Microstructure and chemistry of MgO- and CoO-doped nickel-titanate spinels

Abstract

When NiO-rich powder mixtures of NiO and TiO2 are equilibrated in air above ∼1425°C and subsequently quenched, X-ray diffraction patterns of the resulting specimens can be indexed with a single set of reflections in the Fd3m space group. Originally this result was cited as evidence of a highly nonstoichiometric single phase. However, subsequent TEM investigations showed that these specimens are composed of periclase- and spinelstructured phases with small domains and coherent phase boundaries. Periclase-structured domains are distributed throughout the spinel with two distinct morphologies: cuboidal particles, faceted on {100}, of uniform size that increases from 30 to 150 run with increasing NiO-mole-fraction; and an isotropic interconnected twophase microstructure, also of uniform size, varying from 2 to 5 ran seemingly independent of composition. The finer-scale features strongly resemble those arising from spinodal decomposition in an isotropic medium and can be ascribed to decomposition of a nonstoichiometric spinel phase during quenching. It has recently been suggested that the faceted particles coexist with the nonstoichiometric spinel at the equilibration temperature.