No abstract

The Cambridge Crystallography Subroutine Library (CCSL) is a versatile set of subroutines which will have many different applications for non-standard crystallography. At present it is used extensively at the Institut Laue Langevin for interpreting all polarized neutron measurements, proceeding from the raw flipping-ratio measurements to the final production of spin-density maps. It is also used to analyse simple magnetic and more complicated nuclear structures, and for more general crystallographic applications such as arranging lists of reflections into groups of symmetry equivalents. It finds applications in the analysis of neutron powder diffraction patterns, as well as in more recent developments for the analysis of X-ray powder diffraction photographs.

The Voigt function has been incorporated as a peak‐shape description into a program for the refinement of constant‐wavelength X‐ray and neutron diffraction patterns. The results obtained for neutron diffraction are encouraging and indicate that the Voigt function describes the symmetrical component of the profile peak shape to high accuracy even in the presence of substantial line broadening from particle‐size effects. In contrast with approximations to the Voigt function, such as the pseudo‐Voigt and Pearson VII functions, the present treatment allows the angular dependences of line‐broadening effects resulting from particle‐size and instrumental contributions to be coded independently from each other in the Rietveld technique. The present treatment, which details improvements to the symmetrical component of the peak shape, does not offer a fully rigorous description of the peak shape as asymmetry corrections such as those given by Howard [J. Appl. Cryst. (1982), 15, 615–620] are not included.

AbstraetThe crystal structure of a-A1PO4 (berlinite; P3121) has been refined from Laue photographs taken with synchrotron radiation at the SRS Daresbury Laboratory. The refinement led to an R factor of 0.19 for the integrated intensities (measured by microdensitometer) of 175 non-symmetry-equivalent Laue spots. The refinement program includes explicitly the overlapping diffraction orders and uses an experimentally determined instrument function to describe the incidentbeam intensity and the film response as functions of wavelength. The values of the eight positional parameters obtained in this preliminary assessment of the Laue method are in good agreement with those previously determined by conventional single-crystal X-ray diffractometry (e.g. the mean A1-O and P-O bond lengths correspond to within 0.025 A) suggesting that accurate structure refinements should be obtainable. By using the Laue method large amounts of data can be collected with exposures of only a few seconds. A number of applications for the technique are suggested.

Crystal Structure and Bonding of Ordered C60. --(DAVID, W. I. F.; IBBERSON, R. M.; MATTHEWMAN, J. C.; PRASSIDES, K.; DENNIS, T. J.; HARE, J. P.; KROTO, H. W.; TAYLOR, R.; WALTON, D. R. M.; Nature (London) 353 (1991) 6340, 147-149; School Chem. Mol. Sci., Univ. Sussex, Brighton BN1 9QJ, UK; EN)