Routes to the quaternary aluminum silicides RE₄FE_2+xAl_7-xSi₈ (RE = Ce, Pr, Nd, Sm); exploratory synthesis with molten Al as a solvent

The four new intermetallic aluminum silicides RE₄Fe_2+xAl_7-xSi₈ (RE = Ce, Pr, Nd, Sm) crystallize from the reaction of Si, Fe, and RE (or rare earth oxides) in molten Al at 850 °C. 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) Å, b = 16.265(3) Å, c = 4.0804(8) Å, R1 = 0.0196, and wR2 = 0.0486 for the Sm analogue. The crystal structure is a complex three-dimensional network comprised of repeating layers containing Al, Si, and Fe connected by atoms between the layers. The RE³⁺ ions are then located within tunnels 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 calculations, carried out on the hypothetical analogue Y₄Fe₂Al₇Si₈, predict metallic behavior and suggest Fe to be in a reduced state with almost filled d orbitals. Variable temperature electrical conductivity and thermopower measurements 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.