On the structure of the NiO–Al2O3 systems, studied by diffuse-reflectance spectroscopy

Diffuse-reflectance spectra of the NiO–Al₂O₃ systems in the 350–800 nm spectral domain are analysed. Two types of Ni²⁺complexes in γ-Al₂O₃ have been found at low concentrations (<5%): Ni²⁺6O^2?] and Ni²⁺4O^2?] with octahedral (O_h) and tetrahedral (T_d) symmetries, respectively. Coexistence of these two complexes is discussed in connection with the sample preparation and their thermal treatment.