Distribution of the M-M Distances in the Oxides of the Group 13 Elements and their Spinels and Delafossites

An analysis of the distribution of the M-M distances (M = Al, Ga, In, Tl) in all their binary, ternary and quaternary oxides, contained in the Inorganic Crystal Structure Database (ICSD95), shows that the M-M distances are not distributed uniformly, but present maxima at values which are near the corresponding distances in pure metals. As for boron, this feature reflects the tendency of these elements to form aggregates in their compounds. These metal aggregates maintain distances and, in many instances, also the topology of the elemental structures, as is illustrated here with two families of structures, i.e. spinels and delafossites. In spinels, the group 13 cations form a 3D (three-dimensional) network of truncated tetrahedra, which are also found in the Zintl phases of these elements. In MgAl₂O₄ the Al subnet is identical, in topology and dimensions, to that of CaAl₂ and both reproduce exactly half of the f.c.c. (face-centred cubic) net of Al metal.