Two New Binary Calcium−Aluminum Compounds:  Ca₁₃Al₁₄, with a Novel Two-Dimensional Aluminum Network, and Ca₈Al₃, an Fe₃Al-Type Analogue¹

Abstract

Ca 13 Al 14 and Ca 8 Al 3 are obtained by fusion of the appropriate mixture of the elements in Ta containers at 1100 °C followed by annealing at 600 °C or slow cooling, respectively. The structures of both compounds were determined by single-crystal X-ray means. Ca 13 Al 14 crystallizes with monoclinic symmetry (space group C 2/ m (no. 12), Z = 2, a = 15.551(4) Å, b = 9.873(2) Å, c = 9.726(2) Å, β = 108.09(2)°), and Ca 8 Al 3 has the triclinic Ca 8 In 3 -type structure ( P 1̄ (no. 2), Z = 2, a = 9.484(3) Å, b = 9.592(3) Å, c = 9.671(3) Å, α = 99.02(3)°, β = 101.13(3)°, γ = 119.55(3)°). Ca 13 Al 14 contains a two-dimensional Al network structure composed of planar hexagonal six-membered rings, planar rhombus (four-membered) rings, and trigonal three-membered rings. An electron count on the basis of the simple Zintl−Klemm formalism for three- and four-bonded Al in Ca 13 Al 14 suggests the phase is closed shell. However, full band-structure calculations within the extended Hückel formalism indicate that it is metallic, with considerable Ca−Al covalency and, characterically, with all Al−Al bonding states just filled at E F . The descriptor “metallic Zintl phase” is apt. Ca 8 Al 3 contains isolated Ca and Al atoms and is slightly electron-rich relative to classical valence rules. Both compounds are good metallic conductors with Pauli-paramagnetic-like properties.