Ab-initio structure solution from powders

An interesting example of ab-initio structure solution from powder diffraction data using the high-resolution instrument D2B has been the determination of the low temperature structure of SF6. This simple octahedral molecule has been the subject of extensive molecular dynamics simulations which demonstrated the orientational-disorder in the cubic phase. The molecular dynamics simulations predict the presence of an intermediate trigonal phase, followed by a further transition to a triclinic structure, space group P1 with Z=3. Analysis of the high-resolution diffraction patterns obtained on D2B of SF6 at 5K and 60K showed that the correct spacegroup of the low-temperature phase is C2/m with Z=6. The existence of two crystallographically distinct SF6 molecules (shown below) in the ratio 1:2 in the low-temperature structure is in good agreement with spectroscopic studies.

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Plot of the monoclinic structure of SF6 at 60K showing the two types of molecules and their molecular librations.

Further work on combining X-ray and neutron data in structure solutions has also been carried out. A good example is the structure analysis of lead oxalate, PbC2O4, which crystallizes in space group P1- with two independent oxalate ions. The unit cell was determined from Guinier photographs. After this lead atom was located using data extracted from a synchrotron powder profile recorded at the NSLS, followed by determination of the light atom positions employing difference Fourier techniques. Finally, because the scattering of lead strongly dominates the X-ray spectrum, the structure was confirmed by a Rietveld refinement using data from D2B.

One commonly occurring problem in very high resolution powder work is broadening from grain size, strain or stacking fault effects, and in the present case the X-ray data showed very highly anisotropic broadening. Eventually this could be related to the crystal arrangement, which consists of alternating layers of lead and oxalate ions. In the lead layer slips along the lead-lead direction seem to occur, creating peak broadening in the corresponding Bragg reflections. In addition this could lead to irregularities in the direction orthogonal to the layers, and also here peak broadening is found. High resolution powder data might therefore simultaneously give the solution of non-trivial structural problems and some indication of the nature of the imperfections in the long range order.

Last updated by Andrew Crowe on 06/02/1996