Growth, optical and electrical properties of pure and Mn-doped Na0.5Bi0.5TiO3 lead-free piezoelectric crystals

In this paper, polycrystalline materials of Na0.5Bi0.5TiO3 and 1 at.% Mn-doped Na0.5Bi0.5TiO3 (abbreviated as NBT and Mn:NBT) were synthesized by solid-state reaction techniques. Single crystals were successfully grown by using the top-seeded solution growth (TSSG) method, which dimensions are Φ40 mm × 10 mm for pure NBT crystal and Φ30 mm × 10 mm for Mn:NBT crystal. X-ray powder diffraction results show that the as-grown NBT and Mn:NBT crystal all possess the perovskite structure and belongs to the rhombohedral system and their unit cell constants are almost the same with each other, which are a = b = c = 3.8825 Å, α = β = γ = 89.22° for Mn: NBT crystal and a = b = c = 3.8858 Å, α = β = γ = 89.25° for NBT crystal. The cubic crystallization morphology of NBT crystal can be explained by the model of anionic coordination polyhedral growth-units. Room temperature absorption spectra indicates the existence of Mn3+ ions in the as-grown Mn:NBT crystal. At room temperature, the dielectric constant of 〈0 0 1〉-oriented NBT is 720 at 1 kHz and it decreases to 600 for 〈0 0 1〉-oriented Mn:NBT crystal. The phase transition temperature Tm corresponding to dielectric constants maximum is moderately decreased from 340 °C to 330 °C by Mn doping. Mn substitution in the NBT crystal significantly increases the values of electromechanical coupling factors kt and piezoelectric constants d33, which are 0.24 and 65 pC/N for NBT crystal, 0.52 and 130 pC/N for Mn:NBT crystal, respectively. These results indicate that the Mn substitution in the NBT crystal results in both hardening and softening effects due to the existence of mixed valence states of Mn ions.