Influence of manganese on the colorimetric and magnetic properties of cobalt ferrite inorganic pigments synthesised by solution combustion and their applications as paints on cement, plaster and wood

Abstract

This study investigates the effect of substituting cobalt with manganese on the magnetic and optical properties of cobalt ferrite inorganic pigments synthesised by solution combustion. X-ray diffraction analysis confirms the formation of a substitutional solid solution with manganese replacing cobalt, associated with a shift in diffraction peaks to lower angles due to the larger ionic radius of manganese. As the manganese concentration increases (x = 0 to x = 0.6), lattice parameters and unit cell volumes increase, except at x = 0.5, which could be attributed to changes in cation distribution and lattice micro-strain. Raman spectroscopy shows shifts in the vibrational modes of metal–oxygen bonds, influenced by differences in atomic masses and bond strength constants between manganese and cobalt. Magnetic measurements indicate a decrease in coercive field, saturation magnetisation and remanence with increasing manganese content, due to weaker super-exchange interactions. Diffuse reflectance spectroscopy reveals low reflectance in the visible range due to electronic transitions in the metal ions. The substitution of manganese increases the band gap from 2.1 to 2.6 eV between x = 0.0 and x = 0.5, then decreases to 2.4 eV at x = 0.6. Colorimetric analysis shows significant variations in CIELab colorimetric coordinates, with lightness, chroma and hue changing with manganese content. Paints derived from these pigments, applied to cement, plaster and wood substrates, exhibit varied optical properties, demonstrating the influence of manganese on the optical characteristics of the pigments.