The effect of grain boundary on the visible light absorption of BaTi 1‐x [Ni 1/2 Nb 1/2 ] x O 3‐δ ferroelectric ceramics

National Key RD National Basic Research Program of China, Grant/Award Number: 2015CB654605; National Nature Science Foundation of China, Grant/Award Number: 51672293 and 51831010; Instrument Developing Project of Chinese Academy of Sciences, Grant/Award Number: ZDKYYQ20180004; Chinese Academy of Sciences President's International Fellowship Initiative, Grant/Award Number: 2017VMA0018 and 2011T2G03