SOLUTION OF AN ORGANIC-CRYSTAL STRUCTURE FROM-X-RAY POWDER DIFFRACTION DATA BY A GENERALIZED RIGID-BODY MONTE-CARLO METHOD - CRYSTAL-STRUCTURE DETERMINATION OF 1-METHYLFLUORENE

Many important crystalline materials do not, form single crystals of s ufficient size and quality for single crystal X-ray diffraction studie s, and in such cases it is essential that:the crystal structure can be solved from powder diffraction data; however, there are many difficul ties associated with solving crystal structures directly from powder d iffraction data. In this paper, we report the successful application o f a Monte Carlo technique to solve the previously unknown crystal stru cture of 1-methylfluorene from X-ray powder diffraction data collected on a conventional laboratory diffractometer. The 'structural fragment ' used in the Monte Carlo calculation comprised the non-hydrogen atoms of the fluorenyl (C-13) group, and this rigid structural fragment was subjected to simultaneous translation and rotation within the unit ce ll. The correct position of the structural fragment was discriminated readily (on the basis of the agreement between experimental and calcul ated X-ray powder diffractograms) from wrong positions sampled during the Monte Carlo calculation, and the correct position was then used as the initial structural model in Rietveld refinement and difference Fo urier calculations. The work reported in this paper represents the fir st demonstration of the application of the Monte Carlo approach to sol ve a previously unknown crystal structure from powder diffraction data in the general case requiring simultaneous translation and rotation o f a rigid structural fragment, and paves the way for the future applic ation of the Monte Carlo approach to a much wider array of structural problems.