High resolution powder diffraction at ISIS

This paper reviews the initial commissioning of the high resolution time-of-flight neutron powder diffractometer, HRPD, on the Spallation Neutron Source, ISIS, at the Rutherford Appleton Laboratory. Preliminary results have confirmed both intensity and resolution predictions indicating that (δd/d) lies between 0.04 % and 0.08 % for all d-spacings between 0.02 and 5 A. The scientific potential of this increased resolution over existing time-of-flight diffractometers has been demonstrated in the successful ab initio structure determination of an unknown inorganic material, FeAsO4, and the detailed study of subtle symmetry changes in NiO. The true instrumental resolution, however, has been observed in only a small number of experiments: sample broadening is often seen to play a dominant role in the determination of the peak shape, particularly at longer d-spacings. This leads to additional useful information about macroscopic properties, such as an anisotropic crystallite size, strain distribution and sample homogeneity, but also results in a significant increase in complexity of peak-shape description and data-analysis strategy. (author) 13 refs., 8 figs., 4 tabs