Crystal structure and optical properties of karrooite ceramic pigments

Karrooite, MgTi2O5, is a promising candidate for ceramic pigments, being refractory, with high refractive indices and capable to host transition metals in two different octahedrally-coordinated crystallographic sites, M1 and M2, both strongly distorted. Karrooite samples were synthesized by the conventional ceramic route (firing temperature 1300°C) according to the formula Mg1-xTi2-xM2xO5 (M = Co, Cr, Fe, Mn, Ni or V; x = 0.04 and 0.10) and characterised by XRD, DRS and technological testing in order to assess their potential as ceramic pigments. Optical spectra revealed that transition metals are accomodated in the karrooite lattice as Co(II), Cr(IV), Fe(III), Mn(II) and Mn(III), Ni(II) and V(IV). Structural data showed that dopants slightly varied unit-cell parameters, while their addition affected cation ordering and increasing amounts of dopants in the synthesis batch determined the crystallization of accessory phases (geikielite-type solid solutions such as (Mg,Co)TiO3) testifying a relatively low solubility of these transition metals in the karrooite structure. Furthermore, unit-cell parameters of karrooite are slightly influenced by doping, while the cation ordering is increased by the addition of V and decreased by the addition of Co. Besides, the higher the amount of dopant the stronger the site distortion. Low purity colours are obtained - orange-buff (Cr or V), brown-tan (Fe or Mn), yellow (Ni) and green (Co) - due to the disordered distribution of dopants in the M1 and M2 sites, bringing about a doubling and broadening of optical absorption bands. Testing in ceramic glazes and bodies proved these pigments are stable in low temperature applications.