Optical Study of the Electronic Structure and Lattice Dynamics of NdBaMn₂O₆ Single Crystals

We investigated the electronic structure and lattice dynamics of double perovskite NdBaMn₂O₆ single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The optical absorption band centered at approximately 0.88 eV was assigned to on-site d–d transitions in Mn, whereas the optical feature at approximately 4.10 eV was assigned to charge-transfer transitions between the 2p state of O and 3d state of Mn. Analysis of the temperature dependence of the d-d transition indicated anomalies at 290 and 235 K. The activated phonon mode, which appeared at approximately 440 cm⁻¹ alongside with the enhancement of the 270 cm⁻¹ phonon mode, coupled strongly to the metal–insulator transition at 290 K, which was associated with a charge/orbital ordering. Moreover, the MnO₆ octahedral breathing mode at 610 cm⁻¹ exhibited softening at a temperature lower than 235 K (temperature of the antiferromagnetic phase transition), which revealed the strong coupling between the lattice and magnetic degrees of freedom. The spin–phonon coupling constant obtained was λ = 2.5 cm⁻¹. These findings highlight the importance of charge–orbital–spin interactions in establishing NdBaMn₂O₆ phases with novel properties.