Physical properties and sinterability of pure and iron-doped bismuth sodium titanate ceramics

Abstract Pure (BNT) and iron-doped bismuth sodium titanate (Fe-BNT) ceramics were produced according to the formula Bi_0.5Na_0.5Ti_1−xFe_xO_3−0.5x, where x = 0 to 0.1. The addition of Fe₂O₃ enables decreasing the sintering temperature to 900 °C in comparison with 1075 °C for pure BNT, whilst also achieving lower porosities and greater densities. This is attributed to oxygen vacancy generation arising from substitution of Fe³⁺ onto the Ti⁴⁺ site of the BNT perovskite structure, and the resulting increase in mass transport that this enables during sintering. X-ray diffraction (XRD) analysis of Fe-BNT samples shows single-phase BNT with no secondary phases for all studied Fe contents, confirming complete solid solution of Fe. Rietveld refinement of XRD data revealed a pseudocubic perovskite symmetry (Pm-3m), and unit cell lengths increased with increasing Fe content. Scanning electron microscopy (SEM) showed that average grain size increases with increasing Fe content from an average grain size of ~ 0.5 μm in (x = 0) pure BNT to ~ 5 μm in (x = 0.1) Fe-doped BNT. Increasing Fe content also led to decreasing porosity, with relative density increasing to a maximum > 97% of its theoretical value at x = 0.07 to 0.1. The addition of Fe to BNT ceramics significantly affects electrical properties, reducing the remnant polarization, coercive field, strain and desirable ferroelectric properties compared with those of pure densified BNT. At room temperature, a high relative permittivity (ɛ′) of 1050 (x = 0.07) at an applied frequency of 1 kHz and a lower loss factor (tanδ) of 0.006 (x = 0.1) at an applied frequency of 300 kHz were observed by comparison with pure BNT ceramics.