| Summary: | The influence of the Na/Bi ratio on the electrical properties of nonstoichiometric NBT has been investigated in the Na0.5-xBi0.5+xTiO3+x series. The solid solution limit is very small -0.02 < x < 0.02, but the bulk conductivity has a dramatic change of ~ 4 orders of magnitude at 600 ℃, where it switches from an insulator to an ionic conductor for Na/Bi ≥ 1. Within the solid solution limit, the maximum bulk conductivity is achieved by replacing 1 at% Bi with Na atoms on the A-site (acceptor doping), which generates oxygen vacancies. Furthermore, the limit of oxygen vacancies is identified to be ~ 0.33 at%, which is illustrated by the geometric-independent parameter fmax. In general, ceramics prepared by hand grinding have better control of the nominal starting composition. The bulk conductivity of NBT can also be enhanced by Ga-acceptor doping on the B-site. By decreasing the Na/Bi ratio (donor doping) which compensates for oxygen vacancies, it switches from being an ionic conductor to an insulator. In contrast, by increasing the Na/Bi ratio (acceptor doping) which generates oxygen vacancies, the bulk conductivity remains the same, which is independent of the nominal oxygen content. In fact, the actual composition changes with increasing nominal oxygen vacancies, in order to maintain the actual oxygen content at a certain level. This self-recovery behaviour restricts the enhancement of ionic conductivity in NBT-based ceramics. The influence of B-site acceptor dopants and highly polarised Bi ions on the electrical properties has been investigated in the solid solution of NBT with Bi-based perovskite end members, such as BiGaO3 (BG), BiFeO3 (BF) and Bi(Ni0.5Ti0.5)O3 (BNiT). In general, the ionic conductivity is suppressed with increasing doping level due to the mobile oxygen ions being trapped by the accepter dopants on the B-site. The electronic conductivity is enhanced by transition metal ions, such as Fe3+ and Ni2+. For the BG-NBT series, the solid solution limit is ~ 6%, where the ionic conductivity is eliminated. The actual composition may be influenced by the presence of secondary phases, leading to a slight mismatch of the charge configuration between the A and B-site results in a dramatic change in conductivity. The rhombohedral phase is stabilised by BF-doping, where Tm associated with the permittivity increases from 320 to 445 ℃ when the BF content increases from 0 to 50%. The ionic conductivity is slowly supressed by the BF-doping, where p-type electronic conductivity becomes dominant for samples prepared in air when the BF content is 60%; however, ionic conduction may be dominant when annealed under N2 at 600 ℃. Furthermore, 1 at% Nb-doping for Ti can efficiently suppress the ionic conductivity. For the BNiT-NBT series, the solid solution limit is ~ 60%. Neutron diffraction studies confirm the high temperature tetragonal phase is stabilised at room temperature when the BNiT content is ≥ 40%. The coexistence of rhombohedral and tetragonal phases leads to permittivity peak broadening and a decrease of Tm to a lowest value of 186 ℃ during cooling when the BNiT content is 60%. p-type electronic conductivity becomes dominant when the BNiT content is ≥ 30% for samples processed and measured in air.
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