Structural, electrical and optical properties of (K,Ba)(Nb,Ni)O3-δ electroceramics: oxygen vacancy and grain size effects - Guerra, J. D. S.; Silva, M. C. O.; Silva, A. C.; Oliveira, M. A.; Mendez-González, Y.; Monte, A. F. G.; M'Peko, Jean Claude; Hernandes, Antônio Carlos (Ceramics International, 2020)