In the average structure refinement of molecular materials, split positions are often encountered. As a consequence a molecule on one site can be present in more than one orientation. The typical structure refinement stops there and does not consider diffuse scattering, which allows statements about short range order interactions. Building and refining a short range order model to analyze the diffuse scattering takes the structure refinement to the next level.

A complex molecular crystal will usually consist of several components on several sites within the unit cell. Furthermore, a single site may be occupied by molecules in different orientations e.g. a first molecule in orientations A and B on site 1 and a second molecule in orientations C and D on site 2, see Fig. 1. In such systems short range order is common: On a local scale, the molecules tend to show preferred pair-wise arrangements. Characterizing this local order enables profound statements about molecular interactions [1].

With the help of molecular form factors [1] and the theory of diffuse scattering (e.g. Warren [2]), we developed a method that characterizes correlated chemical short range occupational disorder directly in reciprocal space. The diffuse scattering IDiff can be expressed as a function of the indices *h,k,l* in reciprocal space:

Where *I _{Laue}* is the Laue scattering () and

Here (*u,v,w*) are vectors in direct space, *n _{s}* is the number of different sites and

We apply this formula for the analysis of the diffuse scattering of 9-Bromo-10-Methylanthracene [3]. The model for the short range order can be developed directly in reciprocal space and the formula can be used to perform a least squares regression analysis to fit the short range order parameters quantitatively.

Our method to characterize complex molecular disorder using single crystal diffuse scattering is a powerful tool to understand and model molecular interactions in disordered crystals. As the method treats data directly in reciprocal space and enables least squares fitting of disorder models, calculations can be performed on desktop computers without the excessive use of computation time.

Figure 1:(a) Refined average unit cell with overlayed components A and B on site 1 and 2.

(b) Possible configuration of disordered unit cells.

(c) Diffuse scattering of 9-Bromo-10-Methylanthracene in the 0kl-layer.

[1] Schmidt, E., & Neder, R. B. (2017). Acta Cryst. A73, 231.

[2] Warren, B. E. (1969). X-ray Diffraction. Courier Corporation.

[3] Welberry, T.R., Jones, R.D. & Epstein, J. (1982). Actra Cryst. B38, 1518–1525.Keywords: diffuse scattering, disorder, single crystal