CdS microclusters in sodalite frameworks of different composition: A density functional study

We present a local density functional study of Cd₄S and Cd₄S₄ clusters inside sodalite cages of different compositions (Al:Si ratios). The composition of the framework determines the cluster - cage charge transfer and strongly affects the atomic structure of the inclusion. The energy gap and the character of the highest occupied (HOMO) and lowest unoccupied (LUMO) electronic states depend on the size and stoichiometry of the included clusters, as well as on the overall stoichiometry of the composite. The calculated gap for Cd₄S inclusions in aluminosilicate and aluminate sodalite (at half and full packing respectively) is ∼2.5 eV (i.e., about twice the calculated gap of bulk CdS), while for Cd₄S₄ in aluminosilicate and pure silica sodalite (at half packing) it is 1.7-1.9 eV (i.e., about 1.5 times the gap of bulk CdS). Our results indicate that simple confinement arguments are usually insufficient to predict the behavior of semiconductor - zeolite composites.