Raman Spectroscopic Investigation and Electronic State Calculation for Ca 2 (Mn,Ti)O 4 Black Pigments with High Near-Infrared (NIR) Reflectivity.

Layered perovskite A 2 B O 4 compounds were studied by a combination of X-ray powder diffraction (XRD) analysis, Raman spectroscopy, and density functional theory (DFT) calculations. Ti 4+ -doped Ca 2 MnO 4 ceramics with high near-infrared (NIR) reflectivity were selected as a test case. After elucidating their crystal structures ( I 4 1 / 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 Sr 2 IrO 4 in the same space group. An additional peak was observed at approximately 780 cm -1 for the Ti 4+ -doped samples, indicating that a silent A 2g mode was activated by doping with Ti 4+ , similar to the A 1g (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 Ca 2 MnO 4 . Thus, these results were attributed to the presence of the Ti-Ti correlation with a certain distance. The calculated band gap energies of Ca 2 MnO 4 and Ca 2 Mn 0.75 Ti 0.25 O 4 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 Ti 4+ ions is the reduced density of states (DOS) near the Fermi level.