Experimental and optical studies of the new organic inorganic bromide: [(C3H7)4N]2CoBr4

Abstract

The alkylammonium halogenocobalate families are subjected to diverse studies according to their wide field application. In this work, the [(C3H7)4N]2CoBr4 compound was synthesized using a slow evaporation solution growth method at room temperature and characterized by X-ray powder diffraction (XRPD) and UV-VIS-IR absorption spectrometry. The X-ray powder diffraction (XRPD) data confirmed the formation of a single-phase with monoclinic system (C2/c space group). The optical absorption and reflectance spectra of the named compound were measured at the range [240 nm–1900 nm] at room temperature. Using three methods, we have determined the direct optical band gap estimated at 3.8 eV and the Urbach energy. Different models were used to calculate the refractive index, which was found to be in the range of 2.12–2.58. This spectrum consists also of electronic transition d-d of Co2+ (3 d7) ions. This work examines a detailed crystal-field analysis of the UV–Vis–NIR lines associated to the transitions between the Stark levels of Co2+, occupying a Td site symmetry in [CoBr4]2- (organic part). The theoretical study is based on the Racah tensor algebra methods. This analysis permits us to determine the electronic structure of the cobalt ion Co2+ based on the crystal field theory. As a result, Racah and crystal-field parameters have been reliably obtained. A good agreement between the theoretical and the experimental energy levels are obtained. This theoretical study confirms the Td site symmetry of Co2+ in [CoBr4]2- and leads to a satisfactory correlation between experimental and calculated energy levels. In addition to internal transitions, the absorption spectrum shows vibrational peaks witness of anharmonic behavior.