Routes to the quaternary aluminum silicides RE4Fe2+xAl7-xSi8Si8 (RE = Ce, Pr, Nd, Sm); Exploratory synthesis with molten Al as a solvent

The four new intermetallic aluminum silicides RE4Fe2+xAl7-xSi8 (RE = Ce, Pr , Nd, Sm) crystallize from the reaction of Si, Fe, and RE (or rare earth ox ides) in molten Al at 850 degreesC. All compounds share the same structure type as determined by single-crystal X-ray diffraction analysis. They form in the space group Cmmm (No. 65) with cell constants of a = 10.909(2) Angst rom, b = 16.265(3) Angstrom, c = 4.0804(8) Angstrom, R1 = 0.0196, and wR2 = 0.0486 for the Sm analogue. The crystal structure is a complex three-dimen sional network comprised of repeating layers containing Al, Si, and Fe conn ected by atoms between the layers. The RE3+ ions are then located within tu nnels of the three-dimensional network, running parallel to the c axis with a coordination number of 14. Magnetic susceptibility measurements indicate that the rare earth ions are in a 3+ oxidation state, whereas the Fe atoms are in an effective diamagnetic state. Electronic band structure calculati ons, carried out on the hypothetical analogue Y4Fe2Al7Si8, predict metallic behavior and suggest Fe to be in a reduced state with almost filled d orbi tals. Variable temperature electrical conductivity and thermopower measurem ents confirm the metallic nature of the compounds. The charge transport and magnetic properties of the Ce analogue are anomalous and indicative of f(1 /0) valence fluctuations at T < 100 K.