Co-Doped Hardystonite, Ca2(Zn,Co)Si2O7, a New Blue Ceramic Pigment

Raising cost, limited reserves, and toxicity make a pressing need to reduce the consumption of cobalt in the ceramic industry, trying to improve efficiency and sustainability of pigments. A novel blue colorant, based on the melilite structure, has been developed by searching for a ceramic pigment stable in very aggressive media, like the calcium- and zinc-rich glazes used in porous tiles (stoneware and monoporosa). Hardystonite was selected as a typical crystalline compound found in these coatings, which has just one four-fold crystallographic site where Co2+ ions can be accommodated, thus ensuring its unrivalled blue colour. Five samples (Ca2Zn1-xCoxSi2O7 with x = 0.05, 0.1, 0.2, 0.3 and 0.4) were prepared by solid state synthesis in industrial-like conditions (95% yield) and characterized by XRD, DRS, SEM-EDS and technological testing. Increasing cobalt doping gives rise to a gradual expansion of the hardystonite unit cell, unexpected on the basis of Zn2+ and Co2+ ionic radii, attributed to a change of the covalent character of M–O bonding. Optical spectra are dominated by the strong absorption bands of Co2+ in tetrahedral coordination (crystal field strength Dq = 421 cm-1, Racah B parameter = 793 cm-1). The best compromise between cobalt concentration and optical response was found to be around x = 0.3. The hardystonite pigment bestows a deep blue colour on glazes and glassy coatings, withstanding aggressive media rich in CaO and ZnO better than industrial blue pigments (cobalt aluminate, spinel) with similar colour efficiency than industrial blue dyes (cobalt silicate, olivine) but with the advantage to avoid specking defects of highly staining colorants.