Doping Effects on Multivalence States, Electronic Structure, and Optical Band Gap in LaCrO3 under Varied Atmospheres: An Integrated Experimental and Density Functional Theory Study.

Abstract

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-x Ca _x CrO₃ (LCCx), La_1-x Sr _x CrO₃ (LSCx), and La_0.8Sr_0.2Cr_1-x Mn _x 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.