Effect of the calcination temperature on the cation distribution and optical properties of Mg0.5Co0.5CrAlO4 spinel pigment

Abstract In this study, a blue-green pigment has been prepared by partially replace Co2+ and Cr3+ in CoCr2O4 spinel structure with Mg2+ and Al3+ using a gel casting method The gel precursor was calcined at various temperatures (900-1400 °C) to obtain Mg0.5Co0.5CrAlO4 spinel pigment. Combining the Rietveld refinement method of XRD and peak fitting of XPS high-resolution spectra, the relationships between the cation distributions (Co2+, Mg2+, Al3+, and Cr3+) in the tetrahedron and octahedron of the spinel structure and the calcination temperature were examined. In the octahedron, the contents of Co2+ and Al3+ decreased with increasing calcination temperature, and the Mg2+ and Cr3+ contents exhibited the opposite trend. The bond lengths of A-O and B-O change with increasing calcination temperature, thereby leading to a change in the unit cell. The optical performance of the pigments was investigated via UV-vis and CIE L*a*b* spectrophotometry, and the study shows that the blue-green hue of the pigment powder is caused by the absorption at υ3～υ8 (370 nm ∼640 nm) in the visible light region. The varied contents of Co2+ and Cr3+ in the spinel structure among calcination temperatures cause the absorption spectrum intensity change, thereby resulting in various blue-green tones. This study lays the foundations for subsequent investigations of colour modification in MgxCo1-xAlyCr2-yO4 spinel.