Exploratory Synthesis with Molten Aluminum as a Solvent and Routes to Multinary Aluminum Silicides. Sm₂Ni(Ni_xSi_1-x)Al₄Si₆ (x = 0.18-0.27): A New Silicide with a Ferromagnetic Transition at 17.5 K

A new quaternary silicide, Sm₂Ni(Ni_xSi_1-x)Al₄Si⁶ (x = 0.18-0.27), has been synthesized from Sm₂O₃, NiO, and Si or Sm, Ni, and Si in Al metal flux at 800 °C. The structure, determined by single-crystal X-ray diffraction, is tetragonal, space group P4/nmm (No. 129) with Z = 2, and lattice parameters a = b = 5.8060(3) Å, c = 14.845(1) Å. Refinement based upon F² [I > 2σ(I)] yielded R1 = 0.0252 and wR2 = 0.0634. The compound exhibits a new structure type containing two different alternating layers which are linked together through Si/Ni-Si bonds to form a three-dimensional framework. One layer is formed by edge-shared (the edges parallel to c) NiAl₈ cubes. The other layer is a Si-based net which consists of six-member, five-member, and square planar rings. The structure of this compound cannot be rationalized on the basis of the Zintl-Klemm concept. Extended Hückel, tight binding calculations were carried out for different hypothetical stoichiometries, besides the observed one, of the compound Sm₂Ni(Ni_xSi_1-x)Al₄Si₆. Five models were investigated with x = 0, 0.25, 0.5, 0.75, and 1. The compounds with x = 0 and 0.25 are predicted to be more stable than the others. Electrical conductivity and thermopower data indicate that the compound is p-type metallic. The temperature-dependent magnetic susceptibility exhibits an antiferromagnetic ordering near 60 K and a weak ferromagnetic (WF) transition near 17 K. High temperature (150-300 K) magnetic susceptibility data suggest that Sm is in the 3+ oxidation state.