Internal Electric Field in Co-Doped BaTiO3 With Co2+/3+, Nb5+, Li+, and F−: Impact on Functional Properties and Charge Compensation With Niobium and Fluorine Ions

Dense barium titanate (BaTiO 3 ) ceramics ( d_{\text {rel}} > 95{\%} ) with a microscale grain size are obtained at 800 °C-1100 °C by a solid-state ceramic process. BaTiO 3 (BT) doped with Co 2+/3+ leads to a significant improvement in the properties ( d_{33}> 250 pC/N). Soft and hard characteristics of the piezoceramics are observed depending on the dopant ions. The Co/Li acceptor dopants lead to hard piezoceramics and aging phenomena. Aged BT:Co, Li exhibits double loops and a distorted hysteresis cycle for nonpoled and poled ceramics, respectively. Ceramics poled by the increasing field process at room temperature and the field cooling process present different poled and aged states, which are dependent on the thermal history and poling process. The distorted hysteresis loops for BT:Co, Li indicate an increased internal bias field with aging time. Insertion of donor dopants, such as Nb 5+ ions, significantly reduces the internal field. These behaviors are related to the presence of defect dipoles ( \text{M}_{\text {Ti}} "- \text{V}{_{\mathrm {O}}} {^{\circ }} {^{\circ }})^{x} due to the insertion of acceptor dopants in the B-sites following the oxygen vacancies to equilibrate charge compensation. BT:Co sintered with LiF leads to a quasi-symmetric hysteresis loop, indicating that F − may insert into an oxygen site and counteract the formation of oxygen vacancies. Dielectric drift of BT:Co, Li shows resilience to an ac electric field, which is related to the increased internal field. BT doped with 0.75 mol% Co 2+/3+ and 1 mol% Li 2 CO 3 presents hard piezoelectric behavior with a Rayleigh coefficient \alpha = 2.53\,\,10^{-7} m/V and the capability to handle high electrical stress of up to 400 \text{V}_{\text {rms}} /mm.