Determination of Local Atomic Correlations by Modelling Total Diffraction Data. Nield V M Laboratory, University of Kent at Canterbury,CT2 7NR, UK.

Refinement of Bragg and diffuse scattering simultaneously enables a far greater amount of information to be obtained than is standard at present. While Bragg scattering gives information on the time-averaged structure, and so on the mean atomic positions and the corresponding atomic displacements, it alone cannot tell us about the actual positional relationship between atoms. Diffuse scattering on the other hand contains information on an instantaneous time scale about the system under study. Hence it gives information on a local scale.

This talk addresses the question of how simultaneous refinement of Bragg and diffuse scattering can be performed, using neutron diffraction data from both powder and single crystal samples. In both cases this requires the use of large super-cells (consisting of many thousand atoms). For refining the total scattering from powder diffraction data the problem is tackled via a radial distribution function, which is Fourier transformed for comparison with the experimental diffraction pattern. In the single crystal case the diffuse scattering is calculated directly from the supercell. In both cases a reverse Monte Carlo refinement is used. These techniques will be discussed in detail, with examples of modelling neutron total diffraction data from superionics, in the case of powder data [1], and from ice [2] and urea for single crystal data.

[1] Nield V M et al. Solid State Ionics 66 247 (1993)

[2] Nield V M et al. Acta Cryst. A 51 763 (1995)