Crystal Structures of Metal Silicates and Phosphates from Conventional X-Ray Powder Diffraction Data. Damodara M. Poojary and Abraham Clearfield. Department of Chemistry, Texas A & M University, College Station, TX 77843

We have been interested in the structural chemistry of metal phosphates, phosphonates, and silicates because of their potential applications in the area of ion exchange, intercalation, sorption, catalysis, etc. Most of these compounds can be obtained only in the polycrystalline form and their structural characterization depends mainly on the use of powder diffraction data. It should also be mentioned that these compounds are difficult to crystallize and generally their diffraction intensities fall off very rapidly at higher scattering angles. Nevertheless, over the past few years, a large number of structures of this class of compounds were solved by ab initio methods using mainly conventional X-ray diffraction data. These results formed the basis for explaining their physical properties and to design materials for specific purposes.

In this talk, special attention will be given to the structures of some porous silicotitanates which are highly selective for radioactive ions like Cs⁺. These compounds consist of tetrameric titanium octahedra that are linked by silicate groups to form tunnels. The exchangeable cations and water molecules occupy the centers of the tunnels. X-ray structure analyses reveal that the pores in these compounds are ideally suited for Cs⁺ while smaller ions do not fit well. The structures of these as well as some other framework tetravalent metal silicate compounds that are specific for Cs⁺ ions will be described. The talk also will focus on the structure solution of some new metal phosphates and metallo-organic phosphates by using laboratory X-ray powder diffraction data.