POWDER STRUCTURES FROM LIMITED DATA SETS. Damodara M. Poojary, Abraham Clearfield, Department of Chemistry, Texas A & M University, College Station, TX 7783

Structures of compounds which can be obtained only in the polycrystalline form depend mainly on the use of their powder diffraction data. Substantial progress has been made in instrumentation and computational aspects during the last decade for the application of powder diffraction techniques to solve unknown structures. One of the crucial steps involved in this process is to acquire the best possible diffraction data. Synchrotron sources, with their high brightness, excellent collimation and wavelength tunability provide optimum conditions for extracting individual intensities in the powder pattern. In most cases we have been able to arrive at the solution using data from a rotating X-ray source. However, synchrotron data was required to complete the structures in some cases.

This talk focuses mainly on the structure determination of metal phosphonates and phosphates. These compounds are difficult to obtain in single crystalline form and in most cases even their powder samples are poorly crystalline. The compounds yield only weak diffraction peaks whose intensities fall off very rapidly at higher scattering angles. Despite these difficulties, we have been able to solve the structures of a large number of compounds. Generally, the structures of these compounds are layered, where the metal-phosphate inorganic framework forms a two-dimensional layers which are separated by the organic groups on either side. In some cases metal phosphonate interactions led to unusual linear or porous structures. General methods used in solving the structures will be discussed using some representative examples.

In many cases the structures were solved by the use of 30-70 low angle(CuK[[alpha]]; 2[[theta]]<60deg.) reflections by a combination of direct methods and heavy atom methods. The success comes from experience both in the structural aspects of these materials as well as the systematic application of efficient methods. It is equally important to use the results from other methods like spectroscopy, thermogravimetry, electron diffraction, etc. to arrive at the solution when only a limited number of powder diffraction data are available.