EXPLORATORY SYNTHESIS WITH MOLTEN ALUMINUM AS A SOLVENT AND ROUTES TOMULTINARY ALUMINUM SILICIDES - SM2NI(NIXSI1-X)AL4SI6 (X = 0.18-0.27) - A NEW SILICIDE WITH A FERROMAGNETIC TRANSITION AT 17.5 K

A new quaternary silicide, Sm2Ni(NixSi1-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 OC. The structure, determined by single-crystal X-ray dif fraction, is tetragonal, space group P4/nmm (No. 129) with Z = 2, and lattice parameters a = b = 5.8060(3) Angstrom, c = 14.845(1) Angstrom. Refinement based upon F-2 [I > 2 sigma(T)] yielded R1 = 0.0252 and wR 2 = 0.0634. The compound exhibits a new structure type containing two different alternating layers which are linked together through Si/Ni-S i 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 p lanar rings. The structure of this compound cannot be rationalized on the basis of the Zintl-Klemm concept. Extended Huckel, tight binding c alculations were carried out for different hypothetical stoichiometrie s, besides the observed one, of the compound Sm2Ni(NixSi1-x)Al4Si6. Fi ve models were investigated with x = 0, 0.25, 0.5, 0.75, and 1. The co mpounds with x = 0 and 0.25 are predicted to be more stable than the o thers. Electrical conductivity and thermopower data indicate that the compound is p-type metallic. The temperature-dependent magnetic suscep tibility exhibits an antiferromagnetic ordering near 60 K and a weak f erromagnetic (WF) transition near 17 K. High temperature (150-300 K) m agnetic susceptibility data suggest that Sm is in the 3+ oxidation sta te.