Synthesis and characterisation of SrY 2 (cid:1) x Ce x O 4 as environmentally friendly reddish-brown pigments

Reddish-brown SrY 2 (cid:1) x Ce x O 4 (0 # x # 1.2) solid solutions were synthesized by a citrate sol – gel method as novel environmentally friendly inorganic pigments. The powders obtained were characterized by X-ray powder di ﬀ raction (XRD), UV-vis di ﬀ use re ﬂ ectance spectra and CIE L * a * b * Ch (cid:3) chromatic coordinate measurements. All SrY 2 (cid:1) x Ce x O 4 (0 # x # 1.2) samples were obtained in a single-phase form and the lattice volume increased on increasing the Ce 3+ concentration. The reddish-brown pigments exhibited optical absorption due to the 4f – 5d allowed transition of Ce 3+ . The absorption bands observed in the wavelength region of 400 and 550 nm were due to the Ce 3+ ions in the ideal octahedral Y 3+ site and those in the longer wavelength region above 600 nm were attributed to the transition of Ce 3+ in the distorted octahedral Y 3+ site. The samples gradually became reddish on increasing the Ce 3+ content. The most reddish colour was obtained in SrYCeO 4 ( a * ¼ +21.8).


Introduction
Inorganic pigments are typically applied to ceramic tiles, inks and paints, due to their high hiding power, weather resistance and thermal stability.However, the use of the conventional pigments containing toxic elements, such as Cd, Pb, Hg, Cr, Co, and Sb, has been forbidden or restricted, because they have adverse effects on the human body and the environment.][20][21][22][23][24][25][26][27] Among them, it has been reported that Ca 1Àx La x TaO 2Àx N 1+x and Ce 2 S 3 showed bright red colour. 18,19But, unfortunately, harmful NO x and SO x are generated when these nitride and sulphide pigments are incinerated.Therefore, oxides are desirable as materials for environmentally friendly pigments.
Because of this situation, we focused on a trivalent cerium (Ce 3+ ) ion as a red colouring source.As an example of a popular Ce 3+ -containing material, Ce 3+ -doped yttrium aluminium garnet, (Y, Ce) 3 Al 5 O 12 (YAG:Ce 3+ ), has been well known as a yellow-emitting phosphor widely used in white light emitting diodes.YAG:Ce 3+ absorbs the visible lights in the wavelength region of 410 to 500 nm, [28][29][30] which is attributed to the 4f-5d allowed transition.The absorption wavelength due to the Ce 3+ ions depends on the host crystal structure, because the energy level of the 5d orbital of Ce 3+ is strongly affected by the crystal eld strength around the Ce 3+ ions.In the case of a phosphor, the amount of Ce 3+ is about 1 mol% to prevent concentration quenching, but it is considered that colouring of the sample can be seen by further increasing the Ce 3+ concentration.Furthermore, it is expected that strong reddish colour will be obtained if Ce 3+ is doped at a high concentration in the Y 3+ site in the lattice with a stronger crystal eld.
In this study, we selected SrY 2 O 4 as a host material, because this compound is composed of non-toxic elements.SrY 2 O 4 belongs to the CaFe 2 O 4 -related structure, and it crystallizes into an orthogonal structure with space group Pnma.Four formula units for a total of 28 atoms are contained in the SrY 2 O 4 structure.All of the constituent atoms occupy 4c sites according to the Wyckoff notation. 31The Sr 2+ and Y 3+ ions are coordinated by eight and six O 2À ions, respectively.Y 3+ occupies two nonequivalent sites C s symmetry, where one Y(1) site is nearly a regular octahedron but the other Y(2) one is much distorted. 32ince it has been reported that high calcination temperature is necessary for the synthesis of SrY 2 O 4 by a solid-state reaction, 33,34 SrY 2Àx Ce x O 4 (0 # x # 1.2) pigments were synthesized using a citrate sol-gel method.The optical and colour properties of the samples were evaluated as novel environmentally friendly inorganic reddish-brown pigments.

Materials and methods
The SrY 2Àx Ce x O 4 (0 # x # 1.2) pigments were synthesized using a citrate sol-gel method.Sr(NO 3 ) 2 (Wako Pure Chemical Industries Ltd., 99.9%), Y(NO 3 ) 3 $6H 2 O (Kishida Chemical Co. Ltd., 99.9%) and Ce(NO 3 ) 3 $6H 2 O (Kishida Chemical Co. Ltd., 98.0%) were weighed so as to obtain the objective compositions and dissolved in deionized water to adjust the Sr and (Y + Ce) concentrations to 0.3 and 0.6 mol L À1 , respectively.Aer the solution was stirred homogeneously, citric acid was added as a chelating agent to complex the cations into the solution in the mole ratio 2 : 1 with respect to the total cations (Sr, Y and Ce).The mixed solution was stirred at 80 C until a gel was obtained, and then, the gel was dried at 120 C for 24 h in an oven.The dried gel was calcined in an aluminium silicate (mullite) crucible at 500 C for 6 h in air.Aer the calcination, the sample was heated again at 1300 C for 6 h in a ow of 5% H 2 -95% N 2 gas.Before characterisation, the sample was ground in an agate mortar.

Characterisation
The samples synthesized were characterised by X-ray powder diffraction (XRD; Rigaku, Ultima IV) with Cu-Ka radiation, operated with voltage and current settings of 40 kV and 40 mA, respectively.The sampling width and the scan speed were 0.02 and 6 min À1 .The lattice parameters and volumes were calculated from the XRD peak angles, which were rened using a-Al 2 O 3 as a standard and using the CellCalc Ver.2.20 soware.Rietveld renement of the obtained XRD patterns was performed using the RIETAN-FP soware package to determine the precise crystal structure and the occupancy of the Y(1) and the Y(2) sites for the SrY 2Àx CeO 4 (x ¼ 0, 0.2, 1.0) samples. 35From the Rietveld renement, the following nal R-factors were obtained: R wp (R-weighted pattern), R p (R-pattern), R e (R-expected), S (goodness-of-t indicator), and R F (R-structure factor).
The morphology of the SrYCeO 4 particles was investigated by using eld-emission-type scanning electron microscopy (FE-SEM; JEOL, JSM-6701F).The optical reectance spectra were measured with a UV-vis spectrometer (Shimadzu, UV-2550 with an integrating sphere attachment) with barium sulphate as a reference.The colour properties of the samples were evaluated in terms of the CIE L*a*b*Ch system using a chromometer (Konika-Minolta, CR-300).The L* parameter indicates the brightness or darkness of a colour relative to a neutral grey scale, and the a* (the red-green axis) and b* (the yellow-blue axis) parameters express the colour qualitatively.Chroma parameter (C) represents the colour saturation of the pigments and is calculated according to the following formula: The parameter h ranges from 0 to 360 , and is calculated with the formula, h ¼ tan À1 (b*/a*).X-ray photoelectron spectra measurements (XPS; ULVAC-PHI, PHI5000 VersaProbe II) using Mg-Ka radiation were carried out to investigate the oxidation state of the cerium ion on the surface of the as-synthesized and the calcined SrYCeO 4 samples.

X-ray powder diffraction and SEM image
Fig. 1 shows the XRD patterns of the synthesized SrY 2Àx Ce x O 4 (0 # x # 1.2) pigments.All SrY 2Àx Ce x O 4 (0 # x # 1.2) samples were obtained in a single-phase form, and no diffraction peaks of impurities or other phases were observed in the patterns.The XRD peaks shied to lower angle direction with increasing the Ce 3+ .The lattice volumes of all samples were calculated from the XRD peak angles, and the results are summarized in Table 1.The cell volume increased with increasing the Ce 3+ concentration.These results indicate that Y 3+ (ionic radius: 0.104 nm) 36 ions in the host lattice were partially substituted by larger Ce 3+ (0.115 nm) 36 ions to form solid solutions.
The Rietveld analysis of the XRD data of the SrY 2Àx CeO 4 (x ¼ 0, 0.2, and 1.0) samples was carried out to determine the site occupancy of the Y(1) and the Y(2) sites.The Rietveld renement proles of the samples are shown in Fig. 2, and the detailed crystallographic data and structure renement parameters are summarized in Tables 2 and 3, respectively.sites affects the Ce 3+ occupancy.In fact, the occupancy ratio, Ce2/Ce1, was 0.25 for SrY 1.8 Ce 0.2 O 4 (x ¼ 0.2), while it was 0.56 for SrYCeO 4 (x ¼ 1).These results indicate that the Ce 3+ ions were preferentially located in the energetically favoured ideal octahedral Y(1) site when the Ce 3+ concentration was low, and suggest that the distorted Y(2) site were also begun to be occupied when the Ce 3+ concentration was increased and the solubility in the Y(1) sites were saturated.Fig. 4 shows the FE-SEM images of the SrYCeO 4 (x ¼ 1) sample at different magnications.Since it was synthesized at a high temperature of 1300 C, the primary particles melted to form large secondary particles.

Reectance spectra
The UV-vis reectance and absorption spectra of SrY 2Àx Ce x O 4 (0 # x # 1.2) are depicted in Fig. 5.The absorbance spectra were represented by the Kubelka-Munk function, f(R) ¼ (1 À R) 2 /2R, where R is reectance. 38The non-doped SrY 2 O 4 sample showed high reectance in the visible light region of 400 to 750 nm.30]39 As the concentration of Ce 3+ increased, the absorption due to the 4f-5d transition appeared more intensely.In addition, the reectance at 600 nm and longer wavelengths also decreased when the Ce 3+ concentration became high.splitting of Ce 3+ in the Y(2) site is also larger than that in the Y(1) site.As already discussed, the Ce 3+ preferentially occupies the ideal Y(1) in the low Ce 3+ concentration sample, and the Y(2) occupancy was increased with increasing the Ce 3+ concentration.
Accordingly, the optical absorption at 600 nm and longer wavelengths was observed in the samples with high Ce 3+ concentration.

Chromatic properties
The chromatic parameters of the synthesized SrY 2Àx Ce x O 4 (0 # x # 1.2) pigments are summarized in Table 4.The photographs of these pigments are also displayed in Fig. 7.The L* values increased as the amount of Ce 3+ decreased.The a* and b* values increased in a positive direction.As already discussed above regarding the results in Fig. 3, these relationships can be attributed to the difference of coordination environment around the Ce 3+ ions.When the Ce 3+ concentration is relatively low, the Ce 3+ ions are preferentially located in the ideal octahedral Y(1) site.Since the crystal eld of Ce 3+ in the Y(1) site is relative small, the 4f-5d allowed transition of Ce 3+ in this site is observed at the wavelengths in the region of violet to blue green (400-550 nm).As a result, the samples are yellow, which is complementary colour of blue.On the other hand, the samples containing Ce 3+ at high concentrations additionally absorbed light at wavelengths of 600 nm and above, corresponding to the 4f-5d allowed transition of Ce 3+ in the distorted octahedral Y(2) site.Accordingly, the samples gradually became reddish with increasing the Ce 3+ content.Among the SrY 2Àx Ce x O 4 samples synthesized in this study, SrYCeO 4 is the most reddish (a* ¼ +21.8).

Thermal and chemical stability tests
The thermal and chemical stabilities of the SrYCeO 4 pigment were evaluated using the powder sample.To evaluate the thermal stability, this sample was heated in a mullite crucible at 300 C and 500 C for 3 h under an air atmosphere and cooled to room temperature.The acid/base resistance of the SrYCeO 4 pigment was tested in 4% acetic acid and 4% ammonium bicarbonate solutions, and the pigment was dispersed into the acid/base solutions.Aer leaving them at room temperature for    1 h, the sample was washed with deionized water and ethanol, and then dried at room temperature.The colour of the samples aer the thermal and chemical stability tests were evaluated using the colorimeter.The colour coordinate data are summarized in Table 5.Unfortunately, the heat resistance of this sample was low, and the colour degradation was observed aer heating the present SrYCeO 4 pigment at 300 C and above in air.On the other hand, the SrYCeO 4 pigment has chemical stability.The colour was almost unchanged aer the leaching test in the acetic acid and ammonium bicarbonate solutions.
In order to investigate the reason for the color degradation aer the heating in air, oxidation state of the cerium ions of the SrYCeO 4 samples was identied by the XPS measurement before and aer the heat resistance tests.The Ce (3d 3/2 ) and Ce (3d 5/2 ) XPS obtained from the SrYCeO 4 sample before and aer the heat resistance tests are shown in Fig. 8.In addition to the binding energy peaks for Ce 3+ at 884.2 (V 0 ) and 902.4 (U 0 ) eV, four peaks corresponding to Ce 4+ species on the surface of the non-treatment sample were observed at 881.1 (V), 897.4 (V 000 ), 899.8 (U) and 914.5 (U 000 ) eV. 40,41 The labels U and V refer to the Ce (3d 3/2 ) and Ce (3d 5/2 ) spin-orbit components.The intensities of two peaks assigned to Ce 3+ decreased with increasing the calcination temperature, in comparison with those of the assynthesized sample.Therefore, the colour degradation will be caused by the oxidation of Ce 3+ to Ce 4+ .

Conclusions
SrY 2Àx Ce x O 4 (0 # x # 1.2) were synthesized using a citrate solgel method as environmentally friendly inorganic reddishbrown pigments.The samples exhibited optical absorption due to the 4f-5d allowed transition of Ce 3+ at wavelengths from 410 to 500 nm and at 600 nm and longer.The former is attributed to the 4f-5d transition of Ce 3+ in the ideal octahedral Y(1) site, and the latter is due to that in the distorted octahedral Y(2) site.Since the Ce 3+ ions were preferentially dissolved into the energetically favoured Y(1) site, the colour of the samples gradually changed from yellow to reddish brown with increasing the Ce 3+ concentration.The most reddish colour was observed for SrYCeO 4 (a* ¼ +21.8).Since this compound is consisted with non-toxic elements, it is expected to be an environmentally friendly inorganic reddish-brown pigment.

Conflicts of interest
There are no conicts to declare.

Fig. 3
Fig.3shows the crystal structure of SrY 2 O 4 illustrated using the VESTA program based on the crystallographic data from the Rietveld renement.37As seen in Table2, the low R-factors were obtained for all the SrY 2Àx Ce x O 4 (x ¼ 0, 0.2, and 1.0) samples.The Rietveld renements revealed that the Ce 3+ concentrations at the Y(1) site gradually increased from 16 to 64 mol%, while that in the Y(2) site increased from 4 to 36 mol% as x increased from 0.2 to 1, as seen in Table3.Therefore, in the SrY 2Àx Ce x O 4 structure, Ce 3+ ions occupied both Y(1) and Y(2) sites.Although each Y site is coordinated by six oxide anions, one Y(1) site is located in the ideal octahedral coordination environment and the other Y(2) site is signicantly distorted, as shown in Fig.3.This difference of structural distortion of two non-equivalent Y

Fig. 2
Fig. 2 Observed (red + symbols), calculated (solid line) and difference blue line patterns for the Rietveld refinement from the X-ray powder diffraction data of the synthesized SrY 2 O 4 (a), SrY 1.8 Ce 0.2 O 4 (b) and SrYCeO 4 (c).
These results are considered to be due to the existence of two non-equivalent octahedral Y sites of different coordination environments in the crystal structure of SrY 2 O 4 .As mentioned above, Y(1) site is located in the ideal octahedral coordination environment and the other Y(2) site is signicantly distorted, 31 as illustrated in Fig. 3.The band structure models of the ideal Y(1) and the distorted Y(2) sites in the SrY 2Àx Ce x O 4 (0 # x # 1.2) samples are illustrated schematically in Fig. 6.The valence band (VB) and the conduction band (CB) consist of O 2p and Y 3d orbitals, respectively.When the Ce 3+ ions are doped into the SrY 2 O 4 lattice, the 4f and 5d energy levels of Ce 3+ are introduced between VB (O 2p orbital) and CB (Y 3d orbital).Since the crystal eld energy around the Ce 3+ ions in the distorted Y(2) site is stronger than that in the ideal Y(1) site, the 5d orbital energy

Fig. 3
Fig. 3 Crystal structure obtained by the Rietveld analysis for SrY 2 O 4 (a), and the octahedral coordination environment of Y(1)O 6 and Y(2)O 6 in SrY 2 O 4 (b).

Table 3
Refined structural parameters of SrY 2Àx Ce x O 4 (x ¼ 0, 0.2, 1) pigments from Rietveld refinement using XRD data obtained at room temperature a SrY 2 O 4 (x ¼ 0) b 4 (x ¼ 1) c a All atoms are placed at general 4c positions.b At renement of SrY 2 O 4 , isotropic atomic displacement parameters (U iso ) of four oxygen atoms were constrained to be equal.c Because of the disordering of Y and Ce atoms, the fractional coordinate and U iso were constraint to the same values, respectively.In order to rene the occupation ratio of Ce atoms, U iso parameters of Sr, Y and O atoms were xed to the respective values of each atoms at non-doped SrY 2 O 4 .

Table 5
Colour coordinates of SrYCeO 4 before and after thermal and chemical stability tests Fig. 8 Ce (3d) XPS for SrYCeO 4 samples before and after heat resistance test.