M-doped Al2TiO5 (M= Cr, Mn, Co) solid solutions and their use as ceramic pigments

New ceramic pigments based on the tialite (Al2TiO5) structure, doped with Co (pink), Cr (green) or Mn (brown), were prepared through pyrolysis of aerosols followed by calcination of the obtained powders at 1400°C. The expected decomposition of Al2TiO5 into a mixture of Al2O3 and TiO2 on re-firing was inhibited by Cr-doping and also by co-doping with Mg the Mn or Co doped samples. Microstructure and phase evolution during pigment preparation were monitored by SEM and XRPD. Unit cell parameters of tialite were determined by Rietveld refinement of XRD patterns revealing in all cases the formation of solid solutions where the solubility of dopants in the Al2TiO5 lattice followed the trend: Co Mn Cr. Valence state and possible location of dopants in the tialite lattice were investigated by XPS and DRS spectroscopies, which suggested the presence of Cr3+ ions in a large interstitial site of the tialite lattice with distorted octahedral geometry, and of Mn3+ and Co2+ ions at the Al3+ octahedral sites of the tialite lattice in the former case, and in both Al3+ and Ti4+ octahedral sites, in the latter. Technological behaviour of pigments was assessed by testing in ceramic glazes finding that the color stability was reasonably good for the Mn-doped tialite and the Cr-doped pigment although the latter suffered a small loss of green hue, while the Co-doped pigment was not stable in glazes, undergoing a cobalt-leaching effect.