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    Acceptor Doping and Oxygen Vacancy Migration in Layered Perovskite NdBaInO<sub>4</sub>‑Based Mixed Conductors

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    The Ca<sup>2+</sup> and Ba<sup>2+</sup> solubility on Nd<sup>3+</sup> sites in new layered perovskite NdBaInO<sub>4</sub> mixed oxide ionic and hole conductor and their effect on the oxide ion conductivity of NdBaInO<sub>4</sub> were investigated. Among the alkaline earth metal cations Ca<sup>2+</sup>, Sr<sup>2+</sup>, and Ba<sup>2+</sup>, Ca<sup>2+</sup> was shown to be the optimum acceptor–dopant for Nd<sup>3+</sup> in NdBaInO<sub>4</sub> showing the largest substitution for Nd<sup>3+</sup> up to 20% and leading to oxide ion conductivities ∼3 × 10<sup>–4</sup>–1.3 × 10<sup>–3</sup> s/cm within 600–800 °C on Nd<sub>0.8</sub>Ca<sub>0.2</sub>BaInO<sub>3.9</sub> composition, exceeding the most-conducting Nd<sub>0.9</sub>Sr<sub>0.1</sub>BaInO<sub>3.95</sub> in the Sr-doped NdBaInO<sub>4</sub>. Energetics of defect formation and oxygen vacancy migration in NdBaInO<sub>4</sub> were computed through the atomistic static-lattice simulation. The solution energies of Ca<sup>2+</sup>/Sr<sup>2+</sup>/Ba<sup>2+</sup> on the Nd<sup>3+</sup> site in NdBaInO<sub>4</sub> for creating the oxygen vacancies confirm the predominance of Ca<sup>2+</sup> on the substitution for Nd<sup>3+</sup> and enhancement of the oxygen vacancy conductivity over the larger Sr<sup>2+</sup> and Ba<sup>2+</sup>. The electronic defect formation energies indicate that the p-type conduction in a high partial oxygen pressure range of the NdBaInO<sub>4</sub>-based materials is from the oxidation reaction forming the holes centered on O atoms. Both the static lattice and molecular dynamic simulations indicate two-dimensional oxygen vacancy migration within the perovskite slab boundaries for the acceptor-doped NdBaInO<sub>4</sub>. Molecular dynamic simulations on the Ca-doped NdBaInO<sub>4</sub> specify two major vacancy migration events, respectively, via one intraslab path along the <i>b</i> axis and one interslab path along the <i>c</i> axis. These paths are composed by two terminal oxygen sites within the perovskite slab boundaries
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