Location and oxidation state of iron in Fe-substituted CuInS2 chalcopyrites

CuIn1−xFexS2(x=0–0.30) was synthesized via high-temperature, solid-state synthesis. Phase-pure materials were found in samples where x=0–0.15, after which a secondary phase became apparent. The materials were characterized with the use of X-ray powder diffraction (XRPD), and Reitveld refinement revealed a linear decrease in unit cell volume as the amount of iron substitution increases in accordance with Vegard’s Law. Inductively coupled plasma (ICP) confirms that the actual stoichiometry is close to the nominal composition of the materials. The temperature for both the chalcopyrite-to-sphalerite and the sphalerite-to-wurtzite phase transitions decreases with increasing iron substitution for indium. These findings suggest that the Fe is being randomly incorporated into the crystal structure of the CuInS2. X-ray photoelectron spectroscopy (XPS) measurements were used to determine the oxidation state of the ions (Cu1+, In3+, and S2−), and Fe57 Mössbauer spectroscopy verified that the iron is in the 3+ oxidation state. Band gaps of the solid solution were estimated to be in the range of 0.70–0.85 eV. Rietveld refinement of neutron diffraction data indicates that the iron is occupying the In site within the chalcopyrite structure.