STRU- SOLUTION OF LITEUUM DIBORAm HYDRATE: A COMPARTSON OF POWDERDlFFRACl’TONWlTHSING~YST~ANALYSES

The determination ab initio of crystal structures from powder diffraction data has been one of the major advance of modern powder crystallographyl. It is a consequence of the developments occurred in instrument resolution, powder pattern indexing and fitting techniques, e.g. the problem of peak overlap resulting from the collapse of the three dimensional pattern into one dimensional powder diffraction data has been circumvented by the advent of the Rietveld method. The impact of this important tool for studying the structure of materials which are only available in powder form is considerable, as shown by the increasing number of examples reported to date in literature (see, for example, ref. 2), and the possibility to use conventional X-ray sources has opening up the field to a much wider community of users. For structures with moderate complexity, the technique is becoming applied almost routinely, unless there is a strong limitation due to data quality, e.g. significant intrinsic line broadening arising from the microstructural properties of the material. In the reported examples, different radiation sources (X-rays, from conventional or synchrotron sources, or neutrons) were used for data collection, and a variety of diffraction geometries was also used. For example, a few years ago we reported the crystal structure of lithium diborate hydrate Lii~O~(OH).H~O solved ab initio from powder diffraction using the Debye-Scherrer geometry with monochromatic X-rays3. Diffraction data were collected with the curved Position-Sensitive Detector (PSD) from INEL. Although data were limited in sine/h by the PSD geometry, the structure solution was easily found. However, the angular limitation certainly influenced the precision on atomic coordinates. Recently, single crystals of the same material were obtained and the structure has been solved again from single-crystal diffraction data. The present study deals with a comparison and discussion of the results obtained by the two techniques.