Aging effects in pure and doped barium titanate ceramics

Ferroelectric properties of 9% mol (Hf,Zr)-doped BaTiO3 ceramics have been studied by comparison with those of pure tetragonal BaTiO3 ceramics. The addition of doping ions lead to a shift of the rhombohedral to orthorhombic and orthorhombic to tetragonal transition temperatures above 0 degreesC, and to a decrease of tetragonal to cubic (ferroelectric to paraelectric) transition temperature from 122 degreesC to 95 degreesC on heating and from 120 degreesC to 85 degreesC on cooling. In pure tetragonal ceramics at room temperature, aging develops by a thermally activated process, leading to a linear dependence of the relative permittivity and dielectric loss on the logarithm of time. In case of doped ceramics, the experiments showed a decreasing reversed-S dependence of capacitance and tan delta on the logarithm of time, suggesting that dielectric aging in these materials is related with defect diffusion. It was concluded that in doped BaTiO3 ceramics the aging mechanisms depend on the crystalline symmetry which influences the twinning process and the domain-wall dynamics. Aging of (Hf,Zr)-doped BaTiO3 ceramics depends on temperature and the phase symmetry, which imposes the twinning rate and the rearrangement speed of ferroelectric domains minimizing the elastic energy of the lattice.