(183ah) First-Principles Study on the Electronic, Optical and Thermodynamic Properties of LaxSr1-XCo1-YFeyO3-? (x/y =0.25, 0.5, 0.75) Perovskites

The doped La_xSr_1-xCo_1-yFe_yO_3-δ perovskite systems have shown high electronic and ionic conductivities, making them a promising cathode material in solid oxide fuel cells (SOFCs), oxygen permeable membrane and catalyst, and recently attract great attentions in optical gas sensor application at high temperature. High temperature gas sensor is essential for industrial applications to improve energy efficiency and reduce toxic emissions. However, gas sensors operating at high temperatures encounter many challenging issues, such as thermal and long-term stability, sensitivity, reproducibility and selectivity. Sensing processes at such high temperatures are complicated and the corresponding mechanisms have not well understood. Hence, theoretical modeling could play a role to explore the sensing mechanisms and support experimental development of practical sensor devices. In this study, the electronic, optical and thermodynamic properties of La_xSr_1-xCo_1-yFe_yO_3-δ (x/y = 0.25, 0.5, 0.75) are investigated using density functional theory calculations. The obtained results show that the crystal volume decrease upon La³⁺ doping, due to its smaller ionic radius of La³⁺ over Sr²⁺. Since La³⁺ substitution introduces electrons, the valence bands move downwards to low energy range. Fe doping mainly brings out changes in energy band states near the Fermi level. By correlating the energy band structures with optical absorption, we obtained that the absorption peaks could be adjusted or enhanced by doping, for example, doping La³⁺ in SrCoO₃ cubic structure results in a blue shift of optical absorption at about 300 nm, doping Fe introduces more empty states just above the Fermi level and thus increases the optical absorption at about 1500 nm. Furthermore, to explore the nonstoichiometric nature and oxygen release out of these materials, we investigated the thermodynamic feasibility of oxygen removal from the perovskites according to the equation: 8 La_xSr_1-xCo_1-yFe_yO_3-δ→ 8 La_xSr_1-xCo_1-yFe_yO_2.75 + O₂. Our results show that the parent SrCoO₃ is most favorable to release oxygen, both La and Fe doping will increase the difficulty for oxygen releasing. Such finding is consistent with the experiment that the oxygen deficiency δ could be higher to 0.5 for SrCoO₃.