Diffuse scattering and PDF analysis not only with neutrons

J. Kulda

Institut Laue-Langevin, BP 156, 380042 Grenoble Cedex, France

kulda@ill.eu

Electroceramics, similarly to many other modern functional materials, exhibit a considerable portion of structural disorder, playing a key role in their functionality. The details of local atomic arrangements and their short-range correlations can be revealed by the pair distribution function (PDF) technique [1], whose application consists in taking Fourier transform of the complete powder diffraction pattern, including the slowly varying, diffuse scattering part of the signal between and underneath the Bragg peaks and taking into account instrumental contributions to the line widths. The resulting real-space atomic distance distribution by itself often permits qualitative discussions of changes in nearest neighbor distances as a function of chemical composition and/or of thermodynamic parameter variations.

More involved and more quantitative interpretations of the PDF necessarily call for (much) more computing effort following one of the two possible approaches. In the first approach, a model-free technique of reverse Monte-Carlo (RMC) is used to build up a model structure providing a diffraction pattern coinciding with the observed one within statistical limits [2]. In an ideal case the progress in computing power permits to treat data obtained by various experimental probes (X-ray, neutron and electron scattering, EXAFS, NMR etc.) simultaneously to remove ambiguities inherent in each single technique [3]. Alternatively, molecular dynamics and/or ab-initio computational techniques can be employed to build up a model of the studied system, which can then be further refined to match the observed pattern [eg. 4] and analysed in terms of atomic correlations.

1. T. Egami, S.J.L. Billinge: Underneath the Bragg Peaks: structural analysis of complex materials,

        Pergamon materials series v. 7 (2003)

2. R.L. McGreevy, J. Phys.: Condens. Matter 13 (2001) R877

3. M. Eremenko et al., Nature Comm. 10 (2019) 2728

4. M. Pasciak et al., Phys. Rev. B 99 (2019) 104102