Abstract
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.
Highlights
The quaternary sulfosalt semiconductors Cu2BIICIVX4 (BII = Zn, Fe; CIV = Sn, Ge, Si; X = S, Se) have drawn wide attention for their potential applications in many fields. Depending on their band gaps these materials are interesting for thin film solar cells, high-temperature thermoelectric materials and nonlinear optics (Schnabel et al, 2017; Guo et al, 2012; Heinrich et al, 2014; Rosmus et al, 2014)
Cu2FeSnS4 (CFTS) with a band gap of 1.3 eV can be used for this purpose and in addition has photocatalytic properties (Balazet al., 2017)
Anomalous diffraction data for powder diffraction analysis and Multiple-Edge Anomalous Diffraction (MEAD) were collected at the KMC-2 diffraction station in symmetric reflection geometry, using finely ground powder on a rotating zero-background silicon sample holder
Summary
As the atomic form factor changes at the absorption edges of the respective elements, isoelectronic cations can be distinguished from diffraction data taken with radiation of appropriate energies. Multiple-Edge Anomalous Diffraction (MEAD) is such an approach This method calls for measuring the energy dependency of the intensity of selected Bragg peaks hkl around the X-ray absorption edge of a chemical element. An incomplete variant has been used to study the Cu–Zn distribution in CZTS (Nozaki et al, 2012), confirming the known kesterite over stannite structure type This last work, did use only a small number of different energies, all at the low-energy site of the Cu K absorption edge. This significantly reduces the level of difficulty, making MEAD much more accessible for non-specialists
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
