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 Sn1–xFexO2 (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 (Heffective) on the subspace of states with separately involved t2g 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+–O2––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 Sn1–xFexO2 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 Sn1–xFexO2 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.