2 NEW BINARY CALCIUM-ALUMINUM COMPOUNDS - CA13AL14, WITH A NOVEL 2-DIMENSIONAL ALUMINUM NETWORK, AND CA8AL3, AN FE3AL-TYPE ANALOG

Ca13Al14 and Ca8Al3 are obtained by fusion of the appropriate mixture of the elements in Ta containers at 1100 degrees C followed by anneali ng at 600 degrees C or slow cooling, respectively. The structures of b oth compounds were determined by single-crystal X-ray means. Ca13Al14 crystallizes with monoclinic symmetry (space group C2/m (no. 12), Z = 2, a = 15.551(4) Angstrom, b = 9.873(2) Angstrom, c = 9.726(2) Angstro m, beta = 108.09(2)degrees), and Ca8Al3 has the triclinic Ca8In3-type structure (Pi (no. 2), Z = 2, a = 9.484(3) Angstrom, b = 9.592(3) Angs trom, c = 9.671(3) Angstrom, alpha = 99.02(3)degrees, beta = 101.13(3) degrees gamma = 119.55(3)degrees). Ca13Al14 contains a two-dimensional Al network structure composed of planar hexagonal six-membered rings, planar rhombus (four-membered) rings, and trigonal three-membered rin gs. An electron count on the basis of the simple Zintl-Klemm formalism for three- and four-bonded Al in Ca13Al14 suggests the phase is close d shell. However, full band-structure calculations within the extended Huckel formalism indicate that it is metallic, with considerable Ca-A l covalency and, characterically, with all AI-AI bonding states just f illed at EF The descriptor ''metallic Zintl phase'' is apt. Ca8Al3 con tains isolated Ca and Al atoms and is slightly electron-rich relative to classical valence rules. Both compounds are good metallic conductor s with Pauli-paramagnetic-like properties.