Microstructure and high temperature transport properties of Ca-doped nickel oxide

The high temperature microstructure of Ca-doped NiO single crystals was analysed to clear the experimental data on electrical conductivity, Seebeck's effect, diffusion and kinetic demixing. Combined transmission light microscopy and transmission electron microscopy examinations coupled with EDS analyses revealed the presence of CaO precipitates in localized (eutectic-like) zones for a wide range of analysed oxygen partial pressure. Their presence in the structure was confirmed for equilibrium (air) conditions in NiO-CaO system at about 1700 degrees C (approximately 42 mol% CaO), according to phase diagrams (Smith et al., 1969). For the wide range of oxygen partial pressure and high temperature (1000 / 1200 degrees C) the spinel phase was not observed, which is opposite to our previous results obtained for Ca-doped CoO single crystals (Kusinski et al., 2006). 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(Ca) > D(Ni)), 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.