Cobalt Minimisation in Violet Co3P2O8 Pigment

This study considers the limitations of cobalt violet orthophosphate, Co3P2O8, in the ceramic industry due to its large amount of cobalt. MgxCo3−xP2O8 (0 ≤ x ≤ 3) solid solutions with the stable Co3P2O8 structure were synthesised via the chemical coprecipitation method. The formation of solid solutions between the isostructural Co3P2O8 and Mg3P2O8 compounds decreased the toxically large amount of cobalt in this inorganic pigment and increased the melting point to a temperature higher than 1200 °C when x ≥ 1.5. Co3P2O8 melted at 1160 °C, and compositions with x ≥ 1.5 were stable between 800 and 1200 °C. The substitution of Co(II) with Mg(II) decreased the toxicity of these materials and decreased their price; hence, the interest of these materials for the ceramic industry is greater. An interesting purple colour with a* = 31.6 and b* = −24.2 was obtained from a powdered Mg2.5Co0.5P2O8 composition fired at 1200 °C. It considerably reduced the amount of cobalt, thus improving the colour of the Co3P2O8 pigment (a* = 16.2 and b* = −20.1 at 1000 °C). Co3P2O8 is classified as an inorganic pigment (DCMA-8-11-1), and the solid solutions prepared were also inorganic pigments when unglazed. When introducing 3% of the sample (pigment) together with enamel, spreading the mixture on a ceramic support and calcining the whole in an electric oven, a colour change from violet to blue was observed due to the change in the local environment of Co(II), which could be seen in the UVV spectra of the glazed samples with the displacement of the bands towards higher wavelengths and with the appearance of a new band assigned to tetrahedral Co(II). This blue colour was also obtained with Co2SiO4, MgCoSiO4 or Co3P2O8 pigments containing a greater amount of cobalt.


Introduction
In the ceramic industry, dark blue is obtained by using compounds or solid solutions containing cobalt. The coordination number of the Co(II) cation is different among the crystalline structures used, and the colouration of powdered compositions changes between blue (CoAl 2 O 4 with a spinel structure and 88% tetrahedral Co(II), ICSD-260589; Co x Zn 2−x SiO 4 (0.005 ≤ x ≤ 1) with a willemite structure, and 100% tetrahedral Co(II) ICSD-186367) and violet, blue, or purple (Co 2 SiO 4 with olivine structure and 100% octahedral Co(II) ICSD-260092; Co 3 P 2 O 8 with a related olivine structure and 100% octahedral Co(II) ICSD-9850; stable Co 3 P 2 O 8 with 1/3 octahedral Co(II) and 2/3 Co(II) in C.N. = 5 ICSD-38259) [1].
Cobalt violet orthophosphate, Co 3 P 2 O 8 , is a pigment included in the DCMA Classification of Mixed Metal Oxide Inorganic Colour Pigments (DCMA-8-11-1) [2]. Its use in the ceramic industry is limited because of the large amount of cobalt in this compound. The formation of solid solutions between the isostructural Co 3 P 2 O 8 and Mg 3 P 2 O 8 compounds could be used to avoid the toxically large amount of cobalt in this pigment. In compositions that are rich in magnesium, these solid solutions could decrease the amount of cobalt, thus increasing the interest of these materials for the ceramic industry. The substitution of Co(II) with Mg(II) decreases the toxicity of these materials and decreases their price. The Co 3 P 2 O 8 compound melts at 1160 • C [3,4]. Magnesium orthophosphate melts at 1357 • C [5]. The melting point of some compositions of the solid solutions could be higher than 1200 • C.
The formation of solid solutions through the substitution of ions in a crystalline structure changes the bond strength and modifies the colour of the materials. The colour blue is usually obtained from a tetrahedral CoO 4 geometry. The colour purple is obtained from LiZn 1−x Co x PO 4 (0 ≤ x ≤ 0.4) compositions with a LiZnPO 4 structure, which can be explained by the highly distorted geometry in the CoO 4 tetrahedra. The shorter Co-O bonds increase the ligand field strength and lead to a blue-shifted absorption, thus developing an excellent purple pigment [6]. The increase in the amount of Co in these compositions decreases the negative b* value of the CIE L* a* b* parameters and the colour change to a violet hue due to the presence of the LiCoPO 4 phase with Co(II) ions in octahedral coordination together with the LiZnPO 4 phase with Co(II) ions in tetrahedral coordination [6].
The stable polymorph of the Co 3 P 2 O 8 compound with the monoclinic Mg 3 P 2 O 8 structure (α phase) contains Co(II) ions in both a square planar pyramid and an octahedral coordination, Co1 in the 4e site and Co2 in the 2a site (ICSD-38259) [7]. A polymorph of the Mg 3 P 2 O 8 compound with the Ni 3 P 2 O 8 structure (β phase) has also been reported (ICSD-9849) [1,8]. The transition temperature from β-Mg 3 P 2 O 8 to α-Mg 3 P 2 O 8 (ICSD-261231) is 1055 • C [5]. In the stable Co 3 P 2 O 8 structure, all of the Co(II) ions are distributed in layers (bc planes), and these layers are joined by PO 4 tetrahedra. Figure 1 shows two unit cells of the stable Co 3 P 2 O 8 structure, the projection of nine unit cells in the (001) plane and the details of the oxygens around Co1 (CN = 5) and Co2 (CN = 6), with an edge shared by both polyhedra (two oxygens, O1 and O2, shared between Co1 and Co2). Two Co1-to-Co2 distances-2.896 and 3.165 Å-are shorter than the other Co-to-Co distances (ICSD-38259) [7]. The structure was drawn with the FPStudio program [9][10][11]. In Co2−xZnxSiO4 solid solutions comprising Co2SiO4 and Zn2SiO4 compounds with olivine and willemite structures, the intense blue colour is kept with a considerably lower amount of cobalt [15]. In the same way, the colour of some compositions of solid solutions with the structure of cobalt orthophosphate with a small amount of cobalt could also be similar to the colour of the Co3P2O8 compound. It was expected that the change in the coordination number of the Co(II) ion would modify the colouration of the material with respect to that obtained with the olivine and willemite structures. Cobalt orthophosphate is soluble in Mg 3 (PO 4 ) 2 at about 1100 K (827 • C) over the whole range of compositions [12]. Information about (Co, Mg) 3 (PO 4 ) 2 solid solutions with the α-Mg 3 P 2 O 8 structure prepared from mixtures of Co 3 P 2 O 8 and Mg 3 P 2 O 8 orthophosphates and fired at 800 • C can be found in the bibliography [13]. The catalytic behaviour of Mg 3−x Co x (PO 4 ) 2 solid solutions (compositions fired at 500-700 • C) shows that the substitution of magnesium with cobalt in the Mg 3 (PO 4 ) 2 structure leads to active and selective phases in the oxidative dehydrogenation of ethane and propane [14]. It seems possible to increase the thermal stability of Co 3 P 2 O 8 through the formation of these solid solutions with the substitution of Co(II) ions by Mg(II) ions. As far as we know, no information about Mg x Co 3−x P 2 O 8 solid solutions at T ≥ 1000 • C has been reported.
In Co 2−x Zn x SiO 4 solid solutions comprising Co 2 SiO 4 and Zn 2 SiO 4 compounds with olivine and willemite structures, the intense blue colour is kept with a considerably lower amount of cobalt [15]. In the same way, the colour of some compositions of solid solutions with the structure of cobalt orthophosphate with a small amount of cobalt could also be similar to the colour of the Co 3 P 2 O 8 compound. It was expected that the change in the coordination number of the Co(II) ion would modify the colouration of the material with respect to that obtained with the olivine and willemite structures.
The aim of this study was the formation of solid solutions from compositions comprising Co 3 P 2 O 8 and Mg 3 P 2 O 8 in order to obtain information about the composition and temperature with which the desired colour is developed and to minimise the toxic and expensive amounts of cobalt.  PO 4 in water were added to 100 mL of water. Samples were vigorously stirred for 20 h at room temperature. Then, an aqueous ammonia solution (Panreac, 25%) was added under continuous stirring until reaching pH = 10. The experimental parameters were chosen in order to obtain precipitates of the cations before drying the material. pH = 10 was chosen because, although Co(OH) 2 precipitates at pH > 7, Mg(OH) 2 precipitates at pH > 9.5. Under these conditions, the materials were coprecipitated and dried in a stove at 65 • C to evacuate only the water. The Mg:Co:P molar ratio of the starting materials was preserved in this process. The dry samples were fired at 300, 600, 800, 1000 and 1200 • C for 6 h at each temperature.

Experimental Methods
The development of the crystalline phases at different temperatures was studied by using XRD. The resulting materials were examined using a Panalytical X-ray diffractometer (Malvern Panalytical, Almelo, The Netherlands) with CuK α radiation. A structure profile refinement was carried out using the Rietveld method (Fullprof.2k computer program) [9][10][11]. Diffraction patterns ranging between 6 and 110 (2θ) were collected by employing monochromatic CuK α radiation, a step size of 0.02 (2θ) and a sampling time of 10 s. The unit cell parameters, interatomic distances and Co(II) ion occupation in the two M1 and M2 sites in the stable Co 3 P 2 O 8 structure were determined in order to investigate the possible formation of solid solutions under these synthesis conditions. The initial structural information was taken from the Inorganic Crystal Structure Database [1].
The Co(II) ion sites and the transfer charge bands in the samples were studied by using UV-vis-NIR spectroscopy (diffuse reflectance). The ultraviolet-visible-near-infrared (UV-vis-NIR) spectra in the range of 200 to 2500 nm were obtained using a Jasco V-670 spectrophotometer and BaSO 4 as reference substance.
The CIEL*a*b* colour parameters for the fired samples-L* is the lightness axis (black (0) → white (100)), a* is the green (−) → red (+) axis and b* is the blue (−) → yellow (+) axis [16]-were obtained with an X-Rite spectrophotometer (SP60, standard illuminant: D65, an observer of 10 • , and a reference sample of MgO). To test their possible utility in the ceramic industry, the compositions fired at 1200 • C were enamelled at 3% weight with a commercial glaze (SiO 2 -Al 2 O 3 -PbO-Na 2 O-CaO glaze) onto commercial ceramic biscuits. Many pigments were dissolved in this glaze. The colour of the material was lost when this occurred. Glazed tiles were fired for 15 min at 1065 • C, and subsequently, their UV-vis-NIR spectra and their CIEL*a*b* colour parameters were obtained. Table 1 shows the evolution of the crystalline phases in the Mg x Co 3−x P 2 O 8 (0.0 ≤ x ≤ 3.0) compositions according to composition and temperature. A stable Co 3 P 2 O 8 structure was developed at 800 • C, although small amounts of Mg 2 P 2 O 7 were also detected when x ≥ 2.0 at this temperature. This crystalline phase was the only crystalline phase detected in all of the compositions at 1000 • C and when x ≥ 1.5 at 1200 • C. Compositions with x < 1.5 melted at 1200 • C, and they could not be removed from the crucible.

Results and Discussion
Crystalline phases: C = stable Co 3 P 2 O 8 , Mg 3 P 2 O 8 , or solid solutions with the same structure; M = Mg 2 P 2 O 7 . Diffraction peak intensity: s = strong, vw = very weak.
values were refined, and the results appear in Figure 5 as E1 (experimental occupation in the M1 position) and E2 (experimental occupation in the M2 position). The experimental distribution of Co(II) between the two positions indicates that the Co(II) ion presents a higher preference for the site with C.N. = 5 (4e) than the Mg(II) ion at 800, 1000 and 1200 °C. The octahedral positions (2a) were mostly occupied by Mg(II) ions. This result is in agreement with the literature on compositions at 800 °C [12,13].  Figure 6 shows the UV-vis-NIR spectra of MgxCo3−xP2O8 fired at 800, 1000 and 1200 °C. The three bands at 1100, 580 and 500 nm from the fired MgxCo3−xP2O8 composition were assigned to Co(II) in the octahedral site with Δ/B < 13. These bands could be assigned to the first 4 T1  4 T2(F) transition, to the second 4 T1  4 A2(F) transition and to the third 4 T1  4 T1(P) transition [18]. The bands at 1700-1717, 890 and 480 nm were assigned to Co(II) in a square-planar pyramid coordination ( 4 A2  4 A1(F), 4 A2  4 E(F) and 4 A2  4 E(P) transitions) according to the stable Co3P2O8 solid solutions detected by XRD. In the spectra at 1000 • C, the absorbance at 1100 nm (the 4 T 1 → 4 T 2 (F) transition was assigned to Co(II) in the octahedral site) was slightly smaller than at 890 nm (the 4 A 2 → 4 E(F) transition was assigned to Co(II) in a square-planar pyramid coordination) when x < 1.0 (small amount of Mg(II) in the compositions), although the most noticeable change was the decrease in absorbance with the composition due to the decrease in the total Co(II) in samples.
The slight changes in the Co-O distances with composition ( Figure 4) changed the ligand field strength, and a gradation of purple to violet colour was obtained (Table 5). Purple and violet colours were obtained when the stable Co 3 P 2 O 8 structure was developed from these compositions. These colourations were kept at 1000 and 1200 • C when x ≥ 1.5, so pigments with a smaller amount of Co(II) were obtained.
According to the CIE L* a* b* parameters (Table 5), an increase in the amount of red colour could be detected starting at 600 • C with a* > 20 when 1.0 ≤ x ≤ 2.5 at 1000 • C. Figure 7 shows the variations in a* and b* with composition (x) at T ≥ 800 • C. All of the compositions with x = 3.0 were violet or purple with a positive a* (red amount) and negative b* (blue amount). The composition with x = 1.5 (Mg 1.5 Co 1.5 P 2 O 8 ) showed the greatest amounts of red and blue at 1000 • C, and the composition with x = 2.5 (Mg 2.5 Co 0.5 P 2 O 8 ) did so at 1200 • C. The positions of the third transition band of the octahedral Co(II) ion, 4 T 1 → 4 T 1 (P), and the 4 A 2 → 4 E(P) band of the Co(II) ion in a square-planar pyramid coordination could be related to the variation of the amount of red colour (positive a*). The amount of blue colour (negative b*) could be related to the second transition band of the octahedral Co(II) ion, 4 T 1 → 4 A 2 . The greatest amounts of red colour (positive a*) and blue colour (negative b*) were obtained when the totality of Co(II) was almost entirely in the pentacoordinated site (x > 1.0), with a great distortion at 1200 • C ( Table 3). The optimal compositions could be established when 2.0 ≤ x ≤ 2.5 (highest a* and lowest b*) at this temperature. In the spectra at 1000 °C, the absorbance at 1100 nm (the 4 T1  4 T2(F) transition was assigned to Co(II) in the octahedral site) was slightly smaller than at 890 nm (the 4 A2  4 E(F) transition was assigned to Co(II) in a square-planar pyramid coordination) when x < 1.0 (small amount of Mg(II) in the compositions), although the most noticeable change was the decrease in absorbance with the composition due to the decrease in the total Co(II) in samples.  site (x > 1.0), with a great distortion at 1200 °C ( Table 3). The optimal compositions could be established when 2.0 ≤ x ≤ 2.5 (highest a* and lowest b*) at this temperature.  (Table 5)-were, in absolute value, greater than those of the CoxZn2−xSiO4 compositions (1.5 ≤ x ≤ 2.0 with the olivine structure and 0.05 ≤ x ≤ 1.00 with the willemite structure), which had a* of -9.3 to 4.4 and b* of −1.8 to −20.3 [15], and all of them were fired at 1200 °C. A greater amount of red  (Table 5)-were, in absolute value, greater than those of the Co x Zn 2−x SiO 4 compositions (1.5 ≤ x ≤ 2.0 with the olivine structure and 0.05 ≤ x ≤ 1.00 with the willemite structure), which had a* of -9.3 to 4.4 and b* of −1.8 to −20.3 [15], and all of them were fired at 1200 • C. A greater amount of red colour (+a*) that was comparable with the greater amount of blue colour (-b*) was obtained in the solid solutions with the stable Co 3 P 2 O 8 structure. So, the powdered Mg 2.5 Co 0.5 P 2 O 8 composition fired at 1200 • C considerably reduced the amount of cobalt, keeping a colour comparable with that in the Co 3 P 2 O 8 pigment, and its melting point was higher than 1200 • C. This composition could be used as a violet inorganic pigment in substitution for the Co 3 P 2 O 8 inorganic pigment, thus decreasing its toxicity due to large amount of cobalt. Co 3 P 2 O 8 is classified as a pigment (DCMA-8-11-1, DCMA Classification of Mixed Metal Oxide Inorganic Colour Pigments). Pigments include naturally occurring substances prepared from minerals or their combustion products, as well as synthetic compounds produced from appropriate raw materials. Pigments are insoluble in the surrounding media, and their optical effect arises from selective light absorption [19]. Therefore, the solid solutions prepared here are also inorganic pigments when unglazed. Figure 8 shows the visible spectra in glazed tiles prepared with 3% Mg x Co 3−x P 2 O 8 (1.5 ≤ x ≤ 3.0) materials fired at 1200 • C. The bands assigned to Co(II) in the octahedral site and to Co(II) in a square-planar pyramid coordination were detected at higher wavelengths in the enamelled samples than in the powdered samples. This displacement increased the absorbance in the range of 593-650 nm and slightly decreased the absorbance at about 550 nm, so the colour observed in these enamelled materials was the characteristic cobalt blue colour obtained from purple powdered materials (1.5 ≤ x ≤ 2.5). The absorbance between 450 and 630 nm decreased in the enamelled Co 3 P 2 O 8 composition with respect to the powdered Co 3 P 2 O 8 at 1000 • C. This decrease was not detected in the Mg 2.0 Co 1.0 P 2 O 8 and Mg 2.5 Co 0.5 P 2 O 8 compositions that were fired at 1200 • C with their lower amount of cobalt. The violet colour of the powdered samples changed to the characteristic cobalt blue due to the change in the local environment of the Co(II) ions, which could be visualised in the UVV spectra of the glazed samples with the displacement of the bands towards higher wavelengths and with the appearance of a new band assigned to tetrahedral Co(II). This blue colour was also obtained with Co 2 SiO 4 , MgCoSiO 4 or Co 3 P 2 O 8 pigments containing a greater amount of cobalt. Mg2.5Co0.5P2O8 compositions that were fired at 1200 °C with their lower amount of cobalt. The violet colour of the powdered samples changed to the characteristic cobalt blue due to the change in the local environment of the Co(II) ions, which could be visualised in the UVV spectra of the glazed samples with the displacement of the bands towards higher wavelengths and with the appearance of a new band assigned to tetrahedral Co(II). This blue colour was also obtained with Co2SiO4, MgCoSiO4 or Co3P2O8 pigments containing a greater amount of cobalt. The colour parameters (L* a* b*) of the enamelled samples under the conditions of this study with the commercial glaze used are included in Table 6. A dark blue colour was obtained from the compositions with 1.5 ≤ x ≤ 2.0, and a blue colour with a greater lightness was obtained from the composition with x = 2.5. The MgxCo3−xP2O8 (1.5 ≤ x ≤ 2.5) solid solutions with the stable Co3P2O8 structure may be used as blue pigments in the ceramic industry. The CIE L*/a*/b* colour parameters of classical blue pigments used in the ceramic industry with Co2SiO4 or MgCoSiO4 compositions with an olivine structure (weight The colour parameters (L* a* b*) of the enamelled samples under the conditions of this study with the commercial glaze used are included in Table 6. A dark blue colour was obtained from the compositions with 1.5 ≤ x ≤ 2.0, and a blue colour with a greater lightness was obtained from the composition with x = 2.5. The Mg x Co 3−x P 2 O 8 (1.5 ≤ x ≤ 2.5) solid solutions with the stable Co 3 P 2 O 8 structure may be used as blue pigments in the ceramic industry. The CIE L*/a*/b* colour parameters of classical blue pigments used in the ceramic industry with Co 2 SiO 4 or MgCoSiO 4 compositions with an olivine structure (weight ratio of pigment to glaze equal to 1:5, 20 weight% pigment) were 29.00/11.20/-25. 6 and 29.19/7.97/-17.61, respectively, for the single-fired enamelled samples [20,21]. The glazed tiles from the Mg x Co 3−x P 2 O 8 (1.5 ≤ x ≤ 2.5) solid solutions (including 3% pigment) showed blue colourations with a large amount of blue colour (-15.9 ≤ b* ≤ -20.20) and a low lightness (18.53 ≤ L* ≤ 27.05). The amount of cobalt in the compositions was between 28.1 weight% (x = 1.5) and 10.5 weight% (x = 2.5), while it was 56.1 weight% in Co 2 SiO 4 and 33.6 weight% in MgCoSiO 4 . The use of the Mg x Co 3−x P 2 O 8 (1.5 ≤ x ≤ 2.5) solid solutions with the stable Co 3 P 2 O 8 structure as blue pigments reduced the amount of cobalt used with respect to the amount of cobalt used in Co 2 SiO 4 and MgCoSiO 4 pigments because comparable values of blue (negative b*) were obtained with a smaller amount of cobalt in the composition of the pigment and with a pigment quantity that was lower by 6.7. Table 6. CIE L* a* b* colour parameters from the glazed tiles obtained from the Mg x Co 3−x P 2 O 8 (1.5 ≤ x ≤ 3.0) materials fired at 1200 • C.
x L* a* b* Observed Colour

Conclusions
Mg x Co 3−x P 2 O 8 (0 ≤ x ≤ 3) solid solutions with the stable Co 3 P 2 O 8 structure were synthesised via the chemical coprecipitation method. Their structural characterisation is consistent with the replacement of the Co(II) ion with the smaller Mg(II) ion. A decrease in the b unit cell parameter and the unit cell volume with x was obtained. The slight increase in the a and c unit cell parameters with x indicates that the Co 3 P 2 O 8 structure is distorted when Mg(II) is incorporated into it. When x > 1.0, the decrease in the longest M-O distance with x is remarkable.
The experimental distribution of Co(II) between the two positions in the solid solutions with the stable Co 3 P 2 O 8 structure indicates that the Co(II) ion presents a preference for the site with CN = 5 (4e) and the Mg(II) ion presents a preference for the octahedral position