Raman Spectroscopic Investigation and Electronic State Calculation for Ca₂(Mn,Ti)O₄ Black Pigments with High Near-Infrared (NIR) Reflectivity

Abstract

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.

Local Structural Investigation of Near-Infrared-Reflective Black Ca2(Mn,Ti)O4 Pigments Using Synchrotron Radiation X-Ray and Density Functional Theory Calculations

“…The phonon frequencies of the Raman-active modes of Ca 2 MnO 4 were calculated by DFT using the refined crystal structure. The calculated and experimental Raman frequencies are listed in Table . For the phonon modes at the Γ-point for the I 4 1 / acd (No.…”

“…The valence and conduction bands of Ca 2 MnO 4 are composed of O 2p and Mn 3d orbitals, respectively. 20 Consequently, Ca 2 (Mn,Ti)O 4 absorbed the visible light due to the interband transition between the O 2p and Mn 3d orbitals and reflected NIR light. Recently it was revealed that one of the factors leading to the enhanced NIR reflectivity by doping Ti 4+ was the reduced DOS near the Fermi level.…”

“…Recently it was revealed that one of the factors leading to the enhanced NIR reflectivity by doping Ti 4+ was the reduced DOS near the Fermi level. 20 Figure 3 shows the Mn and Ti K-edge XANES spectra for Ca 2 (Mn 1−x Ti x )O 4 (0 ≤ x ≤ 0.25) pigments. As references, the spectra for MnO (Mn 2+ ), Mn 2 O 3 (Mn 3+ ), Li 2 MnO 3 (Mn 4+ ), TiO (Ti 2+ ), Ti 2 O 3 (Ti 3+ ), and TiO 2 (Ti 4+ ) are also shown in this figure.…”

“…Spin-polarized GGA with a Hubbard U correction (GGA + U) was employed to correct the partially filled d-orbitals of the Mn atoms. As in our previous study, 20 the effective U was set to 3 eV and the antiferromagnetic (AFM) configuration was adopted, as reported in a previous study. 23 The lattice parameters and atomic positions as obtained by Rietveld refinement were used.…”

“…In order to elucidate the reason why the NIR-reflective property of Ca 2 MnO 4 was enhanced by doping Ti 4+ , recently, we investigated the crystal structures and electronic states of Ca 2 (Mn,Ti)O 4 solid solutions using X-ray powder diffraction (XRD) analysis, Raman spectroscopy, and density functional theory (DFT) calculations. 20 From the results of XRD analysis and Raman spectroscopy, a Ti−Ti correlation with a characteristic distance was identified. Furthermore, the DFT calculations revealed that one of the factors leading to the enhancement of NIR reflectivity upon introduction of Ti 4+ was the reduced density of states (DOS) near the Fermi level.…”