Optical band-gap narrowing in perovskite ferroelectric ABO₃ ceramics (A=Pb, Ba; B=Ti) by ion substitution technique

Band-gap narrowing effects have been observed in and Ni-doped PbTiO₃ and Ni-doped Ba(Ti_0.875Ce_0.125)O₃ ferroelectric ceramics. Structural characterizations and microstructural analysis show that the as-prepared PbTi_0.67Ni_0.33O_3−δ (PTN) and Ba(Ti_0.75Ce_0.125Ni_0.125)O_3−δ (BTCN) samples exhibit polycrystalline perovskite structures with dense microstructures. The Ni doping at Ti-site results in lattice expansion and introduces oxygen vacancies. Note that the band-gap values of PTN and BTCN are 2.42 eV and 2.73 eV, respectively, much smaller than that of corresponding parent PbTiO₃ (3.21 eV) and BaTiO₃ (3.37 eV) materials. This optical behavior can be due to new states of both the highest occupied molecular orbital and the lowest unoccupied molecular orbital in an electronic structure with the presence of Ni. With such decreased optical band-gaps, the PTN and BTCN materials will have great potential in solar-energy conversion devices.