Defect Chemistry of Sodium Bismuth Titanate and its Solid Solutions

Abstract

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 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 knowledge, the solid solution NBT-6BT was investigated for A-site non-stoichiometry and B-site acceptor doping to reveal similar defect chemical mechanisms as observed in NBT. B-site acceptor doping leads to similar electrical properties. The induction of high levels of oxygen ionic conductivity is, therefore, following the same mechanism in NBT-6BT. Based on this finding, acceptor doping was revealed to be not a valid approach to enhance the ferroelectric properties in NBT and NBT-based solid solutions. A-site non-stoichiometry featured a significant impact on the non-ergodic/ergodic relaxor transition on NBT-6BT which led to considerably different ferroelectric and piezoelectric properties at room temperature. Additionally, the presence of increased oxygen vacancy concentration is directly related to chemical diffusion during the processing. This circumstance was utilized in the core-shell structured NBT-based solid solution NBT-25ST. In detail, B-site doping and A-site non-stoichiometry were simultaneously applied on NBxT-25ST to elucidate the origin of forming this particular chemical inhomogeneity which could either be facilitated by the occurrence of A-site vacancies or oxygen vacancies. With the help of a detailed SEM analysis, it could be confirmed that the oxygen vacancy concentration is mainly responsible for the formation of core-shell structures. Low oxygen vacancy concentration stabilized fine-grained, core-shell structures, larger concentrations result in grain growth and homogeneous elemental distribution. A remarkable impact on the resulting piezo- and ferroelectric properties could be revealed. As the NBT-rich core was attributed to being responsible for non-ergodic relaxor behavior, a core-shell structured composition should provide non-ergodic behavior. This assumption holds for A-site non-stoichiometry in NBxT-25ST. Acceptor doping with Fe (high vacancy concentration), however, leads to non-ergodic relaxor behavior, Nb-donor doping (low vacancy concentration results in ergodic relaxor behavior at room temperature irrespective on additional A-site non-stoichiometry. This result confirms that, besides changing the occurrence of core-shell microstructures, the ferro- and piezoelectric responses of the NBT-rich cores themselves are affected by the doping. This results in ergodic behavior in the case of Nb-donor doping. The feature of relaxor behavior was utilized to further enhance the application range of NBT. Therefore, the solid solution NBxT-6BT-yCZ was investigated for its temperature-dependent dielectric properties. It could be confirmed that the stoichiometric NBT-6BT-20CZ composition exhibits an exceptionally large application temperature range where the criterion of temperature stable permittivity and low dielectric loss are fulfilled simultaneously. The almost temperature-independent permittivity was attributed to the presence and coexistence of two different kinds of polar nano regions, in the respective low temperature (LT) and high temperature (HT) form. The dielectric losses, however, limited the application range. By the addition of BiAlO3 (BA), a further decrease of the dielectric losses could be achieved. This resulted in an excellent high-temperature dielectric material that can by far outperform commonly used high-temperature capacitors for the given operation temperature window in which the permittivity and loss criterion are fulfilled simultaneously. The here presented results elucidate the defect chemical origin of oxygen ionic conductivity in NBT. By applying the gained defect chemical knowledge, oxygen vacancy formation can precisely be controlled and the application range of NBT and its solid solutions could be enlarged towards tunable piezo- and ferroelectric properties as well as excellent temperature stable dielectric properties.