Doping Effects on Multivalence States, Electronic Structure, and Optical Band Gap in LaCrO₃ under Varied Atmospheres:...

Doping effects on the valence state, electronic structure, and optical band and the effects on electrical conductivity were studied on the doped lanthanum chromite (LaCrO₃) system. The specific compositions studied were La_1- Ca CrO₃ (LCCx), La_1- Sr CrO₃ (LSCx), and La_0.8Sr_0.2Cr_1- Mn O₃ (LSCMx) (0.1 ≤ x ≤ 0.4). The powders were synthesized using a modified Pechini sol-gel method, and the ceramic samples were densified using a reactive sintering method resulting in densities >96% theoretical. X-ray photoelectron spectroscopy (XPS) was completed to characterize the defect states and cationic valence compensation as a result of divalent (Ca²⁺ or Sr²⁺) and trivalent (Mn³⁺) substitutions. XPS was completed for samples tested in oxidizing and reducing atmospheres (up to 1500 °C), which provided insights into the oxidation state transitions induced by the Ca²⁺ and Sr²⁺ dopants. The work notably demonstrated, for the first time, the oxidation/reduction transitions of Cr⁴⁺ to Cr³⁺ in Sr²⁺/Mn³⁺ co-doped samples under reducing atmospheres. Reflectance UV-vis spectrophotometry optical band gap measurements were also completed for the same materials; a decrease in the optical band gap (2.81-3.12 eV) was shown with increased substitution, suggesting electronic structure modifications in the LaCrO₃ perovskite. Density functional theory calculations validated experimental trends, predicting a diminishing band gap with a rising dopant concentration. The transition in Cr oxidation states was attributed to the presence of divalent/trivalent cations. These findings contribute some insights into methods to tune the LaCrO₃ electrical properties for various low- and high-temperature applications.