Total neutron diffraction: the correct way to determine the true structure of crystalline materials?

Crystallography, using conventional Bragg diffraction, and the study of atomic correlation functions, using total diffraction, have historically been carried out separately. There exist two different scientific communities, which in the case of neutron diffraction normally use different instruments. However, modern time-of-flight neutron diffractometers allow data to be collected to high maximum momentum transfer, Q max , and with good reciprocal-space resolution, d / d . The high Q max yields correlation functions with good real-space resolution, whilst the good reciprocal-space resolution yields data well suited to conventional crystallographic analysis. We show how the Liquids and Amorphous Diffractometer, LAD, at the ISIS spallation neutron source at the Rutherford Appleton Laboratory has been used to obtain new information on a number of disordered crystalline molybdates, Li 2 MoO 3 , LiMoO 2 and D 2 MoO 3 . The average crystal structures are determined using Rietveld refinement of the Bragg diffraction data, whilst the local structures are determined by modelling the correlation functions, T ( r ), obtained from total neutron diffraction data. Reconciling the information from the two techniques provides a deeper understanding of structures than is possible using either technique in isolation. Finally, we discuss how the next generation of instruments will allow the development of this technique with specific reference to the new General Materials Diffractometer, GEM, at ISIS.