The Effect of Hydrothermal Synthesis Parameters on the Formation of Sodium Bismuth Titanate

We report the effect of various Bi:Ti ratios x:1 (x = 0.5–3) on the phase formation of sodium-bismuth titanate Na_0.5Bi_0.5TiO₃ (NBT) during hydrothermal synthesis (230°С, 3 MPa, 96 h) from titanium oxide and bismuth hydroxide in 20 M NaOH solution. It was shown that single-phase ceramics with perovskite structure could be obtained at x = 0.5–0.9 using the hot-pressing technique (1025°С, 60 MPa, 1 h). At x = 1, hydrothermal synthesis results in single-phase samples, while, after sintering, the admixture of Na₂Ti₃O₇ was detected. At x = 2 and 3, samples contained admixtures after hydrothermal synthesis. The x–value does not significantly affect the composition of perovskite-like products despite the excess of bismuth in the system. The chemical composition of the samples with x = 0.5–0.9 corresponds to the general formula Na_0.60–0.69Bi_0.40–0.31TiO_2.9–2.81δ_0.10–0.19 (where δ represents vacancies in oxygen sublattice). The Bi³⁺ content in the samples is underestimated due to the solubility of its hydroxide in a 20 M alkali solution. The properties of ceramics are not significantly affected by x values, and materials exhibit following average characteristics: electrical resistivity 1.3 × 10⁹ Ohm∙cm, tanδ = 0.15, К_р = 0.1, d₃₁ = 6.7 and d₃₃ = 9.5 pC/N, g₃₁ = 2.34 and g₃₃ = 3 mV∙m/N.