Photoacoustic determination of the band gaps of Ca2MnO4 and Ca3Mn2O7

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

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.

Enhanced Charge Transport in Ca2MnO4-Layered Perovskites by Point Defect Engineering

Coupling between thermal and charge transport in crystalline materials has always been one of the greatest challenges in understanding the underlying physics of thermoelectric materials. In this sense, CaO(CaMnO₃)_m Ruddlesden-Popper layered perovskites, comprising m perovskite subcells separated by CaO planes, exhibit intriguing thermal and electronic transport properties that can be tuned by altering their crystal periodicities. Applying the well-established phonon glass electron crystal (PGEC) concept enables us to increase the transparency of these CaO planes to electron transport at the same time while preserving their opacity to phonon transport. First-principles calculations indicate that the total local potential at CaO planes, where Y substitutes for Ca, is lower by ca. 50% compared to La substitution. Measurements of the electrical conductivity and Seebeck coefficients for Ca_2-xR_xMnO₄ (R = La or Y; x = 0.01, 0.05, 0.1, and 0.15) bulk materials in the range of 300-1000 K confirm that compounds doped with Y exhibit higher electrical conductivity values than their La-doped counterparts. We attribute this to lower polaron hopping energy values (up to 23%) evaluated using the small polaron hopping model. This study introduces an original way to employ the PGEC approach for thermoelectric oxides.

Synthesis and characterization of black pigments based on calcium manganese oxides for high near-infrared (NIR) reflectance

Ca₂Mn_0.85Ti_0.15O₄ 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.