Increase in depolarization temperature and improvement in ferroelectric properties by V⁵⁺ doping in lead-free 0.94(Na_0.50Bi_0.50)TiO₃-0.06BaTiO₃ ceramics

A detailed study was carried out to investigate the effects of poling on structure, vibrational, dielectric, and ferroelectric properties of donor-doped (V⁵⁺ at Ti⁴⁺-site) lead-free Na_0.47Bi_0.47Ba_0.06Ti_(1-x)V_xO₃, (x = 0, 0.01, and 0.03) ceramics fabricated via a modified sol-gel method. Rietveld refinement of synchrotron radiation source powder x-ray diffraction data showed that unpoled samples are in rhombohedral R3c phase whereas poled samples showed a mix rhombohedral R3c and tetragonal P4mm phases at ambient temperature, due to a long-range order established in lattice system after poling. V⁺⁵ doping increases the rhombohedral distortion in unpoled and poled samples while it reduces the tetragonality in poled samples. Vibrational study revealed that unpoled samples have more lattice disorder compared to poled samples. X-ray absorption near edge spectroscopy measurement confirmed that Ti and V are in 4+ and 5+ oxidation states, respectively, for all poled and unpoled samples. The average grain size was found to decrease from 5.6 ± 0.5 μm for x = 0 to 1.0 ± 0.2 μm for x = 0.03. Depolarization temperature was found to increase significantly in poled samples from ∼104 °C for undoped sample to 150 °C for the sample with 1% vanadium substitution. Drastic improvements in ferroelectric and dielectric properties are explained in terms of structural changes. High remnant polarization P_r ∼ 31.4 μC/cm² and moderately low coercive field E_c ∼ 20 kV/cm have been observed at an applied electric field of ∼35 kV/cm for the sample with 1% vanadium substitution which makes it an attractive candidate for ferroelectric applications.