In recent years extended research has been focusing on improving the properties of capacitors for energy storage applications. In this context, lead-free antiferroelectric materials (AFE) are excellent candidates due to their ability of displaying high energy density, high energy efficiency and low environmental impact. In this class of compounds, NaNbO₃ is amongst the most promising materials, due to the possibility to obtain double P-E loops at room temperature. One of the mostly used methods to obtain narrower antiferroelectric P-E loops is via doping. In fact, it has been shown that the P-E loops of NaNbO₃ can be improved upon doping with SrSnO₃^[1]. When impurities are introduced into a system, they will interact with the already present intrinsic defects and can lead to formation of defect dipoles, which will affect the switching behaviour of the electric dipoles and therefore the P-E loops. Therefore, we have studied both the equilibrium and quenched defect thermodynamics of intrinsic and extrinsic defects, including defect complexes, in the promising SrSnO₃-doped Sodium Niobate, using Density Functional Theory. In particular, we have computed the formation energies accounting for all possible charge states, in different regions of the stability diagram. Moreover, we have developed an original approach to account for charge state transitions in the defect quenching model, in order to more properly reproduce the experimental conditions.
We show that the binding energies of the defect complexes are negative, indicating their presence in large concentrations. Therefore, they will most likely influence the switching mechanisms and the P-E loops. Moreover, we show that defect quenching considerably shifts the Fermi level, indicating that the Fermi level could be engineered to obtain better AFE properties. 

References

[1] Zhang et al. Chemistry of Materials 33, no. 1 (January 12, 2021): 266–74