Cation-anion versus cation-framework interactions in sodalites: First-principles study of model Cu-exchanged sodalites

We present Local Density Functional calculations of the structural and elec tronic properties of halosodalite systems in which the Na+ cations are prog ressively exchanged by Cu+. The calculated lattice constant decreases with increasing Cu loading up to about two Cu/cage, while it is approximately in dependent of Cu content at higher loadings, in qualitative agreement with e xperimental observations for silver-exchanged sodalites. The reduction of t he lattice parameter is accompanied by the formation of partially covalent Cu-X bonds (X=Cl, Br). The length of these bonds increases with increasing the number of Cu ions per cage, resulting in a simultaneous reduction of Cu -framework distances. For fully exchanged sodalites, the competition betwee n Cu-X and Cu-framework interactions leads to a splitting of the enclosed C u4X3+ aggregates in fragments. In the electronic density of states, Cu 3d s tates give rise to a narrow band above the top of the valence band of natur al sodalite. Thus the band gap is strongly reduced w.r.t. that of natural s odalite, consistent with experiments for silver-exchanged sodalites. This r eduction of the gap is largely independent of the Cu content, i.e., it alre ady occurs at low Cu loading. (C) 1999 American Institute of Physics. [S002 1-9606(99)31117-X].