Counterfeit band gaps caused by microstructural voids in photo-ferroelectric ceramics

In recent years, conventional wide band gap ferroelectric ceramics have been engineered by doping to obtain reduced band gaps. The band gaps in these engineered ceramics are usually determined according to the optical absorption measurement. However, this raises a potential problem in the research of these presumed narrow band gap ferroelectric ceramics as the results of the absorption measurement may give incorrect information of band gaps. This paper demonstrates how microstructural voids, i.e. micron-sized intra- and inter-granular pores in the ceramics formed with improper sintering conditions, can create additional absorption peaks thus leading to counterfeit band gaps. Two types of Ba/Ni co-doped (K,Na)NbO₃ (KNN) photo-ferroelectric ceramics with relatively wide (~3 ​eV) and narrow (~2 ​eV) band gaps are used to explain the potential problem. This paper hopes to encourage photo-ferroelectric ceramics researchers to use methods other than the optical absorption to determine band gaps in future works.