Synthesis, stability and coloring properties of yellow–orange pigments based on Ni-doped karrooite (Ni,Mg)Ti2O5

Solids solutions of Ni in MgTi2O5 pseudobrookite (karrooite) were investigated to develop new yellowish ceramic pigments or dyes. Ni-Karrooite samples (Mg1−xNixTi2O5) were prepared by ceramic method and through a citrate-gel metalorganic decomposition route, using rapid firing conditions (800–1500 °C). Fired pigments were characterized by XRD, SEM/EDX, UV–vis–NIR and color measurement (CIE-L*a*b*) techniques. The entropy-stabilization of MgTi2O5 with respect to ilmenite and rutile decreased with Ni doping, and Ni solid-solubility increased with temperature (40 mol% at 1200 °C, 60 mol% at 1400–1500 °C). Optical absorptions of Ni2+ in karrooite octahedral sites (mainly M1 sites) produced intense yellowish-orange colors, increasing saturation and red/yellow hues with Ni doping (up to x = 0.4–0.5). The pigments showed a limited stability within low-temperature (1000–1050 °C) ceramic glazes, although glazed samples developed intense yellowish colorations. Noteworthy, metalorganic decomposition route enabled the formation of Ni-karrooite at lower temperatures (1000 °C, 20 mol% Ni) and produced finer-grained powders, although the colors of powders and enameled samples were similar.