Optical study of the electronic structure and lattice dynamics of SmBaMn2O6 single crystal

Abstract

A-site ordered double perovskites have gained attention due to their unique magnetic, structural, and electronic phase transitions. These complex phase transitions are due to the competing spin, charge, and phonon degrees of freedom. We investigated the electronic structure and lattice dynamics of double perovskite SmBaMn₂O₆ single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. This interesting material exhibits complex structural, magnetic, and charge/orbital ordering phase transitions. The room temperature optical absorption spectrum of SmBaMn₂O₆ shows three bands around 1.29, 4.00, and 5.72 eV. The lowest optical absorption band centered at approximately 1.29 eV was assigned to on-site d–d transitions in Mn, whereas the optical features at approximately 4.00 and 5.72 eV were assigned to charge-transfer transitions between the 2p state of O and the 3d state of Mn. Room temperature Raman scattering spectrum shows 15 phonon modes at approximately 105, 123, 154, 195, 260, 288, 310, 329, 359, 412, 447, 482, 513, 558 and 615 cm^−1. Phonon modes below 200 cm^−1 involve the motion of Sm and Ba atoms. Phonon modes between 200 and 400 cm^−1 are due to mix vibrations of stretching, bending, and tilting. The strong phonon peaks at 447 and 482 cm^−1 are mainly due to Jahn-Teller stretching while the peaks at 614 cm^−1 are assigned to the breathing modes of the oxygen atoms in the MnO₆ octahedra. With decreasing temperature, these phonon modes show a shift of peak position to higher frequencies and a narrowing of the resonance linewidth. Notably, new phonon modes appear in the spectra range between 200 and 400 cm^−1 below 200 K which can be associated with charge ordering. Furthermore, magnetic order-induced changes were observed in the breathing modes with the splitting of 615 cm^−1. Anomalies in the phonon frequency and linewidth observed near the phase transition temperatures highlight the importance of charge–orbital–spin interactions in this material.