Structure and dielectric properties of double A-site doped bismuth sodium titanate relaxor ferroelectrics for high power energy storage applications

Abstract

The structural and dielectric properties of barium/strontium substituted Bi_0.5Na_0.5TiO₃ were examined in compositions of general formula (Ba_0.4Sr_0.6TiO₃)_x(Bi_0.5Na_0.5TiO₃)_1−x. An average classic cubic perovskite structure is maintained from x = 0.5 to 1.0. The temperature dependence of dielectric properties of studied compositions shows relaxor-ferroelectric behaviour attributed to the existence of polar nano-regions. Ferroelectric measurements under variable temperature demonstrated two transitions, from normal ferroelectric to relaxor-ferroelectric and relaxor-ferroelectric to paraelectric, at the dipole freezing temperature, T_f, and temperature of maximum permittivity, T_m, respectively. The obtained value of T_f coincides with the onset of linear thermal expansion of the cubic unit cell parameter obtained from high resolution powder neutron diffraction data. Careful analysis of the neutron diffraction data revealed no significant change in the average cubic structure from −263 to 150 °C. However, changes in the Gaussian variance component of the neutron peak shape, reveal three distinct regions with transitions at about −100 and 100 °C corresponding to the beginning and end of the dielectric dispersion seen in the permittivity and loss spectra. This variation in the Gaussian variance parameter is attributed to the activity of the polar nano-regions. The composition (Ba_0.4Sr_0.6)_0.5(Bi_0.5Na_0.5)_0.5TiO₃ was found to exhibit the maximum recoverable energy storage density, with a value of 1.618 J cm⁻³ and 76.9% storage efficiency at a field of 17 kV mm⁻¹.