Pair Distribution Functions and Reverse Monte Carlo modelling of disordered metal hydrides

Abstract Total X-ray and neutron scattering experiments aim to measure both diffuse scattering and, in the case of crystalline samples, Bragg scattering on an absolute scale. The total scattering data can be transformed to the Pair Distribution Function (PDF) which shows short-range correlations between the atoms. Reverse Monte Carlo (RMC) modelling is a technique to make three-dimensional model of the atomic arrangements from total scattering data or PDFs. These models are not constrained by space group symmetry, and can give structural insight that is not available from the time- and space averaged models obtained from Bragg scattering alone. RMC modelling has been used to investigate the short-range structure correlations of the disordered crystalline metal deuterides α -VD 0.8 }, β -VD {0.5} and ZrCr 2 D 4 . It was found that the local structures are similar to those found in corresponding ordered low-temperature phases at length scales up to 3–4 Å and that abnormally short D–D separations do not occur despite very short separation between possible D sites. For amorphous metal hydrides, RMC modelling shows that the local atomic configurations can differ substantially from those found in the corresponding crystalline phases.