Local Structural Investigation of Near-Infrared-Reflective Black Ca₂(Mn,Ti)O₄ Pigments Using Synchrotron Radiation...

Layered perovskite black Ca₂(Mn,Ti)O₄ ceramics were studied by using synchrotron radiation X-rays (SRX) and density functional theory (DFT) calculations to investigate the valence states of the cations, average/local structures, and electronic states. The crystallographic data were obtained by the Rietveld refinement of the obtained synchrotron radiation X-ray powder diffraction patterns. X-ray absorption near edge structure (XANES) spectroscopy measurements revealed that the ratios of Mn⁴⁺ and Ti⁴⁺ were about 82 and 95%, respectively, in all samples, and the Mn/Ti valences were not affected by the introduction of Ti⁴⁺. In addition, two pre-edge peaks were observed in the Mn-XANES spectra, but their peak positions and intensities were affected by doping with Ti⁴⁺, indicating that the symmetry of the MnO₆ octahedra was changed. A comparison of the atomic distances estimated from the Rietveld analysis and radial distribution function (RDF) revealed that there were large differences between the M-M distances (M = Mn, Ti). Therefore, XANES simulations were carried out to obtain models of the local structure. The experimental and theoretical data indicate that the Mn atoms were configured in a zigzagging arrangement, and the distortion of the MnO₆ octahedra increased with the increase in the degree of Ti⁴⁺ doping. The origin of the changes to the pre-edge peaks was not only the crystal field strength around Mn but also the symmetry of the MnO₆ octahedra.