Ca2Mn0.85Ti0.15O4 was synthesized as an inorganic black pigment with thermal barrier properties, and its NIR solar reflectance value was much greater than those of the conventional NIR reflective black pigments.
Layered perovskite A
2
BO4 compounds were studied
by a combination of X-ray powder
diffraction (XRD) analysis, Raman spectroscopy, and density functional
theory (DFT) calculations. Ti4+-doped Ca2MnO4 ceramics with high near-infrared (NIR) reflectivity were
selected as a test case. After elucidating their crystal structures
(I41/acd) by XRD analysis,
Raman spectroscopy was applied. Raman peaks were observed at approximately
178, 290, 330, 463, 500, and 562 cm–1, which were
confirmed by DFT calculations, and were in modes identical to those
reported for Sr2IrO4 in the same space group.
An additional peak was observed at approximately 780 cm–1 for the Ti4+-doped samples, indicating that a silent
A2g mode was activated by doping with Ti4+,
similar to the A1g (breathing) mode found in B-site-substituted simple perovskite and B-site-ordered
double perovskite structures. The XRD patterns of the doped samples
did not exhibit any additional X-ray reflections, except for the pattern
typical of nondoped Ca2MnO4. Thus, these results
were attributed to the presence of the Ti–Ti correlation with
a certain distance. The calculated band gap energies of Ca2MnO4 and Ca2Mn0.75Ti0.25O4 were approximately 1.8 eV, which was in reasonable
agreement with the experimental value. The DFT calculations also revealed
that one of the factors contributing to the enhancement of NIR reflectivity
upon introduction of Ti4+ ions is the reduced density of
states (DOS) near the Fermi level.
Eu 2+ -doped calcium zirconates, Ca 1−x Eu x ZrO 3 (0 ≤ x ≤ 1), were synthesized as novel environmentally friendly inorganic yellow pigments by the conventional solid-state reaction method. The crystal structure, morphology, optical properties, and color were characterized. The Eu 2+ -doped samples strongly absorbed blue light in the wavelength range of 435−480 nm, which was caused by the 4f−5d allowed transition of Eu 2+ . The color of the sample gradually became brilliant yellow with increasing the Eu 2+ content. Among the samples synthesized in this study, the brightest yellow color was obtained with the Ca 0.7 Eu 0.3 ZrO 3 (a* = +11.5 and b* = +70.7) sample. Compared with the commercially available praseodymium yellow pigment (a* = −3.28, b* = +70.3), the yellowness value (b*) of Ca 0.7 Eu 0.3 ZrO 3 was comparable and the redness value (a*) was higher. As a result, this pigment exhibited a reddish yellow color as compared with praseodymium yellow. In addition, this pigment was chemically stable. Therefore, the Ca 0.7 Eu 0.3 ZrO 3 pigment has the potential to become a novel environmentally friendly inorganic yellow pigment.
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