Raman Spectroscopic Investigation and Electronic State Calculation for Ca₂(Mn,Ti)O₄ Black Pigments with High...

Layered perovskite A₂BO₄ 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 A_2g mode was activated by doping with Ti⁴⁺, 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 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 Ca₂Mn_0.75Ti_0.25O₄ 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.