The role of counterions (Mo, Nb, Sb, W) in Cr-, Mn-, Ni- and V-doped rutile ceramic pigments Part 1. Crystal structure and phase transformations

Rutile is widely used as ceramic pigment for its excellent optical properties, high melting point and intense coloration when doped with transition elements. Industrial ceramic pigments are manufactured from anatase plus chromophore elements (Cr, Mn, Ni or V) and counterions (Nb, Sb or W). Several solid state reactions occur during the synthesis, involving both the anatase-to-rutile transformation (A->R) and the formation of accessory phases. The A->R transition is heavily affected by chromophores with a lowering of the onset temperature: V < Cr < Ni < Mn; the effect of counterions is almost completely hidden by that of chromophores, even if the sequence Mo < Sb < W < Nb may be inferred. The crystal structure of rutile pigments is modified by chromophores and counterions doping; in fact, the doping varies the cell parameters, implies a progressive distortion of the octahedral site and a peculiar variation of the mean Ti-O bond length, with longer basal Ti-O distances and a shorter apical Ti-O distance. The pigment co-doped with V and W is different for its minimum Ti-O bond length distortion (BLD), an almost regular TiO6 octahedron, and the occurrence of Ti3+ within the accessory compound Ti5O9.