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Environmental-friendly inorganic green pigments based on (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+) solid solutions were synthesized and the dopant element was optimized to give the most greenish color. Among the samples, it was found that Lu-doped sample showed brilliant green hue. In response to the results, (Y1−xLux)2BaCuO5 (0 ≤ x ≤ 0.3) pigments were further synthesized and their color properties were characterized in an attempt to find a new environmentally friendly pigment that has more sufficient green chromaticity. Among the samples, the most vivid green hue was obtained for (Y0.9Lu0.1)2BaCuO5 with a greenness value (−a*) of 48.6 in the CIE L*a*b* system, which was significantly larger than those of commercially available Cr2O3 (−a* = 18.2) and CoO∙ZnO (−a* = 25.3) pigments. The Y2BaCuO5 and (Y0.9Lu0.1)2BaCuO5 pigments were used in paints for porcelain. The green colors of the overglazed decoration panels were brilliant. Therefore, these pigments could be the potential candidates for novel green pigments of over-glazed decoration of Arita ware. Key-words: Y2BaCuO5, Environment-friendly, Green pigment, Solid solution, Arita ware Original Research Paper J. Jpn. Soc. Colour Mater., 88〔7〕,203–207(2015) 203 © 2015 Journal of the Japan Society of Colour Material -1the XRD peak angles, which were refined using α-Al2O3 as a standard. Particle size and morphology were examined by field-emission scanning electron microscopy (FE-SEM; JEOL, JSM-6700FSS). The optical reflectance was measured using a UV-vis spectrometer (Shimadzu, UV-2600) with barium sulfate as a reference. The bandgap energies of the samples were determined from the absorption edge of the absorbance spectra represented by the Kubelka-Munk function, f (R) = (1−R)2/2R, where R is reflectance7). The color properties of the green pellets of the samples were estimated in terms of the CIE L*a*b*CH° system with a colorimeter (Konica-Minolta, CR-400). The parameter L* represents the brightness or darkness of a color relative to a neutral grey scale, while the parameters a* (the redgreen axis) and b* (the yellow-blue axis) qualitatively express the color. Since the a* value in the negative direction corresponds to the green component, it is desirable for a* to be small (for −a* to be large) as possible on the development of green pigments. The parameter C (chroma) represents saturation of the color and H° represents the hue angle. The chroma is defined as C = {(a*)2 + (b*)2}1/2. The hue angle, H°, is expressed in degrees and ranges from 0 to 360° and is calculated using the formula H° = tan−1(b*/a*). For pure green, H° = 180°. 2.3 Evaluation of overglazing pigment for Arita ware The Y2BaCuO5 or (Y0.9Lu0.1)2BaCuO5 pigment was added to a commercially available lead-free frit. Pigment concentrations of 5 and 7 wt% were used for painting. The mixture was mechanically mixed using a mortar grinder for 10 min to prepare overglazed enamels. The overglazed decoration pigment was painted on porcelain panels coated with limestone glazes and the panels were heated in an electric furnace at 1073 K for 30 min at a heating rate of 100 K h−1. The final glazes were evaluated in terms of their color hue by the CIE L*a*b*CH° system with a colorimeter (Nippon Denshoku, SQ-2000). The acid resistance of the Y2BaCuO5 pigment was evaluated using a small dish (diameter: 8 cm; capacity: 30 cm3). A 4-cm-square was painted or drawn inside the dish using the overglazed enamel and the dish was heated at 1073 K for 30 min. 4% acetic acid solution was then poured into the dish. After leaving it at 296 K for 24 h, the Cu and Y concentrations in the 4% acetic acid solution were measured by inductively-coupled plasma spectroscopy (ICP; Shimadzu, ICPS-8100). 3.Results and Discussion 3.1 Y2BaCuO5 and (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+) Fig. 1 shows XRD patterns of the Y2BaCuO5 and (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+) samples. A single-phase orthorhombic structure was observed for all samples, and no diffraction peaks of impurities were evident in the patterns. The XRD peaks shifted to lower angle direction when Sm3+ or Gd3+ was doped into the Y3+ site, because the ionic radii of Sm3+ (0.0958 nm)8) and Gd3+ (0.0938 nm)8) are larger than that of Y3+ (0.0900 nm)8). On the other hand, the peaks shifted to higher angle direction in the cases of Yb3+ and Lu3+ doping, due to smaller ionic radii of Yb3+ (0.0868 nm)8) and Lu3+ (0.0861 nm)8). The orthorhombic lattice volumes of the Y2BaCuO5 and (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+) samples calculated from the diffraction angles in the XRD patterns are summarized in Table 1. The lattice volume of the orthorhombic phase depends on the ionic radius of the doped rare earth ion, indicating the formation of solid solutions. Fig. 2 depicts UV-vis diffuse reflectance spectra for the Y2BaCuO5 and (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+) samples. All samples exhibited strong optical absorption at wavelengths shorter than 400 nm and longer than 600 nm, while strong reflection was recognized between 400 to 600 nm including the green region (500 ‒ 560 nm). As a result, the samples are green. The optical absorption shorter than 400 nm is attributed to the charge transfer transition from O2− to Cu2+, while that longer than 600 nm corresponds to the d-d transitions of Cu2+ 9-12). Fig. 1 XRD patterns of Y2BaCuO5 and (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+). Table 1 Lattice volumes of Y2BaCuO5 and (Y0.9R0.1)2BaCuO5 (R = Sm3+, Gd3+, Yb3+, and Lu3+). Sample Lattice volume / nm3 Y2BaCuO5 0.4912 (Y0.9Sm0.1)2BaCuO5 0.4939 (Y0.9Gd0.1)2BaCuO5 0.4924 (Y0.9Yb0.1)2BaCuO5 0.4905 (Y0.9Lu0.1)2BaCuO5 0.4896 Original Research Paper 204 J. Jpn. Soc. Colour Mater., 88〔7〕(2015)

Novel Environment-friendly Green Pigments for Over-glazed Decoration of <i>Arita</i> Ware