TRANSFORMATIONS IN NEUTRON POWDER DIFFRACTION

The major milestone in neutron powder diffraction occurred thirty years ago with the advent of the Rietveld method of structure refinement. The combination of advances in both instrumentation and computing power allowed moderately complex low-symmetry structures to be analysed by powder methods for the first time. Over the past five years there have been further transformations in neutron powder diffraction again resulting from combined developments in instrumentation and data analysis methods. In this paper we shall focus on high resolution, high intensity time-of-flight neutron powder diffraction and the impact that this technique has had on our understanding of the structural aspects of phase transitions and phase transformations. The essentially constant DELTA-d/d resolution that is intrinsic to a time-of-flight diffractometer implies that peak splittings arising from a structural phase transition are observed with equal resolution at all orders of reflection, thus maximising the information content associated with structural distortions. Additionally, the fixed diffraction geometry permits the use of sophisticated sample environments such as high pressure cells and special atmosphere furnaces without contamination of the sample diffraction pattern.