Grigori V. Vajenine | PubMed Health

PubMed1999

Magic Electron Counts for Networks of Condensed Octahedral Niobium Clusters in Oxoniobates.

Chemical bonding in networks of octahedral oxoniobate clusters condensed through vertex-sharing is analyzed at the extended Hückel level of theory. Crystal or molecular orbitals of the condensed cluster networks are derived from the Nb-Nb bonding molecular orbitals of the monomeric Nb(6)O(18) cluster. Extended networks are treated by considering crystal orbitals at special points of the corresponding Brillouin zones. We find that cluster electron counts corresponding to optimal bonding in these networks decrease from 14 for the Nb(6)O(18) cluster to 12 per Nb octahedron for a dimer, 11 for a linear chain of clusters, 10-10(1)/(2) for a square network of clusters, and 7(3)/(4)-8(1)/(2) for cubic NbO in which all cluster vertices are shared. These results are in good agreement with both the counts derived from the experimentally observed structures and a previously devised counting scheme. The loss of Nb-Nb bonding at the shared cluster vertices and the Nb-O-Nb pi antibonding involving Nb atoms of neighboring octahedra are the reasons for the reduction of the magic cluster electron counts in the considered networks. It is proposed that similar counting schemes can be developed for other condensed cluster networks.

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PubMed1996

Compounds Containing Copper-Sulfur Layers: Electronic Structure, Conductivity, and Stability.

Compounds of the general formula MCu(2n)X(n)(+1), where M is a monovalent metal and X is a chalcogen, exhibit relatively high conductivity and an interesting structural pattern of copper-chalcogen layers. The electronic structure of a series of copper-sulfur layers with the Cu(2n)S(n)(+1) stoichiometry was studied using the extended Hückel method. Attention was focused on the unoccupied states at the top of the valence band. These states are Cu-S and Cu-Cu antibonding, which accounts for the observed contraction in the plane of the layers. The same states turn out to be strongly delocalized in the plane of the layers, with both copper and sulfur contribution; high mobility of holes in these states is responsible for the substantial conductivity observed in the corresponding materials. The idea of isodesmic reactions, borrowed from computational organic chemistry, was developed to address the relative stabilities of the copper-sulfur layers. We found the Cu(2)S(2)(-) layer to be less stable than the Cu(4)S(3)(-) layer, in accord with experiment.

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