CRYSTAL STRUCTURES FROM POWDER DATA BY DIRECT METHODS. W. Lasocha(1), R. Peschar(2), J. Jansen(2) & H. Schenk(2) (1)Faculty of Chemistry, Jagiellonian Univ. Krakow, Poland, (2)Lab.v.Kristallografie, Univ.v.Amsterdam, The Netherlands.
In our structural investigations from powder data we used the Direct Method Powder Diffraction Package POWSIM[1] to solve BOTH the problem of overlapping reflections and subsequently the crystal structure. Crystal structure determination from powder diffraction data by means of 'ab initio' methods requires the knowledge of intensities of the majority of the reflections. However, even the best fitting procedure cannot retrieve accurate intensities for reflections whose diffraction angles differ less than some critical value. In POWSIM several relations from Direct Methods, and one relation based on the Patterson function, are employed to predict which overlapping reflections are strong or weak. This information and the sum intensity of overlapping peaks can be used to redistribute the intensity in most of the clusters and to assign statistical weights to the individual decomposed intensities. Using POWSIM we solved structures of several fibrillar trimolybdates of silver, potassium, ammonium and anilinum[2], and anilinum pentamolybdate[3]. The most complicated structure in this family of compounds is anilinum trimolybdate with 27 atoms in the asymmetric unit. The intensity-redistribution procedure allowed straightforward location of 18 atoms by the structure solution program SIMPEL (part of POWSIM). The results clearly indicate the potential of the method we used. In our work we use X-ray data collected by a conventional powder diffractometer.
W.L. acknowledges support of the Polish Comm. of Science Res.
1 J. Jansen, R. Peschar, H. Schenk. J. Appl. Cryst., 25, 231 (1992), ibid 25,237(1992) and Z. Kristallogr. 206,33(1993)
2 W. Lasocha, J. Jansen, H. Schenk, J. Solid State Chem,109, 1(1994),ibid 115,225(1995),ibid 116,422(1995),ibid 117,(1995)
3 W. Lasocha, H. Schenk, J. Appl. Cryst, submitted