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 the 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 refiring 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 scanning electron microscopy and XRPD. Unit cell parameters of tialite were determined by Rietveld refinement of the X-ray diffraction patterns, revealing in all cases the formation of solid solutions where the solubility of dopants in the Al₂TiO₅ lattice followed the trend Co3+ ions in a large interstitial site of the tialite lattice with a distorted octahedral geometry, and of Mn³⁺ and Co²⁺ ions in the Al³⁺ octahedral sites of the tialite lattice in the former case, and in both Al³⁺ and Ti⁴⁺ octahedral sites in the latter. Testing the ceramic glazes assessed the technological behavior of pigments, which found 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. The Co-doped pigment was found to be not stable in glazes, undergoing a cobalt-leaching effect.