Electrical, luminescent properties and electronic structure of (Ho, Nb) co-doped BNT-based multifunctional ceramics

Abstract

The effects of (Ho, Nb) co-doping on the electrical, luminescent properties and electronic structure of Bi0.5(Na0.82K0.18)0.5Ti1-x(Ho0.5Nb0.5)xO3 (x = 0, 0.010, 0.015, 0.020, 0.025, and 0.030, abbreviated as BNKT-xHN) ceramics were investigated. XRD analysis showed that Ho3+ and Nb5+ ions completely entered the crystal structure of BNT-based ceramics, forming a stable perovskite structure. SEM confirmed the polycrystalline nature and relatively uniform grain structure of the samples. The doping of (Ho, Nb) destroyed the long-range ferroelectricity of BNT-based ceramics, resulting in the transition from ferroelectric phase to relaxor ferroelectric phase. The maximum unipolar strain of 0.32% was obtained by introducing (Ho, Nb) with a concentration of x = 0.020. Under excitation at 453 nm, a strong green emission peak appears at 550 nm, corresponding to the energy level transition of 5S2→5I8, while the weak red emission centered at 658 nm is due to the energy level transition from 5F5→5I8. As the amount of doping increases, the luminous intensity first increases and then decreases. The first-principle method based on density functional theory was used to calculate the energy band structure and density of the system states. The calculated results show that the band gap width (Eg) decreases with the increase of doping amount, and the electron cloud density distribution of the energy level orbit is very stable. Reducing the band gap makes it easier for electrons to absorb photon energy, which may explain the micro-mechanism of changing luminous intensity.