New layered perovskite family built from [CeTa2O7]- layers: coloring mechanism from unique multi-transitions

Color Controllable Inorganic Pigments with Ce3+ as a Color Source

The Ba2RE1-xCexTaO6 (RE = La, Gd, Y; 0 ≤ x ≤ 1) pigments were synthesized by a conventional solid-state reaction method to develop environmentally friendly reddish inorganic pigments. The host was the double-perovskite-type Ba2RETaO6 (RE: rare-earth elements), and the color source was Ce3+. All Ba2RE1-xCexTaO6 samples were obtained in a single-phase form as solid solutions. Rietveld refinement analysis of the Ba2RETaO6 (RE = La, Ce, Gd, and Y) samples revealed that the average bond length between RE3+ and O depends on the ionic radius of RE3+, and the shorter the RE-O length, the stronger the crystal field surrounding Ce3+. A photon energy at the maximum 4f-5d absorption of Ce3+ depended on the weighted average ionic radius (rRE) at the RE3+ site (i.e., the crystal field energy around the Ce3+ ions). In response to this phenomenon, the sample color was changed in order to orange, red, pink, and violet with a decrease in the rRE value, and a hue angle (h°) was roughly linearly related to that. For validation of the tendency, we demonstrated the synthesis and characterization of Ba2La0.5-yYyCe0.5TaO6 to obtain a more reddish color. As we exactly expected, a more reddish color was obtained while maintaining a high C value. Furthermore, the h° values for y = 0.1 and 0.2 were in good agreement with our estimation. In light of the above results, by controlling the intensity of the crystal field surrounding Ce3+ and/or the concentration of Ce3+, the optical absorption wavelength and absorption intensity of the Ba2RE1-xCexTaO6 system can be changed to adjust the color.

Ce(iv)-centered charge-neutral perovskite layers topochemically derived from anionic [CeTa2O7]- layers

Layered perovskites have been extensively investigated in many research fields, such as electronics, catalysis, optics, energy, and magnetics, because of the fascinating chemical properties that are generated by the specific structural features of perovskite frameworks. Furthermore, the interlayers of these structures can be chemically modified through ion exchange to form nanosheets. To further expand the modification of layered perovskites, we have demonstrated an advance in the new structural concept of layered perovskite "charge-neutral perovskite layers" by manipulating the perovskite layer itself. A charge-neutral perovskite layer in [CeIVTa2O7] was synthesized through a soft chemical oxidative reaction based on anionic [CeIIITa2O7]- layers. The Ce oxidation state for the charge-neutral [CeIVTa2O7] layers was found to be tetravalent by X-ray absorption fine structure (XAFS) analysis. The atomic arrangements were determined through scattering transmission electron microscopy and extended XAFS (EXAFS) analysis. The framework structure was simulated through density functional theory (DFT) calculations, the results of which were in good agreement with those of the EXAFS spectra quantitative analysis. The anionic [CeIIITa2O7]- layers exhibited optical absorption in the near infrared (NIR) region at approximately 1000 nm, whereas the level of NIR absorption decreased in the [CeIVTa2O7] charge-neutral layer due to the disappearance of the Ce 4f electrons. In addition, the chemical reactivity of the charge-neutral [CeIVTa2O7] layers was investigated by chemical reduction with ascorbic acid, resulting in the reduction of the [CeIVTa2O7] layers to form anionic [CeIIITa2O7]- layers. Furthermore, the anionic [CeIIITa2O7]- layers exhibited redox activity which the Ce in the perovskite unit can be electrochemically oxidized and reduced. The synthesis of the "charge-neutral" perovskite layer indicated that diverse features were generated by systematically tuning the electronic structure through the redox control of Ce; such diverse features have not been found in conventional layered perovskites. This study could demonstrate the potential for developing innovative, unique functional materials with perovskite structures.