The crystal structure of Fe(3)O(4) below the 122 K Verwey transition has been refined using high-resolution x-ray and neutron powder diffraction data. The refinements give direct evidence for charge ordering (CO) over four independent octahedral Fe sites, two with a charge of +2.4 and the other two of +2.6. CO schemes consistent with our model do not meet the widely accepted Anderson condition of minimum electrostatic repulsion. Instead we propose that CO is driven primarily by a [001] electronic instability, which opens a gap at the transition through a charge density wave mechanism.

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Abstract.Following the seminal work of Von Dreele, high quality powder X-ray diffraction studies on proteins are being established as a valuable complementary technique to singlecrystal measurements. Several studies using a variety of experiments approaches have been reported in the literature, including high-resolution studies employing parallel beam geometry and high intensity measurements using position sensitive detectors. The choice of the optimum instrumental configuration depends on a number of competing factors such as the amount of sample available, its radiation sensitivity, and the quality of the data required for data analysis, e.g. angular resolution, the extent of the data in d-spacing, or the number of patterns required to explore the protein's behaviour at different temperatures, or under different crystallisation conditions, etc. Here we discuss several advantages and disadvantages of different data collection methods followed for selected examples of small proteins.

The transformation of a powder diffraction profile into a set of correlated integrated intensities allows crystallographic calculations to be performed without modelling raw diffractometer counts. By carrying out this transformation it is possible to preserve the information about peak overlap without needing to retain the multitude of experimental details and description of the peak shape function. An implementation is described which allows problems of significant size to be tackled. Least squares refinement, issues of data quality and the generation and importance of free

Abstract. X-ray induced anisotropic variations of cell parameters in porcine pancreatic elastase (PPE) were used in a multi-Pawley refinement in order to improve the deconvolution of overlapping peaks occurring in the high-angle region of the powder pattern. The benefit of combining scans is demonstrated by an improvement in the quality of the isomorphous difference Patterson maps used to detect the positions of heavy atoms in a uranyl derivative of PPE.

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High-resolution time of flight neutron powder diffractometry has been used to determine the antiferromagnetic structure of ferric arsenate at 4.2 K. The structure is incommensurate with a magnetic propagation vector (0.04994(3), 0, -0.24725(2)) in the a*-c* plane of the monoclinic unit cell. The spins of 4.01(2) µB on the Fe3+ ions rotate in a plane containing b* and making an angle of 34.1(7)° to [001] in β obtuse. The structure shows that the dominant antiferromagnetic coupling is between Fe3+ ions which are linked through tetrahedral arsenate groups giving a frustrated exchange network.