Gray–blue Al2O3–MoOx ceramic pigments: Crystal structure, colouring mechanism and performance

Greyish-blue coloured alumina-molybdena was synthesised by high temperature reduction firing and tested for use as a ceramic pigment. Synchrotron light and conventional XRD, ICP-OES and BET analyses proved that molybdenum spreads over the alumina surface, without entering the corundum lattice, with a high superficial concentration (i.e. 27-99 Mo atoms/nm2) implying a prevailing octahedral coordination and a certain long-range order, eventually resulting in the formation of crystalline MoO2. XPS and DRS suggested the occurrence of different valences, Mo(VI) being predominant over Mo(V) and Mo(IV). The colouring mechanism is mainly due to electronic absorption of Mo(V) in the red-orange region of the visible spectrum, giving rise to a characteristic blue shade; the grey cast is caused by extensive light absorption by Mo(IV) crystal field transitions and particularly Mo-O charge transfer. The colouring performance of Al2O3-MoOx in ceramic glazes and porcelain stoneware bodies for wall and floor tiles is excellent, being equivalent to the pigment colouration. Thermal stability in these matrices seems to be mostly affected by Na2O and MgO. However, the distinctly blue shade of these pigments is much less intense than that of the most used ceramic colorants. The alumina-molybdena system develops a peculiar colour that cannot be reproduced unless using the most expensive blue and black cobalt-bearing ceramic pigments. 26 refs.