Structure and optical properties of CdS superclusters in zeolite hosts

Direct synthesis of CdS within the pore structure of zeolites leads to a novel supercluster with a structural geometry superimposed by the host framework. Detailed x-ray powder diffraction and EXAFS analysis together with optical absorption data reveal discrete (CdS, O){sub 4} cubes located within the small sodalite units of the structure which begin to interconnect as the loading density within the zeolite rises. The discrete cube building blocks consist of interlocking tetrahedra of Cd and S with a CdS bond length of 2.47 {angstrom}. At higher loadings these cubes begin to occupy adjacent sodalite units where the Cd atoms point toward each other through the double six-rings linking the sodalite moieties with a Cd-Cd distance of {approximately} 6 {angstrom}. As this three-dimensional interconnection proceeds, the corresponding changes in optical properties indicate a progression toward a semiconductor supercluster with behavior intermediate between that of the discrete CdS cubes and bulk semiconductor. Semiconductor superclusters of this type represent a novel class of materials where the three-dimensional structure and electronic properties can be controlled by using different zeolites as the template. The unique stability of the semiconductor clusters inside the sodalite units is due to the coordination of Cd atoms with the framework oxygen atoms of the double six-ring windows. The stability of the supercluster comes from the interaction between clusters in the adjacent sodalite units. It is suggested that through-bond coupling is responsible for the interaction between clusters. 43 refs., 12 figs., 5 tabs.