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

Abstract

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

Polymorphism in Na2(Co/Zn)P2O7 and Na2(Co/Fe)P2O7 Pyrophosphates: A Combined Diffraction and 31P NMR Study

“…The cell volume decreases with the increasing amount of Zn. This trend might seem counterintuitive, as the Shannon ionic radius of Zn 2+ in tetrahedral coordination (0.60 Å) is slightly larger than that of Co 2+ (0.58 Å); however, a similar trend has been already reported for other solid solutions containing the Zn/Co couple in tetrahedral coordination with oxygen (e.g., (Zn,Co)Al 2 O 4 , 39 (Zn,Co) 2 SiO 4 , 40 and Ca 2 (Zn,Co)Si 2 O 7 41 ) and attributed to a larger covalence degree of the Zn−O bond, due to the strong hybridization of sp 3 orbitals. 42 It is particularly interesting to note the close similarity between the orthorhombic and tetragonal structural parameters for the samples Co50Zn50, Co25Zn75, and Zn1, confirming that the two polymorphs are well related.…”

Synthesis and characterisation of Ba(Zn1−xCox)2Si2O7 (0 ≤ x ≤ 0.50) for blue-violet inorganic pigments

“…35 According to the Tanabe-Sugano diagram, the bands from 550 to 650 nm are assigned to the 4 A 2 (F) / 4 T 1 (P) transition of the tetrahedral coordinated Co 2+ . [33][34][35][36] Table 4. It is notable that the absolute values of a* and b* for the Ba(Zn 0.85 -Co 0.15 ) 2 Si 2 O 7 were signicantly larger than those for the commercial violet pigments.…”

Karrooite green pigments doped with Co and Zn: Synthesis, color properties and stability in ceramic glazes

“…The main research interests were to ascertain whether these Co-doped karrooite pigments were or not sensitive to ceramic glazes rich in alkalis (CaO and ZnO) [2], and also to analyze the effect of Zn-codoping on the stability of these Co-karrooite pigments. Indeed, Zn-containing pigments have been found in some cases to be more stable against chromophore (Co 2+ ) dissolution in the glaze in these more aggressive environments of Zn-Ca-borosilicate melts [35].…”

“…This fact is indicative of much lower pigment stability in these glazes. Indeed, this bluish coloration is well-known in the literature to be associated to Co 2+ ions dissolved and incorporated mainly in tetrahedral environments of the glassy phase, the commonly named "cobalt leaching" or "cobalt bleeding" [4,6,9,10,35,[40][41][42][43].…”