New structures formed by R_3Au_4Sn_3, R_5Au_8Sn_5 and R_3Au_6Sn_5 compounds (R = rare earths)

摘要

In a narrow range from 40 to 45 at% Au three phases were found in the systems R-Au-Sn (R = rare earth): R_3Au_4Sn_3 with R = La-Nd, R_5Au_8Sn_5 with R = Sm, Gd and R_3Au_6Sn_5 with R = La-Nd, Sm. The structures of Pr_3Au_4Sn_3 and Gd_5Au_8Sn_5 were determined from X-ray single crystal diffractometer data. Pr_3Au_4Sn_3 is trigonal, R3m, a = 4.7676(4) A, c = 35.965(6) A, Z = 3, wR2 = 0.060, with refined composition Pr_3Au_(4.15)Sn_(2.85); Gd_5Au_8Sn_5 is hexagonal, P6_3/mmc, a = 4.6812(3) A, c = 42.497(7) A, Z = 2, wR2 = 0.078, with refined composition Gd_(4.970)Au_(8.213)Sn_(4.96). Both represent new structures, belonging to the series R_(m+n)T_(2m+n)Sn_(m+n) (R = rare earth; T = Ni, Cu, Au) built up by intergrowth of m GdPt_2Sn and n SrPtSb slabs. Lattice parameters of the isotypic phases were obtained by X-ray powder diffraction analysis. The R_3Au_6Sn_5 compounds crystallize with the previously determined Pr_3Au_6Sn_5 structure, isopointal with the Ce_3Au_6Sn_5 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 R_3Au_6Sn_5 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 R_3Au_6Sn_5 compounds do not form with heavy lanthanides.