Chemical solution deposition as a route to narrow-band gap and room-temperature ferromagnetic perovskite [K_0.5Na_0.5NbO₃]_1-x[BaNi_0.5Nb_0.5O_3-δ]_x films

[K_0.5Na_0.5NbO₃]_1-x[BaNi_0.5Nb_0.5O_3-δ]_x (KNBNNO, 0 ≤ x ≤ 0.3) films have been fabricated on different substrates for the first time, using a modified chemical solution deposition method. The microstructure, optical properties, ferromagnetism, and substrate effects of KNBNNO films were assessed, and we found that BaNi_0.5Nb_0.5O_3-δ (BNNO) content was a key factor in determining the properties of the final products. The lower band gap of KNBNNO is due to the band-to-band transition from hybridized Ni 3d and O 2p to Nb 4d states. Moreover, with increasing x from 0 to 0.3, the magnetism transition process of the samples from diamagnetism to ferromagnetism may originate from the competition between ferromagnetic exchange interactions in Ni²⁺-VO^2--Ni²⁺ and superexchange interactions in Ni²⁺-Ni²⁺. Notably, absorption behaviors in the visible light wave band for KNBNNO films have been realized, which makes it possible to use KNBNNO films for perovskite solar cell applications.