Neutron diffraction studies of the structure of water at ambient temperatures, revisited [a review of past developments and current problems]

Neutron diffraction techniques used in the study of the structural characteristics of liquid water under ambient conditions are reviewed, showing the way that developments in the experimental and analytic procedures have led to improvements in the extracted results. The three partial pair correlation functions (OH, HH and OO) can be isolated from a set of diffraction measurements with varying hydrogen/deuterium isotopic composition. Although isotopic substitution is used routinely in many other investigations there are particular difficulties with hydrogenous systems due to the high incoherent scattering from hydrogen and the possibility that the H/D substitution is not a true isomorphic replacement. These features are discussed in some detail to analyse the systematic errors and the precision of the final results. An additional aspect of the comparison of data taken by different groups concerns the use of either reactor or pulsed neutron methods. Studies using the Orphee reactor with an over-determined set of five datasets are considered in detail, using analytic correction procedures based on the Powles formalism. Various consistency checks are applied to the data such that smoothing or iteration routines are not required. Two independent sets of the OD and DD pair correlation functions are obtained and are compared with the latest results from Soper et al. The small discrepancies between the various datasets are discussed in terms of propagating systematic errors and also possible variations from H/D equivalence. The relevance of the new results for the interpretation and modelling of ambient water structure are presented and the review ends with a brief comment on likely future developments that incorporate additional experimental information.