Powder X–ray diffraction analysis.
XRD patterns of Ca0.1 Pr0.9 PO4
The phase formation and phase purity of the pigment Ca0.1 Pr0.9 PO4 were characterized and shown in Fig 3. PrPO4 crystallizes in monazite structure, in which the praseodymium ion is nine-coordinated by oxygen atoms with a distorted monocapped square antiprism geometry and phosphorus is four-coordinated by oxygen [22] Isolated irregular PO4 tetrahedra are linked by PrO9 polyhedra to form a network structure. It is possible to substitute praseodymium in PrPO4 by calcium to form a solid solution, as the ionic radii are similar. The powder XRD patterns show a monoclinic monazite structure and we have not observed any weak reflections due to α-Ca2P2O7 for any of the compositions, contrary to earlier observations [23]. It is probably difficult to detect α-Ca2P2O7 reflections by X-ray diffraction as the phase is monoclinic and contains ions of low atomic number. All the peaks are indexed on the basis of monoclinic unit cell with space group P21/n.
(b) XRD patterns of Ca0.1 Y0.9 PO4
The phase formation and phase purity of the pigment Ca0.1 Y0.9 PO4 were checked by powder X-ray diffraction. XRD pattern of calcium doped YPO4 is shown in Fig. 4. The diffraction peak of calcium doped YPO4 match well with the tetragonal phase of YPO4. The quantity of dopant ion into YPO4 structure slightly modifies the crystalline structure.
(c) XRD patterns of Ca0.1 RE0.9 PO4
The phase formation and phase purity of the pigment Ca0.1RE0.9 PO4 were characterized by powder X-ray diffraction. XRD pattern of calcium doped rare earth phosphate is shown in Fig.5. As the rare earth comprises the major elements like Ce, La, Nd, Pr and Sm, XRD pattern of Ca0.1RE0.9 PO4 is obtained with the presence of many phases like monazite monoclinic, hexagonal structures. But the major contributed structure may be monazite monoclinic due to the presence of 47.5% CePO4.
(d) XRD patterns of Ca0.1 Di0.9 PO4
The XRD pattern of Ca0.1 Di0.9 PO4 is shown in fig.6. These are also the same case as that of Ca0.1 Di0.9 PO4. But here the major contributed structure may be monazite monoclinic due to the presence of 45% LaPO4.
Optical properties: Diffuse reflectance and chromatic properties
(a) Ca doped PrPO4 or Ca0.1 Pr0.9 PO4
The effect of alkaline-earth metal ion Ca2+ doping on the optical properties of the pigments Ca0.1 Pr0.9 PO4 was analyzed from the diffuse reflectance spectra (DRS) and the results are shown in Fig 20. It is well known that Pr3+ has several metastable multiplets such as 3P0, 1, 2, 1D2, and 1G4 [24]. In the present study, we observe several bands in the visible region and these could be assigned to the electronic transitions between 4f 2 → 4f 1 5d1 states. There are bands around 445, 471, 485, 591 and 602 nm, which can be assigned based on the energy levels 3H4à 3P2, 3H4à3P1, 3H4à3P0, 3H4à3D2 (upper) and 3H4 à3D2 (lower), respectively [25]. The, the pigment exhibits a green color, since red is complementary color to green. Photograph of synthesized pigment of Ca doped Pr0.9PO4 or Ca0.1 Pr0.9 PO4 is shown in the below fig.7.
Ca doped YPO4 or Ca0.1 Y0.9 PO4
The diffuse reflectance spectra (DRS) of Ca-doped YPO4 is also shown in Fig. 7. A strong absorption noted below 465 nm in the UV–vis reflectance spectrum. This absorption in the blue region is responsible for the yellow color of Ca0.1 Y0.9 PO4, since blue is a complimentary color to yellow. The band gap of the pigment Ca0.1 Y0.9 PO4 is found to be 1.78 eV, which has been calculated by a straight forward extrapolation method from the corresponding absorption spectrum [26]. Picture of prepared pigment Ca doped Pr0.9PO4 or Ca0.1 Pr0.9 PO4 is shown in the below figure 23.
(c) Ca doped REPO4 or Ca0.1 RE0.9 PO4 (RE=Rare earth oxide)
The effect of alkaline-earth metal ion Ca2+ doping on the optical properties of the pigments Ca0.1 RE0.9 PO4 was analyzed from the diffuse reflectance spectra (DRS) and the results are shown in Fig 7. A strong absorption in the red region can be noted from the absorption spectrum of the pigment Ca0.1 RE0.9 PO4. Thus, the pigment exhibits dark green color, since red is complementary color to green. The band gap of the pigment Ca0.1 Y0.9 PO4 is found to be 2.26eV, which has been calculated by a straight forward extrapolation method from the corresponding absorption spectrum [26].
(d) Ca doped DiPO4 or Ca0.1 Di0.9 PO4 (Di = Didymium)
The diffuse reflectance spectra (DRS) of Ca-doped DiPO4 compositions and that of parent DiPO4 are shown in Fig. 7.A weak absorption in the blue region and a strong absorption in the red region can be noted from the absorption spectrum of the pigment Ca0.1 Di0.9 PO4. Thus, the pigment exhibits yellowish green color, since red is complementary color to green and blue is complementary color to yellow.
Color Coordinate Values
The chromatic properties of the synthesized Ca0.1 Ln0.9 PO4 (Ln = Y, Pr, RE, Di) powder pigments can be assessed from their CIE 1976 color coordinate values depicted in table 3. The L*, a* and b* parameters were computed from DRS for the parent and the Ca-doped composition Ca0.1 Pr0.9 PO4 and the results show that the brightness as well as green and yellow components are much better as compared to chromium oxide [27] and the values are presented in Table 1. The color coordinates of the typical Ca2+-doped pigment, Ca0.1 Y0.9 PO4 (L* = 86.7, a* = −9.2, b* = 54.7), especially yellow hue was found to be higher than that of the commercially available pigment (L* = 87.5, a* = −26.2, b* = 34.6) Zircon Yellow [24].
Table 1. The color coordinates (±0.1) of the Ca0.1 Ln0.9 PO4 (Ln = Y , Pr , RE , Di ) pigments and band gap values.
Pigment composition
|
Color coordinates
|
Band gap (eV)
|
L*
|
a*
|
b*
|
C*
|
hº
|
Ca0.1 Y0.9 PO4
|
86.7
|
–9.2
|
54.7
|
55.5
|
80.5
|
1.78
|
Ca0.1 Pr0.9 PO4
|
87 .5
|
-26.2
|
34.6
|
45.6
|
118.4
|
2.43
|
Ca0.1 RE0.9 PO4
|
67.2
|
-22.3
|
13.8
|
39.6
|
98.2
|
2.26
|
Ca0.1 Di0.9 PO4
|
87.0
|
–10.9
|
52.0
|
53.1
|
78.2
|
2.38
|
Cr2O3[27]
|
53.58
|
-16.21
|
14.48
|
|
|
|
Zircon Yellow [24]
|
89.93
|
−3.49
|
43.34
|
|
|
|
Particle size and morphological analysis
Color depends on several material properties of a pigment, among which particle size of the pigment is of prime importance. Decrease in particle size of the pigment increases the surface area which further contributes to high surface coverage, higher number of reflectance points and hence more scattering. The particle size distribution of the typical pigments Ca0.1 Y0.9 PO4 and Ca0.1 Pr0.9 PO4, were investigated in water with calgon as the dispersing agent. The results reveal a distribution with 90% of the particles with size smaller than 7.54 µm, 50% smaller than 2.59 µm and 10% smaller than 0.36 µm. The mean particle diameter of the pigment sample was found to be 5.65 µm. The average particle size of the pigments as observed from the SEM images shown in Fig. 9. is approximately 10 μm, which matches well with the results obtained from particle size analysis.
Thermal stability of the pigments.
The thermal stability (TG analysis) of the typical synthesized pigments, namely Ca0.1 Y0.9 PO4 and Ca0.1 Pr0.9 PO4 were examined in the temperature range of 30–1000 °C and the results are depicted in Fig. 10. The thermo gravimetric analysis results clearly indicate that there is negligible weight loss and phase transition of the pigments up to 1000 °C.
Chemical stability of the pigments
To facilitate the use of these colorants in various applications, it is necessary to establish its chemical and thermal stability. Chemical resistance of the pigment samples Ca0.1 Y0.9 PO4 and Ca0.1 Pr0.9 PO4 were assessed using 10% HCl/H2SO4/NaOH. A known weight (0.1g) of the pigment was taken in beaker containing acid/alkali, soaked for 30 min with constant stirring using a magnetic stirrer. It was then filtered, washed with water, dried and weighed. Negligible weight loss was noted in all the cases. The color coordinates of the ensuing powders were then measured and the results are given in Table 2 and 3. The total color difference ( ) is negligible indicating an imperceptible color change to the human eye.
Table 2. The color coordinates (±0.1) of the Ca0.1 Y0.9 PO4 powder pigments after chemical resistance test.
10% Acid/Alkali
|
Color coordinates
|
a
|
L*
|
a*
|
b*
|
HCl
|
87.1
|
-8.6
|
55
|
0.78
|
H2SO4
|
87.0
|
-8.5
|
55.3
|
0.97
|
NaOH
|
87.6
|
-9.4
|
54.9
|
0.94
|
a = [(DL*) 2 + (Da*) 2 + (Db*) 2] ½
Table 3. The color coordinates (±0.1) of the Ca0.1 Pr0.9 PO4 powder pigments after chemical resistance test.
10% Acid/Alkali
|
Color coordinates
|
a
|
L*
|
a*
|
b*
|
HCl
|
87.7
|
-26.5
|
55.5
|
0.62
|
H2SO4
|
88.3
|
-26.4
|
55.4
|
0.91
|
NaOH
|
87.6
|
-26.5
|
55.2
|
0.87
|
a = [(DL*) 2 + (Da*) 2 + (Db*) 2] ½
Applications of inorganic rare earth pigments: Coloring of plastics
The coloring performance of the typically synthesized pigments (a) Ca0.1 Y0.9 PO4 and (b) Ca0.1 Pr0.9 PO4 was tested for its coloring application in a substrate material like PMMA Poly(methyl methacrylate). Typically, 10 wt. % pigment sample was dispersed in PMMA and compressed to a cylindrical disc (Fig. 11).