Structurally Complex Cobalt Intermetallics Grown from Liquid Aluminum:  Co₁₉Al₄₅Si_10-x (x = 0.13) and Co₅Al₁₄Si₂

The cobalt aluminum silicides Co₁₉Al₄₅Si_10-x (x = 0.13) and Co₅Al₁₄Si₂ were synthesized in liquid aluminum and characterized by single-crystal X-ray diffraction. Co₁₉Al₄₅Si_10-x (x = 0.13) crystallizes in the monoclinic space group C2/c with lattice parameters a = 19.991(2) Å, b = 19.143(2) Å, c = 12.8137(15) Å, β = 123.583(2)°. Co₅Al₁₄Si₂ adopts the orthorhombic space group Pnma with cell parameters a = 13.8948(19) Å, b = 23.039(3) Å, c = 7.3397(10) Å. Both structures are exceptionally complex with the Co₂Si₂ rhombus being a common building motif. The coordination environments of cobalt atoms resemble those of the transition metals in typical quasi-crystal approximants. Co₅Al₁₄Si₂ shows oxidation resistance in air up to 1000 °C by forming a dense-packed Al₂O₃ layer on the surface of the crystal.