Ab initio structure determination from X-ray powder diffraction data
Many important crystalline solids cannot be prepared in the form of single crystals of sufficient size and quality for single crystal X-ray diffraction studies, and in such cases it is essential that structural information can be extracted from powder diffraction data. In this thesis, a number of crystal structures have been determined directly from X-ray powder diffraction data recorded on a conventional laboratory instrument, and the limitations of this technique explored using both conventional and new more sophisticated methods of structure solution. This work has focussed mainly on the more complex problem of molecular systems. The Patterson method has been applied to the determination of a simple unknown inorganic structure, lithium perchlorate, whereas conventional direct methods have been used in the determination of a number of organic structures, including the previously unknown crystal structure of 1,3,4,6-tetrathiopentalene-2,5-dione and formylurea - the first previously unknown organic structure containing only light atoms to be solved by this technique. The combined maximum entropy and likelihood method has been applied to determinate two crystal structures, lithium triflate and p-toluenesulphonhydrazide. Further developments of this technique are also discussed and illustrated in the structure solution of a previously known system. A Monte Carlo algorithm for ab initio crystal structure determination from powder diffraction data has also been developed, and the success of this method demonstrated by its application to the determination of several known structures, and the previously unknown crystal structure of p-bromophenylacetic acid. The effect of data range on the quality of structure solution obtained from both direct methods and the maximum entropy and likelihood method is also discussed.