Formation and the colour development in cobalt spinel pigments

Abstract

Nanocrystalline cobalt spinels were synthesized under hydrothermal conditions, starting from metal chlorides. The color properties in these systems mainly depend upon the preparative method and cation distributions, which have been determined by X‐ray powder diffraction using Rietveld refinements and visible near infrared absorption spectra. We observe the influence of tetrahedral and octahedral preference of cations such as Co 2+ , Cr 3+ , Mn 3+ upon the formation and the color development of cobalt spinel pigments. All the samples prepared revealed the formation of single spinel with particle size at about 0.8‐2.1 μ m; CoAl 2 O 4 reveals king's blue, CoMn 2 O 4 dark blue and CoCr 2 O 4 bluish green. Cobalt blues developed giving deep absorption characteristic of tetrahedral Co 2+ ions at about 550‐680 nm; therefore, color changed from king's blue to bluish green in CoCr 2 O 4 , showing the absorption band of octahedral Cr 3+ ions owing to the large excess octahedral crystal field stabilization energy.