Effects of sintering process on the structural, dielectric, and optical absorption properties of KNbO3‐based ceramics

The influence of the sintering temperature and the dwell time on the structural, optical, and dielectric properties of [KNbO3]0.8[Ba(Ni0.5Nb0.5)O3−δ]0.2 ceramics (KBNNO) was systematically investigated. X‐ray diffraction measurements of the ceramic bodies sintered at different temperatures indicated that all samples presented a cubic‐like structure. However, the results remarkably revealed that both the sintering temperature and the dwell time are crucial factors that must be taken into account for “tuning” the optical response of the KBNNO ceramics. It was concluded that fine distortions in the (Nb,Ni)O6 octahedral site, induced by sintering temperature and dwell time, promoted subtle changes in the local crystal field and, consequently, in the optical properties of the KBNNO. The mechanisms responsible for the observed spectral features of KBNNO were discussed by considering absorbance spectra of NiNb2O6 ceramics and allowed ligand‐field transitions. The comparison between both optical spectra revealed a notable similarity between them. Since the octahedral site in KBNNO is equivalent to the nickel columbite structure, it was inferred that the spectral behavior is mainly driven by the same two contributions: one major associated to a distorted octahedral configuration and one minor related to spin‐orbit coupling effects. In accordance with X‐ray diffraction data, the temperature dependence of the dielectric properties corroborated that the KBNNO present a cubic‐like phase in the whole temperature range investigated. Relative densities of KBNNO ceramic bodies as a function of sintering temperature and of dwell time present maximum values of densities at 1145°C and at 120 min. Color changes of the ceramics by increasing both the parameters are observed, as well as remarkable changes in the intensity of the optical absorption response. An optimum is also achieved for sample sintered at 1145 °C with a dwell time of 120 min.