb and a=460.12(3), c=1642.7(1), wR2=0.0617, 243 F² values, 12 variables for NdZnPb. The plumbides with RE=La, Ce, Sm, Gd and Tb are isotypic while the Zintl phase EuZnPb crystallizes with the orthorhombic TiNiSi type, Pnma, a=796.6(2), b=482.53(9), c=822.9(2) pm. The zinc and lead atoms build up polyanionic networks: slightly puckered Zn₃Pb₃ layers in AA^'BB^' stacking sequence in the YPtAs type plumbides and a three-dimensional [ZnPb]^2- network with distorted tetrahedral ZnPb_4/4 coordination in EuZnSn. The different crystal structures are a consequence of the valence electron count, i.e. 18 for EuZnSn and 19 for the hexagonal plumbides. The crystal chemical details and bonding peculiarities are discussed.