Spectroscopic and theoretical investigation on the origin of color in jarosites

This work aims to understand the origin of the electronic spectra of Fe³⁺ (d⁵), Cr³⁺ (d³), and V³⁺ (d²) containing jarosites. The electronic spectrum of the Fe-jarosite is currently assigned to spin forbidden transitions. This work shows that the spectra essentially arise due to the tetragonal distortion of the coordination symmetry of the Fe³⁺ ion in the jarosite crystal, and thereby obviates the need for invoking spin forbidden transitions. The absorption spectra of Cr- and V-jarosite are also assigned to transitions predicted for the tetragonal distortion of the metal ion coordination. The electronic term symbols are worked out using the correlation diagram and Tanabe-Sugano diagram for orbital splitting for all three systems employing ab initio and DFT methods. The bandgaps were computed and corroborated with the experimentally measured values to support the low symmetry at the metal center.