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

Abstract

(Ni2+, Nb5+) co‐doped BaTiO3 ceramics BaTi1‐x(Ni1/2Nb1/2)xO3‐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 [Vo‐NiTi‐Vo] 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.

Photoresponsive Piezoelectrics

“…Although having been proven by KN-, KNN-and NBT-based compositions, the effectiveness of the Ni 2+ -oxygen vacancy doping strategy for band gap engineering of perovskite oxides was questioned by a BT-based composition. It was found that by doping 1-4 mol % BNNO into BT, the sintered ceramics showed band gaps of 1.4-1.9 eV with d 33 values of 70-130 pC/N (Lou et al, 2019). However, the powders made by crushing the sintered ceramics showed the same band gap of 3.1 eV which was identical to that of the undoped BT (Lou et al, 2019).…”

“…Figure 1 compares the band gaps and d 33 values of different compositions discovered to date for both wide and narrow gap piezoelectrics. It can be noticed in the figure that, although the currently experimented band gap engineering methodologies are believed to degrade ferroelectricity to some extent (Bai et al, 2018a;Bai et al, 2019a (Bai et al, 2017b;Xiao et al, 2019a;Lou et al, 2019;Zhong et al, 2019). This fact gives the narrow band gap piezoelectrics an obvious advantage over their wide band gap counterparts, i.e., the narrow band gap piezoelectrics can effectively absorb the majority of visible lights while exhibiting an uncompromized piezoelectric response, which can clearly be seen in Figure 1.…”

“…In addition, in KNBNZNO Ni 2+ was added individually without associated Ba 2+ , which might eliminate the negative effect of Ba 2+ on the ferroelectricity and thus piezoelectricity as presented above. (Yue and Yi-jian, 2003;Bavbande et al, 2012;Kumar and Kim, 2012;Mamoun et al, 2013;Bowen et al 2014;Paillard et al, 2016;Wu et al (2016); Bai et al, 2017a, Bai et al, 2018aDurruthy-Rodríguez et al, 2018;Xiao et al (2019a); Zhong et al 2019;Lou et al, 2019).…”

“…It was found that by doping 1-4 mol % BNNO into BT, the sintered ceramics showed band gaps of 1.4-1.9 eV with d 33 values of 70-130 pC/N (Lou et al, 2019). However, the powders made by crushing the sintered ceramics showed the same band gap of 3.1 eV which was identical to that of the undoped BT (Lou et al, 2019). Based on this observation, it was proposed that although the Ni 2+ -oxygen vacancy combination was able to largely reduce the band gap, the composition containing such a combination was in fact accumulated at grain boundaries and was not diffused into the unit cells of the grains (Lou et al, 2019).…”

“…However, the challenge is that the introduction of non-d 0 metals is likely to degrade the polarizations and hence piezoelectricity due to lattice distortion and resultant decreased unit cell asymmetry (Bai et al, 2017a;Shafir et al, 2020). In addition, the solubility of Ni 2+ is found to be low, or the diffusion of Ni 2+ is arduous, in some perovskite unit cells like BaTiO 3 (BT), KNbO 3 (KN) and (K 0.5 Na 0.5 )NbO 3 (KNN) (Bai et al, 2019b;Lou et al, 2019). This makes it difficult to form single perovskite phases containing Ni 2+ on the B-sites of the unit cells rather than having Ni-rich regions, grain boundaries or secondary phases in the microstructure.…”