This paper is focusing on the samples elaborated from sol gel synthesis known as Pechini route where powders have been annealed under air and on oxides prepared from co-precipitation method followed by an argon thermal treatment. For all the synthesis routes, annealing treatment under air below 1000 ☌ leads to the obtaining of dark green – black powders whereas blue powders are obtained only for synthesis temperatures above 1200 ☌. Preliminary works, not reported herein, have dealt with the exploration of several CoAl 2O 4 synthesis routes (co-precipitation, solid state, tartrate, sol–gel and polyol 21) and their thermal annealing under air or argon (only for the co-precipitation) between 600 ☌ and 1400 ☌. These effects can be explained considering various and diverging hypotheses: (i) the occurrence of amorphous carbon, (ii) the presence of a Co 3O 4 (Co 3+/Co 2+ mixed valences)/Co 2AlO 4 (Co 3+) impurity phases with almost similar unit–cell parameter as CoAl 2O 4 (Co 2+) taking into account the similar ionic radii of Co 3+(LS) and Al 3+ six-fold coordinated to oxygens, 4,8–11,14–16 (iii) a grain shape influence (taking into account diffusion phenomena) 12,13 or, (iv) the Co 2+ distribution in the network between octahedral and tetrahedral sites. 4,7–13 Depending on the pigment thermal history, all the authors 4,7–9,11 have observed that the samples are darker with a greening of the coloration when a low synthesis temperature is applied. 4,7–12 Indeed, the CoAl 2O 4 composition (known as Thenard's blue) exhibits an all the more intense sky-blue hue than the annealing temperature is high. 7 Nevertheless, the colour of CoAl 2O 4 pigment can strongly vary depending on the thermal history of the material. 4–6 Tetrahedral coordination of Co 2+ is often preferred to octahedral coordination and leads to a strong blue colour in CoAl 2O 4 spinel whereas Co 2+ in octahedral coordination is known to exhibit a purple hue. Hence, in such frameworks, the crystal field of Co 2+ (3d 7) can increase from 0.5 eV in a tetrahedral environment to 0.8–1.3 eV in a 5-fold or octahedral coordination leading to various d–d transitions on the basis of Tanabe–Sugano diagrams. The spinel structure exhibits both octahedral and tetrahedral cationic sites, which lead to large colour scales. The traditional source of the blue colour in ceramic pigments remains the cobalt ion (Co 2+), 3 particularly incorporated into spinel networks derived from rock salt and crystallizing in the Fd m space group. 1400 ☌, under air in order to get the strongest blue color stabilized in ceramics. However, these inorganic pigments needed to be annealed at high temperatures, i.e. synthetic ultra-marine blue Na 7Al 6Si 6O 24S 3 and cobalt blue CoAl 2O 4. Through the years, additional inorganic compounds have been added to the spectrum of blue pigments, e.g. 1,2 The first blue modern synthetic pigment, discovered at the beginning of the 18 th century, was Prussian blue an organic pigment. Introduction Synthetic blue pigments are widely used in the ceramic industry as colouring agents in glazes or porcelain stoneware. A dual-colour display was filled with the as-prepared inks and tested under ☑50 V. Hybrid blue positively charged particles were mixed with white negatively charged particles to formulate dual-colour inks. A top-down (mechanical grinding) and a bottom-up approach lead to the definition of a synthesis route (co-precipitation in basic medium followed by annealing at medium temperatures under Ar) allowing the design of strong blue pure nano-sized pigments to be incorporated in inks. EELS coupled with TEM analyses (occurrence of multiple phases with various Al/Co atomic ratios) lead to us to conclude definitively about the variation of Co valence states. Starting from a sol–gel process, the so-called Pechini route, the correlation between the structural features (inversion rate, Co over-stoichiometry, Co valence states) of the spinel network and its thermal history under air up to high temperatures ( T = 1400 ☌) allows concluding that the stabilization of CoIII in octahedral sites is at the origin of the blackening of the pigment annealed at low and medium temperatures. Blue cobalt inorganic pigments with spinel-type structure have been revisited in order to understand the origin of blackening at low temperatures and to design strong blue nanosized materials.
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