Karrooite green pigments doped with Co and Zn: Synthesis, color properties and stability in ceramic glazes

Abstract

The solid-state synthesis and stabilization of Co doped (Mg1−xCoxTi2O5), Zn doped (Mg1−xZnxTi2O5) and Co- and Zn-codoped karrooite solid solutions (Mg0.8−xZn0.2CoxTi2O5 and (Mg0.5Zn0.5)1−xCoxTi2O5) were investigated. In addition, the optical spectra, color properties and technological performance of (Co,Zn)-karrooite compositions as new green ceramic pigments were also analyzed. XRD characterization revealed for the first time the high solid solubility of Zn2+ in MgTi2O5 karrooite at 1200 ºC (between 60 and 80 mol% per Mg or karrooite formula unit). In contrast, the reactivity and stabilization of karrooite phase decreased in the case of Co2+ doping. Interestingly, codoping with Zn2+ ions at high molar ratios (Zn:Mg ratio equal to 1:1) enhanced the reactivity and enabled the stabilization of (Co,Zn)-MgTi2O5 karrooite solid solutions, even with high Co2+ loadings (20 mol% per karrooite formula unit). The (Co,Zn)-MgTi2O5 pigments exhibited yellowish-green colors associated to Co2+ ions allocated in octahedral M1 and M2 sites of karrooite lattice, and becoming more intense and less yellow the higher the Co content. However, Zn2+ codoping produced less saturated green colors with similar green but lower yellowish hues. The obtained pigments were not stable enough within the tested ceramic glazes, giving rise to turquoise colorations due to cobalt leaching and incorporation into tetrahedral sites of the glassy phase. The stability of Co-karrooite green pigments was higher in a Ca- and Zn-enriched ceramic glaze (B) fired at a higher temperature (1050 °C).