Cation distribution in Cu₂ZnSnSe₄, Cu₂FeSnS₄ and Cu₂ZnSiSe₄ by multiple-edge anomalous diffraction

Multiple-Edge Anomalous Diffraction (MEAD) has been applied to various quaternary sulfosalts belonging to the adamantine compound family in order to validate the distribution of copper, zinc and iron cations in the structure. Semiconductors from this group of materials are promising candidates for photovoltaic applications. Their properties strongly depend on point defects, in particular related to cation order-disorder. However, Cu⁺, Zn²⁺ and Fe²⁺ have very similar scattering factors and are all but indistinguishable in usual X-ray diffraction experiments. Anomalous diffraction utilizes the dependency of the atomic scattering factors f' and f'' of the energy of the radiation, especially close to the element-specific absorption edges. In the MEAD technique, individual Bragg peaks are tracked over an absorption edge. The intensity changes depending on the structure factor can be highly characteristic for Miller indices selected for a specific structural problem, but require very exact measurements. Beamline KMC-2 at synchrotron BESSY II, Berlin, has been recently upgraded for this technique. Anomalous X-ray powder diffraction and XAFS compliment the data. Application of this technique confirmed established cation distribution in Cu₂ZnSnSe₄ (CZTSe) and Cu₂FeSnS₄ (CFTS). In contrast to the literature, cation distribution in Cu₂ZnSiSe₄ (CZSiSe) is shown to adopt a highly ordered wurtz-kesterite structure type.