Cubic aluminum silicides RE8Ru12Al49Si9(AlxSi12-x) (RE = Pr, Sm) from liquid aluminum. Empty (Si,Al)(12) cuboctahedral clusters and assignment of theAl/Si distribution with neutron diffraction

Two new quaternary aluminum silicides, RE8Ru12Al49Si9(AlxSi12-x) (x similar to 4; RE = Pr, Sm), have been synthesized from Sm (or Sm2O3), Pr, Ru, and Si in molten aluminum between 800 and 1000 degreesC in sealed fused silica tubes. Both compounds form black shiny crystals that are stable in air and NaOH. The Nd analog is also stable. The compounds crystallize in a new stru ctural type. The structure, determined by single-crystal X-ray diffraction, is cubic, space group Pm3m with Z = 1, and has lattice parameters of a = 1 1.510(1) Angstrom for Sm8Ru12Al49Si9(AlxSi12-x) and a = 11.553(2) Angstrom for Pr8Ru12Al49Si9(AlxSi12-x) (x similar to 4) The structure consists of oc tahedral units of AlSi6, at the cell center, Si2Ru4Al8 clusters, at each fa ce center, SiAl8 cubes, at the middle of the cell edges, and unique (Al,Si) (12) cuboctohedral clusters, at the cell corners. These different structura l units are connected to each other either by shared atoms, AI-AI bonds, or AI-Ru bends. The rare earth metal atoms fill the space between various str uctural units. The Al/Si distribution was verified by single-crystal neutro n diffraction studies conducted on Pr8Ru12Al49Si9(AlxSi12-x) Sm8Ru12Al49Si9 (AlxSi12-x) and Pr8Ru12-Al49Si9(AlxSi12-x) show ferromagnetic ordering at T -c similar to 10 and similar to 20 K, respectively. A charge of 3+ can be a ssigned to the rare earth atoms while the Ru atoms are diamagnetic.