Frameworks of clusters in alkali metal-gallium phases: Structure, bonding and properties

Abstract

Abstract The intermetallic compounds of gallium are at the border of Zintl phases and metallic systems. Like boron, gallium behaves as a hypoelectronic element forming clusters and fused clusters as well as fragments that need to be reduced by electropositive elements. Actually, owing to its electron paucity, gallium is not able to form isolated clusters because the anionic charges on such units would be too large. In intermetallic phases with alkali metals, this is palliated by intercluster bonding which reduces the charge by one every time an exo-bond is formed. Frameworks may be simple or very complicated, such as in ternary phases where atom defections, disorder and non-stoichiometry obscure the understanding of electronic and bonding requirements. While classical Zintl phases are generally insulators or semiconductors in which the octet and (8-N) rules are fulfilled, gallium intermetallic phases exhibit locally delocalized electrons since coordination numbers within clusters are larger than available bonding electrons pairs. Nevertheless, clusters have well defined electron counts with adapted structures. For clusters that are not too large, these counts conform to Wade's rules, while EHMO calculations are useful for determining counts of higher nuclearity clusters. The three-dimensional networks usually have overall electron counts which are balanced by the total number of valence electrons in the structure, in which the alkali metals are completely ionized. Sometimes the number of available valence electrons exceeds the number required for the anionic lattice stabilization. In this case, with interatomic distances which are in the order, or even less, than in pure metals, alkali metal atoms keep electrons in the conduction band and intermetallic phases behave like poor metals. Such properties have been recently described by Corbett203 for the phase Na7In11.8 with ϱ295 ∼ 540 μ Ω.cm and a Pauli paramagnetism χm of ∼ 1.75 10−4 emu mol−1. The valence bond theory developped by Pauling for metals or intermetallic compounds hardly applies to gallium phases. Bond orders calculated with Pauling's formula are generally larger than those deduced from Extended Huckel Molecular Orbital calculations and lead to valences greater than 4 in certain cases. This would imply the participation of d-electrons to the bonding, which is quite unlikely. Fluctuations of lengths of which would be considered as single GaGa bonds are important. Dramatic shortening of GaGa bonds occurs for small clusters of which the shape is far from being spherical. In this case, tangential atomic orbitals deviate from the atomic edges and the overlap along these edges has been found less σ bonding than in large and nearly spherical clusters. The presence or the absence of non-bonding pairs on atoms that lack exo-bonds is very important for the understanding of versatility and modelling of the more reduced phases. As shown by Burdett and Canadell160,161, the highest bonding molecular orbitals are usually isolated lone pairs on atoms without exo-bonds. In certain cases, and as it is well exemplified in this work for the triply fused defective icosahedron in Na13K4Ga49.57, the lone pair orbitals are raised in the antibonding region. This can lead to fractional occupancies, or even, to expulsion of atoms from their sites, which is why the triply fused “twice or triply-nido” icosahedron has been found to be more stable than the defect-free one. According to these findings, reduction by electrochemical insertion of alkali metals should be able to modify the intermetallic phases of gallium. Topotactic reactions would be possible only within a restricted range of stoichiometry. Deep reduction breaks up the original structures to form non-stoichiometric phases which can be considered as intermediary steps towards the formation of more packed structures. Like boron, and probably indium123,203–205, gallium may be regarded as an “icosogen” element able to form icosahedral extended structures and presumably quasi-periodic materials under certain conditions. In most cases, formation of Samson's polyhedra occurs as a stage in making quasi-crystals whenever isolated atoms or weakly coordinated spacers are not present. In the opposite, the presence of the latter should be regarded as efforts to create periodic lattices.