title: Impact of doping conditions on the Fermi level in lead-free antiferroelectrics
Creators: name: Villa, Lorenzo
Publisher: name: UNSPECIFIED
Year: 2023

description: Extended research, aimed at improving the properties of dielectric capacitors for energy storage applications, has fostered the interest in antiferroelectric materials. In this class of materials, sodium niobate (NN) and silver niobate (AN) are amongst the most promising alternatives to the toxic lead-containing compounds, such as lead zirconate-based ceramics, due to the possibility to obtain double P-E loops at room temperature. Chemical modification has been extensively used in order to improve their antiferroelectric properties and energy efficiency. However, the defect chemistry and its connection with the antiferroelectric properties are still unknown, and a systematic approach for doping these systems is still missing. Moreover, while processing conditions and kinetics are known to play a role in the final properties of the material, their impact has not yet been systematically studied. Lastly, 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. Nonetheless, their presence induced by doping has not yet been investigated. The scope of this doctoral thesis is therefore to study with first-principles calculations the thermodynamics of point defects in NN and AN. We investigate with density functional theory (DFT) how doping and synthesis conditions modify the electronic properties of NN and AN, with focus on the Fermi level. In particular, we determine the thermodynamic defect equilibrium by solving the charge neutrality condition, accounting for the impact of extrisic defects on the compensation mechanisms. Moreover, we develop a novel scheme to account for quenching of defects within the established point defect thermodynamics. In pure NN, the acceptor Na vacancies and donor O vacancies are present in large concentrations, thus dictating the position of the Fermi level. When doped with Sr and Sn, at high temperature and low oxygen partial pressure NN is an n-type semiconductor, while for high oxygen partial pressure the system becomes p-type. At room temperature the material is p-type. Quenching from high temperature all defects, or just O vacancies, shifts the Fermi level towards the conduction band minimum, while quenching Na and Nb vacancies produces a less pronounced shift towards the valence band maximum. Four defect complexes are found to be stable and present in high concentrations. This suggests they play an important role in the field switching mechanism, ultimately influencing the antiferroelectric P-E loops. In AN, the formation energies of the acceptor Ag vacancies are so low that the semiconductor is always p-type. For extremely low oxygen partial pressure, the material is found to be unstable due to the unphysically high concentration of Ag vacancies, which matches the experimental results reported in the literature. Doping with Mn and/or quenching does not produce a significant shift of the Fermi level, and the system remains p-type in all conditions. In summary, our approach allows to systematically study defect thermodynamics and electronic properties in semiconductors, accounting for multiple compensation mechanisms as well as quenched defects that are motivated by the synthesis conditions.