Improved piezoelectric and bright up-conversion photoluminescent properties in Ho-doped Bi_0.5Na_0.5TiO₃–BaTiO₃ lead-free ceramics

Lead-free ceramics of 0.94(Bi_1−x Ho _x Na)_0.5TiO₃–0.06BaTiO₃ have been prepared by a conventional solid reaction method and their piezoelectric, ferroelectric and photoluminescent properties have been studied. After the partial substitution of Ho³⁺ for Bi³⁺, a new Bi_0.5Na_0.5TiO₃-based solid solution with a pure perovskite structure is formed. The doping of Ho³⁺ has no obvious influence on the crystal structure of the ceramics. After the addition of a small amount of Ho³⁺ (x = 0.015), the piezoelectricity (d ₃₃ = 187 pC/N, k _p = 41.1 %, respectively) and ferroelectricity (P _r = 25.2 μC/cm²) of ceramics are enhanced. The down-conversion and up-conversion photoluminescence of the ceramics are observed. A green down-conversion emission band at 548 nm is observed which is from the ⁵S₂ → ⁵I₈ transition of Ho³⁺; the main down-conversion excitation bands located at 410–500 nm is due to the ⁵I₈ → ⁵G_{5, 6} transitions, and the maximum brightness of down-conversion photoluminescence are obtained at x = 0.015. The ceramic with x = 0.01 exhibits the maximum value of the up-conversion emission at 475 nm, which is attributed to the ⁵F₃ → ⁵I₈ transition. Our study shows that the ceramics with x = 0.01–0.02 exhibit simultaneously good piezoelectricity, strong ferroelectricity and excellent photoluminescence, suggesting a potential application in the electro-optic devices.