High-pressure High-temperature Synthesis, Magnetic Properties and X-ray Absorption Spectroscopy of Phases RE3Sn7 and hp-RESn3-x (RE = Tb, Ho, Er)

The high-temperature high-pressure syntheses of binary phases hp-RESn3-x (RE = Tb, Ho, Er) was reinvestigated showing good agreement with previously determined stability fields concerning pressure and temperature conditions. Phases hp-RESn3-x adopt a cubic Cu3Au arrangement (a = 466.52(6) pm for hp-TbSn3-x; a = 465.44(8) pm for hp-HoSn3-x and a = 464.98(9) pm for hp-ErSn3-x). Superstructure reflections observed in experiments with synchrotron radiation indicate an ordered arrangement for HoSn3-x prepared at 8.4(8) GPa and 1270 K (space group Pm (3) over barm). hp-HoSn3-x transforms exothermally at 460 K and ambient pressure into the low-pressure modification HoSn3-x indicating that the ordered Cu3Au-type arrangement is a metastable high-pressure phase. A second set of phases is identified as compounds RE3Sn7 (RE = Ho, Er) being isotypic to the ambient pressure modifications Gd3Sn7 and Tb3Sn7 (space group Cmmm; lattice parameters a = 434.95(7) pm, b = 2615.6(6) pm, c = 442.09(7) pm for Ho3Sn7 and a = 434.29(5) pm, b = 2607.2(4) pm, c = 441.49(5) pm) for Er3Sn7). The crystal structure of compounds RE3Sn7 is interpreted as an intergrowth of AlB2-, CaF2- and AuCu3-like segments. Analysis of the chemical bonding in Er3Sn7 by means of the electron localizability approach reveals two-center Sri Sit bonding in the AlB2-like segment and multi-center bonding in the AuCu3- and CaF2-like motifs. The electrical resistivity of rho approximate to 31 mu Omega cm at 300 K reveals that hp-HoSn3-x is a metallic conductor. The phases hp-RESn3-x and RE3Sn7 (RE = Ho, Er) show paramagnetic Curie Weiss behavior of the rare earth metal ions Ho3+ and Er3+, respectively. Ho3Sn7 and Er3Sn7 order antiferromagnetically at 10.0 K and 8.2 K, respectively. The high-pressure phases RESn3-x show no magnetic order above 2 K. XAS measurements on the compounds Tb3Sn7 and hp-TbSn3-x indicate a 4f (8) configuration evidencing the oxidation state +3 for terbium.