Pseudobrookite ceramic pigments: Crystal structural, optical and technological properties

摘要

Pseudobrookite pigments were synthesised by the conventional ceramic route, calcining at 1300 deg C four mixtures, with a Fe_2O_3:TiO_2 ratio ranging from 47:53 to 40:60, and were characterised by XRPD, DRS and colouring performance in several ceramic matrices. Titania in moderate excess of the Fe_2TiO_5 stoichiometry, necessary to minimise the occurrence of unreacted precursors, induced lattice parameters smaller than ideal pseudobrookite, in agreement with the different radii of Ti~(4+) and Fe~(3+) ions. These pigments exhibit a peculiar, intensely brown coloration originated by several light absorptions in the visible spectrum due to both d~5 electronic transitions and a magnetically-coupled paired transition between iron ions in adjacent lattice sites. A doubling of the ~6A_1 -- > ~4T_1 and ~4T_2 bands is related to the occurrence of Fe~(3+) in both octahedral sites of pseudobrookite. Besides, distinct metal-oxygen distances imply different energy absorptions in good accordance with the crystal field theory, despite the strongly covalent character of the Fe-O bonding. Although an entropy-stabilised phase, pseudobrookite persists dispersed in glazes and glassy coatings even after fast firing at 1200 deg C, so being suitable as ceramic pigment. However, its colouring performance depends on the chemico-physical properties of ceramic matrices: saturated brown shades achieved in low temperature glasses shift to a lighter brown in opacified glazes and fade to a light gray in wall tile glazes, where the high CaO and ZnO content contributes to rapidly dissolve pseudobrookite.