On the microstructures of equilibrated and quenched spinel-containing nickel titanates

Electron microscopy has been used to study the microstructures of equilibrated and quenched NiO-TiO₂ mixtures containing 5, 10, 15, 20 and 25wt% TiO₂. All the specimens contained rocksalt- and spinel-structured phases on a nanometre scale. The microstructure suggests that a nonstoichiometric spinel decomposed during quenching. The 25wt% TiO₂ specimen exhibited a lamellar microstructure composed of NiTiO₃ and spinelNiO. These phases had the usual orientation relationships, i.e., (111)_cubic||(0001)_hex and (110)_cubic|| (1100>_hex. Faceted NiO solid-solution regions with coherent cube-on-cube interfaces with the spinel were found in the four specimens of lower TiO₂ contents. When large (15 and 20wt% TiO₂), these regions were faceted cuboidal particles. With lower TiO₂ contents, these regions were no longer equiaxed and could be interconnected. The specimens were chemically homogeneous on a length scale of about 1mum. Morphological evidence is presented to explain which part of the microstructure most probably already existed in this form during equilibration. In particular, the large-scale NiTiO₃ and the spinel lamella in the 25wt% TiO₂ material form during long-term equilibration.