The effect of grain boundary on the visible light absorption of BaTi_1-x[Ni_1/2Nb_1/2]_xO_3-δ ferroelectric ceramics

(Ni²⁺, Nb⁵⁺) co-doped BaTiO₃ ceramics BaTi_1-x(Ni_1/2Nb_1/2)_xO_3-δ (BTNN-100x) are prepared by a solid-state reaction method, and their electrical and light absorption properties are investigated. The results show that BTNN-100x can generate oxygen vacancies which pin the domain walls. BTNN-100x bulk ceramics show strong visible light absorption. However, the phenomenon of visible light absorption disappear for BTNN-100x ceramic powders, and the band gap of doped ceramic powders are nearly unchanged. The experiments demonstrate that stress and density have little effect on the band gap. And the grain boundary shows stronger cathodoluminescence (CL) emission. Actually, oxygen vacancies can be enriched at grain boundaries, and defect [V_o-Ni_Ti-V_o] complex structures can be form and give rise to the visible light absorption as demonstrated by First-principles calculations. Thus, the engineering design of ferroelectric grain boundaries may pave the way for the application of coupled ferroelectric-photovoltaic processes.