Spectroscopic study of a Ni-bearing gahnite pigment

In the course of the synthesis of a blue Ni-bearing gahnite (ZnAl2O4), a green phase was often observed contaminating the blue products of the synthesis, thus affecting the chromatic stability of the material. Both the blue and the green phases were characterised by means of X-ray powder diffraction (XRD) and diffuse reflectance spectroscopy (DRS), in order to understand the nature of the blue colour and of the contamination.For both phases, the Rietveld refinement of the XRD powder patterns indicates that gahnite is indeed the main phase present. The blue product also contains minimal amounts of corundum (Al2O3). The colour of the blue phase was related to the tetrahedral crystal field surrounding Ni2+ in gahnite. Nevertheless, only a small fraction of tetrahedrally coordinated Ni was determined, being this ion mostly distributed over the octahedral sites. The green phase is apparently homogeneous, both under the electron microscope and from interpretation of XRD. However, from the DRS spectrum, it is suggested that the green colour could result from a mixture of the blue Ni-bearing gahnite and a small fraction of yellow–green NiO. Probably because of its low amount and/or poor crystallinity, this latter phase is undetectable by XRD. The nature of the chromatic instability of the final products of the synthesis is, therefore, ascribed to an incomplete Ni incorporation in the spinel structure.