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.

Electronic transitions in layered hydroxides: Consequences of trigonal distortion of octahedral symmetry.

This work describes the assignment of the electronic spectra of metal ions in D3d coordination symmetry. Layered hydroxides are a class of materials that host transition metal ions such as Ni2+, Co2+, and Cr3+ in D3d coordination symmetry. The electronic spectra of these ions in the layered hydroxides exhibit significant fine structure which is assigned to transitions arising from D3d coordination symmetry. Towards this end, the correlation diagrams- complete or partial, and the resultant Tanabe-Sugano like diagrams for D3d symmetry are obtained from first principles and supported by DFT based computations. The approach engendered here helps in better understanding of the electronic transitions arising due to lower symmetry.