New structures formed by R3Au4Sn3, R5Au8Sn5 and R3Au6Sn5 compounds (R= rare earths)

In a narrow range from 40 to 45 at.% Au three phases were found in the systems ReAueSn (R ¼ rare earth): R3Au4Sn3 with R ¼ LaeNd, R5Au8Sn5 with R ¼ Sm, Gd and R3Au6Sn5 with R ¼ LaeNd, Sm. The structures of Pr3Au4Sn3 and Gd5Au8Sn5 were determined from X-ray single crystal diffractometer data. Pr3Au4Sn3 is trigonal, R3m, a ¼ 4.7676(4) A, c ¼ 35.965(6) A, Z ¼ 3, wR2 ¼ 0.060, with refined composition Pr3Au4.15Sn2.85; Gd5Au8Sn5 is hexagonal, P63/mmc, a ¼ 4.6812(3) A, c ¼ 42.497(7) A, Z ¼ 2, wR2 ¼ 0.078, with refined composition Gd4.970Au8.213Sn4.96. Both represent new structures, belonging to the series RmþnT2mþnSnmþn (R ¼ rare earth; T ¼ Ni, Cu, Au) built up by intergrowth of m GdPt2Sn and n SrPtSb slabs. Lattice parameters of the isotypic phases were obtained by X-ray powder diffraction analysis. The R3Au6Sn5 compounds crystallize with the previously determined Pr3Au6Sn5 structure, isopointal with the Ce3Pd6Sb5 type, and their lattice parameters derived from powder data are also given. The first principles electronic structure calculations, at 0 K, show increased enthalpy of formation across the lanthanide series in R3Au6Sn5 compounds caused by the decreased integrated density of states at the Fermi level. The systematic increase of the enthalpy of formation across the lanthanide series is the main reason why experimentally R3Au6Sn5 compounds do not form with heavy lanthanides.