Defect Chemistry of Sodium Bismuth Titanate and its Solid Solutions

Sebastian Steiner
2020
In this work, the perovskite structured system Na0.5Bi0.5TiO3 (NBT) and its solid solutions (1-x)(Na0.5Bi0.5TiO3)-xBaTiO3 (NBT-BT), (1-x)(Na0.5Bi0.5TiO3)-xSrTiO3 (NBT-ST) and [(1-x)(1-y)(Na0.5Bi0.5TiO3)-yBaTiO3)]-xCaZrO3 (NBT-BT-CZ) have been investigated. In detail, the impact of A-site non-stoichiometry and B-site doping on the electrical, dielectric, ferroelectric and piezoelectric properties was discussed. The main aim was to reveal the defect chemical origin of extremely high oxygen ionic
more » ... high oxygen ionic conductivity in NBT and to apply the gained knowledge to control and enhance the properties of NBT-based solid solutions. This could result in a large application range of NBT and its solid solutions from excellent solid ionic conductors to high-temperature dielectric materials. High levels of oxygen ionic conductivity were rather unexpected in NBT and highlight that the already established defect chemical models for lead- or barium-based systems do not hold for this system. It was assumed that defect complexes form in NBT between a B-site defect and a generated oxygen vacancy, resulting in a non-linear increase of the effective oxygen vacancy concentration. By performing temperature dependent impedance spectroscopy, the electrical properties of NBT have been investigated in more detail. With the help of density functional theory (DFT) calculations, an analytical model was established with regards to a possible defect complex formation in acceptor doped NBT. Further, the conducted work delivers proof that the association energy of the defect complex is dependent on the doping element (in particular mechanical contributions from the differing ionic radii, Coulomb interactions with concerning the valence state and covalent contributions), doping concentration and crystal phase. With a precise adjustment of the A-site and B-site defect chemistry, controllability of the ionic conductivity in NBT could be reached in such a way, that either high ionic conducting NBT or low, semiconducting NBT can be processed. Based on the gained [...]
doi:10.25534/tuprints-00011552 fatcat:kb5pz3uvwrculmpdehxumu6fei