Electronic structure and lattice dynamics of Ba₂CuTeO₆ single crystals

The electronic structure and lattice dynamics of Ba₂CuTeO₆ single crystals were investigated through spectroscopic ellipsometry and Raman scattering measurements. The room-temperature optical absorption spectrum of Ba₂CuTeO₆ presented a direct optical band gap at approximately 1.04 eV and exhibited four bands at approximately 1.45, 3.43, 4.65, and 5.79 eV. The optical absorption band at 1.45 eV was attributed to on-site Cu²⁺ d-d transition. The other bands were attributed to charge-transfer transitions between the O 2p and Cu 3d or Te 5p states. The room-temperature Raman scattering spectrum of Ba₂CuTeO₆ exhibited 16 phonon modes at approximately 85, 97, 104, 119, 160, 194, 380, 396, 404, 409, 492, 568, 574, 606, 679, and 751 cm^-1. When the temperature decreased to less than 287 K, which is the temperature at which structural phase transition occurs from the monoclinic phase to the triclinic phase, additional phonon modes appeared at approximately 124, 128, 152, and 601 cm^-1. On further cooling to lower than 75 and 15 K, which are the temperatures at which short- and long-range antiferromagnetic phase transitions occur, respectively, the phonon modes at approximately 97, 104, 124, 128, 152, 160, 194, 380, 396, 409, 568, 574, 606, and 679 cm^-1 exhibited softening, which indicates a coupling between the magnetic and lattice degrees of freedom. The stretching vibration of CuO₆ octahedra located at 679 cm^-1 had the largest spin-phonon coupling constant (1.67 mRy Å^-2).