Oxygen diffusion in the orthorhombic FeNbO4 material: a computational study

Abstract

ABO₄-type materials have shown significant potential for applications as luminescence and photocatalytic materials, and the orthorhombic FeNbO₄ (o-FeNbO₄) material has also shown excellent promise in catalytic electrodes, unlike other common ABO₄ materials. However, little computational work has been carried out on the o-FeNbO₄ structure, potentially because it is disordered and thus not straightforward to simulate. In this work, we first confirmed the accuracy of the force field parameters obtained from previous studies through optimizations carried out using the GULP code. Next, we found that one ordered configuration of the stoichiometric o-FeNbO₄ structure dominates when analysing the probabilities of cation disorder in three supercells (2 × 2 × 1, 2 × 1 × 2, and 1 × 2 × 2). We then studied the bulk properties of this selected o-FeNbO₄ through DFT calculations, including the lattice parameters, the mechanical properties and the electronic structures, where no remarkable differences were observed compared to the monoclinic FeNbO₄ structure. Finally, because oxygen mobility is key to the successful application of o-FeNbO₄ as an electrode material, we have simulated the diffusion pathways of oxygen through both the stoichiometric and non-stoichiometric structures, where the results show that the existence of oxygen vacancies enhances diffusion and the distribution of the Fe and Nb inside the lattice affects the energy barriers and could therefore impact the oxygen diffusion.