Influence of tetragonality (c/a) on dielectric nonlinearity and direct current (dc) bias characteristics of (1- x )BaTiO3- x Bi0.5Na0.5TiO3 ceramics

The incorporation of Bi0.5Na0.5TiO3 (BNT) into BaTiO3 (BT) in a (1-x)BT-xBNT system, with 0 ≤ x ≤ 0.3, creates an increase in the Curie temperature and the tetragonality (c/a) of the system. This causes (1) a decrease in the dielectric constants in the entire alternating current driving field range up to ∼4.5 V/μm; (2) an increase in the threshold field beyond which the dielectric constants begin to increase; and (3) a steep decrease in the dielectric constants when a direct current (dc) bias field is applied. The Preisach analysis shows that the enhancement of c/a, and its supposed increase in the lattice incompatibility during the domain wall motion, reduces the reversible domain wall contribution to polarization. This enhancement also reduces the mobility of the irreversible domain walls, thus requiring a higher driving field to trigger motion. The steep decrease in the dielectric constants under a dc bias field caused by the enhanced c/a can be associated with an increase in the anisotropy of the dielectric constants, which results in their reduction in the crystallographic c-axis direction. These results demonstrate that a high c/a is not desirable for capacitor applications, although it is the origin of ferroelectricity in perovskite materials.