To get detailed information about the cation and anion distribution within the materials, the neutron powder diffraction data has to be analysed by the Rietveld refinement method [19, 20]. To obtain reliable results it is important to perform the Rietveld procedure in a physically-reasonable sequence. In the case of the chal — copyrite-type compound semiconductors the chalcopyrite-type crystal structure was used as basis model. The free structural parameters of the fit were the lattice constants, the anion position parameter x, the cation site occupancy factors (SOF) of the 4a and 4b site (SOF4a, SOF4b) and atomic displacement parameters (ADP). The following example sequence of free parameters can be applied for the analysis of Cu(In, Ga)(Se, S)2 compounds:

1. Refined parameters in the first step were the global parameters, zero shift and the scaling factor. Profile parameters like the u, v, w, x and y parameters, defining the full width at half maximum (FWHM), were fixed at the values of a previ­ously-measured standard sample such as Y2O3. Also the background values were fixed in the beginning of the refinement. Other fixed parameters were the structure parameters: lattice constants, ADPs, SOFs, and the atomic position parameters. The SOFs were fixed at values according to the chemical compo­sition as known from chemical analysis. The isotropic ADPs (bSo; A = 4a, 4b; 8d) were kept at 1.0

2. Subsequently, lattice constants were refined, the u, v, w, x and y parameters remained fixed, as well as ADPs, SOFs, and position parameters.

3. The bAso of 4a, 4b, and 8d positions are refined, keeping the SOFs of the respective positions fixed.

4. The cation site occupancy factors (SOF4a, SOF4b) are then refined whilst biso were fixed to their previously-refined values.

5. Step 3 and 4 are repeated until no change of parameters occurs. This procedure has been expanded with the use of anisotropic ADPs, bAA, (A = 4a, 4b; i = 1-3), since the tetragonal system leads to the need for anisotropic atomic displacement.

6. The bA and the SOFs are refined simultaneously until convergence.

7. If necessary, background and profile parameters: u, v, w, x and y as well as asymmetry values are refined.

This refinement strategy results in reliable values for SOFs of the respective species. Using the following method of average neutron-scattering length [22], it is possible to determine the point defect concentration in complex off-stoichiometric compounds over a large sample volume.