Band-gap narrowing and magnetic behavior of Ni-doped Ba(Ti_0.875Ce_0.125)O₃ thin films

Band-gap narrowing and magnetic effects have been observed in a Ni-doped Ba(Ti_0.875Ce_0.125)O₃ (BTC) thin film. Structural characterizations and microstructural analysis show that the as-prepared Ba(Ti_0.75Ce_0.125Ni_0.125)O_3-δ (BTCN) thin film exhibits a cubic perovskite structure with an average grain size of 25 nm. The Ce doping at the Ti-site results in an increasing perovskite volume to favour an O-vacancy-stabilized Ni²⁺ substitution. Raman spectroscopy, however, shows the cubic symmetry of crystalline structures is locally lowered by the presence of dopants, significantly deviating from the ideal Pm3m space group. Moreover, BTCN presents a narrowed band-gap, much smaller than that of BaTiO₃ and BTC, 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. Also, magnetic enhancement driven by co-doping has been confirmed in the films, which mainly stems from the exchange interaction of Ni²⁺ ions via an electron trapped in a bridging oxygen vacancy. These findings may open an avenue to discover and design optimal perovskite compounds for solar-energy devices and information storage.