New R3Pd5 Compounds (R = Sc, Y, Gd-Lu): Formation and Stability, Crystal Structure, and Antiferromagnetism

The phases reported in the literature as "R2Pd3" (R = rare earth element) have been reinvestigated. The exact stoichiometric composition of this series of compounds, which form for R = Sc, Y, and from Gd to Lu, including Yb, was found to be R3Pd5. All of them crystallize in the orthorhombic Pu3Pd5 structure type (oS32-Cmcm). The crystal structure has been refined from both single crystal (for Tb3Pd5) and powder X-ray diffraction data (for Tb3Pd5, Ho3 Pd5, and Tm3Pd5). These compounds represent the first example of a binary phase formed by R and Pd adopting the Pu3Pd5-type featuring two crystallographic nonequivalent sites for the R atoms in the unit cell (the Wyckoff sites 4c and 8e). The variation of the lattice parameters and unit cell volume along the series strictly follows the trend of the lanthanide contraction. An extrapolation of the volume contraction versus the R3+ ionic radius gives an atomic volume of 29.74 Å3 for Yb in the hypothetical trivalent metallic state (under normal conditions). The formation temperatures and mechanisms, a peritectic reaction, and stability ranges have also been investigated. It turns out that Gd3Pd5 is a high temperature phase; it was not possible to quench this compound as a metastable phase, at room temperature, to be measured. In the light of our results, most of the R-Pd phase diagrams need to be revised. The magnetization, heat capacity, and electrical resistivity have been measured for Tb3Pd5, Dy3Pd5, Ho3Pd5, and Er3Pd5. They order antiferromagnetically at low temperatures, each undergoing two transitions, TN1 and TN2 (with TN1 going from 13.5 to 5.1 K and TN2 going from 6.5 to 3.6 K, respectively for Tb and Ho compounds). From our data we cannot distinguish whether the two rare earth sublattices sequentially order magnetically at TN1 and TN2, respectively, or whether they are simultaneously involved in both transitions. The electronic structure calculations predict antiferromagnetic ordering also for Gd3Pd5. Y3Pd5 is a Pauli paramagnet