Calcium doping in rock-salt transition metal oxides: defect structure, microstructural evolution, and transport properties in FeO, CoO, and NiO

Abstract

This review presents the results of experimental study conducted over three decades on calcium doping in three isostructural iron-group rock-salt oxides: wüstite (Fe₁–δO), cobalt (CoO), and nickel (NiO) monoxides. In the case of calcio-wüstite, the large Ca²⁺/Fe²⁺ ionic radius mismatch (1.00 vs 0.77 Å) lowers the stacking fault energy, generating ordered stacking faults that transform into CaFe₅O₇ ferrite precipitates during reduction in CO atmosphere. In Ca-doped cobalt oxide single crystals, oxygen activity controls extensive precipitation of Ca-free Co₃O₄ spinel which electron energy-loss spectroscopy links to changes in the Co³⁺/Co²⁺ ratio and to the abrupt conductivity decrease for the partial pressure of oxygen near p ≈ 10^–3-10^–4 atm. In Ca-doped nickel oxide, CaO-rich eutectic-like zones form near subgrain boundaries without precipitation of spinels The chemical diffusion and electrical field kinetic demixing experiments have been combined to prove that the presence of Ca leads to an increase of the chemical diffusion coefficient of NiO. This unexpected result (D(Ca²⁺) > D(Ni²⁺)), confirmed during electrical conductivity experiment, indicates that correlation effects between Ni and Ca cations play a decisive role in diffusion processes in (Ni,Ca)O.

The study were conducted using TEM, SEM, ELELS, TML and EDS techniques, as well electrical conductivity measurements.