Effect of self-doping on the charge state of Fe ions and crystal field transitions in YFeO₃: Experiments and theory

Detailed investigations on the appearance of d-d transitions in optical absorption spectra of YFeO₃ samples have been done. For this purpose, pure and self-doped (vacancy at Y, Fe, and O sites) YFeO₃ samples have been synthesized using sol–gel, the solid-state reaction route, and hydrothermal synthesis methods. It has been observed that the annealing of prepared samples at different temperatures leads to mixed oxidation states of Fe ions (i.e., the coexistence of Fe ions in mixed valence states such as Fe²⁺/Fe³⁺ or Fe³⁺/Fe⁴⁺). Furthermore, soft x-ray absorption (at the Fe L_2,3 edge) and diffuse reflectance spectra analysis revealed that the intensity due to d-d transition is a prominent feature and is strongly associated with the annealing temperatures and the defects present in the samples. Experimental results followed by theoretical analysis carried out using density functional theory suggest that nonstoichiometry of the samples leads to the mixed valency (charge states) of Fe ions and hence to d-d transition in the YFeO₃ (d⁵) system.