Cerium(III) Niobate Layered Perovskites: Abnormal Optical Absorption Modulations by Tuning of B-Site Composition and...

The cerium-introduced layered perovskite of RbCeTa₂O₇ has exhibited a specific optical absorption due to metal-to-metal charge transfer transitions between Ce 4f and transition metal d-orbitals to show the unique pale-green coloration, which is different from conventional coloration mechanisms. To further extend the coloring state based on the same mechanism, in this work, a series of the [Ce(Ta,Nb)₂O₇]^- layered perovskites, Rb_1-Cs[Ce(Ta_1-Nb)₂O₇] (x = 0∼1), with Nb substitutions in the perovskite units have been prepared and investigated in terms of those crystal structures and optical absorption mechanism. The Rietveld analysis using the XRD profile and EXAFS analyses well refined those structures as the Dion-Jacobson-type layered perovskite. The color of solid solutions gradually changed from pale-green to dark reddish-brown with increasing amount of substituted niobium. The unique coloring state change behavior of solid solutions from pale-green to dark reddish-brown depending on the amount of the substituted niobium is not observed in the other layered perovskite analogues (e.g., La and Pr analogues). The first-principles calculation based on the density functional theory method indicated that the band structural change should be a key factor for the coloration modulation. Furthermore, the redox ability through the charge modulation of the perovskite layer, which is a specific function of the cerium-based layered perovskite, was also investigated for the niobate [CeNb₂O₇]^- perovskite layer, resulting in the anisotropic lattice changes similar to those of a Ta analogue with different structural changes in the stacking and in-plane directions. The accompanying change in electronic structure led to a clear modulation in optical absorption, yielding a drastic change in the coloring state from dark brown to yellow.