Optical properties of antiferroelectric Cs₂Nb₄O ₁₁: Absorption spectra and first-principles calculations

We report a joint experimental and theoretical investigation of the optical properties of Cs₂Nb₄O₁₁. In room temperature optical absorption spectra, we found a direct gap about 3.55 0.05 eV and charge transfer excitations at about 4.96 and 6.08 eV, which are in good agreement with the first-principles calculations. Upon passing through the 165 C antiferroelectric to paraelectric phase transition, the peak energies of two charge transfer bands display almost no temperature dependence, yet they become even broader and exhibit enhanced oscillator strength. We infer this intriguing behavior as the manifestation of Nb cation distortions due to the charge-lattice interaction.