First-Principles Calculations to Investigate Coupling Fe Doping with Oxygen Vacancies in Stannic Oxide and Their Physical Properties

We study the effects of Fe doping on the structural, electronic conductivity, elastic constants, and thermal conductivity of Sn_1-xFe_xO₂ (0 ≥ x ≤ 1) oxides by orbital hybridization as a consequence of charge remuneration of Fe²⁺/Fe³⁺ and charge transfer using density functional theory calculations. We derive an effective spin-pseudospin Hamiltonian (H_effective) on the subspace of states with separately involved t_2g orbitals at each Fe site and visualize the combined atomic orbitals to form various molecular orbitals. Here, we determined that the mixed-valence electrons empowered by Fe²⁺-O^2--Fe³⁺ pairings in higher concentrations prevail through the double exchange-coupled pair. Specifically, we examined the oxygen positional parameters, average bond length, octahedral distortion, electronic structure, bulk modulus, shear modulus, Young’s modulus, Poisson ratio, and thermal conductivity, which were significantly affected by the concentrations of x (0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1), leading to negative shear and Young’s moduli, a large Grüneisen parameter, and a lower Debye temperature. We estimate an ultralow thermal conductivity for Sn_1-xFe_xO₂ of around 0.002-1.5 W m^-1 K^-1 at 300 K. We also found that fixations of the 0.5 and 0.3 dopants lead to ultralow thermal conductivity. The electronic structure of SnO₂ is 3.582 eV, and the maximum valence band is composed of the O-2p and Sn-5p states, while the conduction band minimum is between the O-2p and Sn-5s states. Fe doping modulates the bond strengths in Sn_1-xFe_xO₂ by introducing intermediate energy levels and increasing the degree of hybridization of the Fe-d, Sn-p, and O-p electron states. The reduced thermal conductivity at ambient temperature makes the material an effective candidate for use in thermal management materials and optoelectronic devices. © 2023 American Chemical Society.