Cubic Aluminum Silicides RE8Ru12Al49Si9(AlxSi12-x) (RE = Pr, Sm) from Liquid Aluminum. Empty (Si,Al)12Cuboctahedral Clusters and Assignment of the Al/Si Distribution with Neutron Diffraction

Abstract

Two new quaternary aluminum silicides, RE₈Ru₁₂Al₄₉Si₉(Al_xSi_12-x) (x ∼ 4; RE = Pr, Sm), have been synthesized from Sm (or Sm₂O₃), Pr, Ru, and Si in molten aluminum between 800 and 1000 °C 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 structural type. The structure, determined by single-crystal X-ray diffraction, is cubic, space group Pm3̄m with Z = 1, and has lattice parameters of a = 11.510(1) Å for Sm₈Ru₁₂Al₄₉Si₉(Al_xSi_12-x) and a = 11.553(2) Å for Pr₈Ru₁₂Al₄₉Si₉(Al_xSi_12-x) (x ∼ 4). The structure consists of octahedral units of AlSi₆, at the cell center, Si₂Ru₄Al₈ clusters, at each face center, SiAl₈ cubes, at the middle of the cell edges, and unique (Al,Si)₁₂ cuboctohedral clusters, at the cell corners. These different structural units are connected to each other either by shared atoms, Al−Al bonds, or Al−Ru bonds. The rare earth metal atoms fill the space between various structural units. The Al/Si distribution was verified by single-crystal neutron diffraction studies conducted on Pr₈Ru₁₂Al₄₉Si₉(Al_xSi_12-x). Sm₈Ru₁₂Al₄₉Si₉(Al_xSi_12-x) and Pr₈Ru₁₂Al₄₉Si₉(Al_xSi_12-x) show ferromagnetic ordering at T_c ∼10 and ∼20 K, respectively. A charge of 3+ can be assigned to the rare earth atoms while the Ru atoms are diamagnetic.