Transitional Area of Ce4+ to Ce3+ in SmxCayCe1-x-yO2-δ with Various Doping and Oxygen Vacancy Concentrations: A GGA + U Study

摘要: In this work, we perform DFT + U periodic calculations to study geometrical and electronic structures and oxygen vacancy formation energies of SmxCayCe1-x-yO2-δ systems (x = 0.0312, 0.0625, 0.125 and 0.250; y = 0.0312, 0.0625, 0.125 and 0.250; δ = 0.0312, 0.0625, 0.125, 0.250 and 0.50) with different oxygen vacancy and doping concentrations. The calculated results show that the V1-Sm3+-V2 structures where there is a position relationship of the face diagonal between V1 and V2 both nearest to Sm3+ have the lowest energy configurations. The study on electronic structures of the SmxCayCe1-x-yO2-δ systems finds that excess electrons arise from oxygen vacancies and are localized on f-level traps of their neighbor Ce, and Ca2+ and Sm3+ co-doping effectively restrains the reduction of Ce4+. In order to avoid the existence of Ce3+, x and y must be both larger than 0.0625 as δ = 0.125 or δ must be smaller than 0.125 as x = y = 0.0625. The Ce3+/Ce4+ change ratio k has an obvious monotonous increase with increasing the vacancy oxygen concentration. The introduction of Sm3+ decreases k. In addition, the doped Sm3+ can restrain the reduction of Ce4+ when the V1-Sm3+-V2 structure with a face diagonal position relationship in lower reduced atmosphere exists. It need be pointed out that the Sm0.25Ce0.75O1.5 system should be thought of as a Sm-doped Ce2O3 one.