Abstract: Solid solutions of Na_0.5Bi_0.5TiO₃ (NBT) and BiNi_0.5Ti_0.5O₃ (BNiT) were prepared by a solid-state reaction route, and their electrical properties investigated by a combination of impedance spectroscopy and electromotive force measurements to explore the possibility of developing mixed ionic-electronic conductors based on NBT. Phase analysis showed that BNiT has a large solid solution limit in NBT (60 mol% based on X-ray diffraction), and the room temperature crystal structure changes from rhombohedral to pseudo-cubic with increasing BNiT content. Neutron diffraction revealed the coexistence of rhombohedral and tetragonal phases when the BNiT content ≥ 40 mol%. Electrically, incorporation of BNiT induces p-type electronic conduction into NBT by hopping of holes between Ni²⁺ ($\mathrm{Ni}_{\mathrm{Ni}}^{X}$) and Ni³⁺ ($\mathrm{Ni}_{\mathrm{Ni}}^{·}$), and therefore changes the electrical conduction mechanism systematically from predominant oxide-ion conduction to mixed ionic-electronic conduction and then to predominant p-type electronic conduction. The total conductivity of the solid solutions showed a “V-shape” variation with increasing BNiT content. Possible mechanisms for the phase evolution and the conductivity-composition relationships are discussed. Achieving high levels of ionic and electronic conductivity simultaneously in NBT by introducing elements with variable oxidation states remains challenging due to the competition between an enhanced electronic component and a suppressed ionic component. Low levels of BNiT incorporation are, however, beneficial to reducing the dielectric loss of NBT for dielectric applications.

Key words: Na_0.5Bi_0.5TiO₃, BiNi_0.5Ti_0.5O₃, Oxide-ion conduction, Mixed ionic-electronic conduction, Phase coexistence