An improved method for analysing single crystal diffuse scattering using the Reverse Monte Carlo technique

An improved method for the analysis of single crystal diffuse scattering using the Reverse Monte Carlo (RMC) simulation technique is presented. Previous RMC studies showed that with respect to the size of the model crystal used for the simulation there are two conflicting requirements. A large crystal size gives relatively noise-free calculated diffraction patterns, but it is found that the 'fit' that is obtained has been achieved by adjusting the large number of high-order correlations in the structure rather than the relatively few low-order correlations of interest. On the other hand a small crystal size necessarily gives a defect structure characterised by short-range correlations, but gives a diffraction pattern so noisy that it cannot meaningfully be fitted to the observed data. The procedure described here overcomes this problem by using a quite different way of calculating the diffuse scattering intensity. Rather than computing the Fourier transform of the complete model crystal, the intensity is taken as the average of scattering intensities of many small crystal volumes (lots) chosen at random. This produces high quality diffraction patterns and at the same time restricts the effect of the RMC refinement to correlation vectors no greater than the lot size. The viability of this modified RMC method compared to the 'normal' RMC simulation technique is investigated using as input the calculated diffuse intensity of simple two dimensional (2D) model examples with known disorder properties.