A Coloring Study of the Ga Richest Alkali Gallides: New In- and Hg-Containing Gallides with the RbGa7- and the K3Ga13-Type Structure

The Ga-rich gallides of the alkali metals present an interesting, yet still scarcely investigated case of polyanionic cluster compounds with subtle variations in the character of their chemical bonding. In the present work, the Ga richest phases K3Ga13, RbGa7, and CsGa7, which are formally electron-precise Zintl/Wade cluster compounds, are systematically studied with respect to a partial substitution of Ga by In and Hg. The pure hepta-gallides AGa7 (A = Rb/Cs; R3¯m), which were formerly obtained from Ga-rich melts in powder form only, were crystallized from Hg-rich melts. Herein, up to 9.9/13.6% (Rb/Cs) of Ga could be substituted by In, which partly takes the four-bonded [M2] dumbbells connecting layers of Ga-icosahedra. Even though the structures are electron precise, the pseudo band gap does not coincide with the Fermi level. In the most Ga-rich potassium compound K3Ga13 (Cmcm) only 1.2% of In and 2.7% of Hg could be incorporated. Although Rb3Ga13 remains unknown, ternary variants containing 5.2 to 8.2% In could be obtained; this structure is also stabilized by a small Hg-proportion. The likewise closed-shell 3D polyanion consists of all-exo-bonded Ga-icosahedra and closo [Ga11] clusters, which are connected by two tetrahedrally four-bonded Ga− and a trigonal-planar three-bonded Ga0. The aspects of the electronic structures and the site-specific Ga↦Hg/In substitution in the polyanion (“coloring”) are discussed for the title compounds and other mixed Ga/In trielides.