We present the optical studies of electronic structure and lattice dynamics in SrFeO3-d, Ba2CuTeO6, and Li2Ni(WO4)2. SrFeO3-d single crystals were grown using the floating zone method;Ba2CuTeO6single crystals were grown using the flux method; and polycrystalline Li2Ni(WO4)2 was grown using the solid-state reaction method. These materials are unusual given their complex magnetic phase transitions.
At room temperature, the optical absorption spectra of SrFeO3-d displayed three maxima in the spectral range of 1.8–5.1 eV and exhibited the direct band gap at approximately 1.98–2.08 eV. SrFeO2.86 exhibited 14 Raman-active phonon modes, and the phonon frequencies of SrFeO2.75 were close to those of SrFeO2.86 with the disappearance of one feature at approximately 226 cm−1. The optical absorption spectrum of Ba2CuTeO6 showed a direct optical band gap at approximately 1.04 eV and exhibited four bands at higher photon energies. The room-temperature Raman scattering spectrum of Ba2CuTeO6 displayed 16 phonon modes. The optical absorption spectrum of Li2Ni(WO4)2 presented a direct optical band gap at 2.25 eV and displayed one band at approximately 5.6 eV. The Raman scattering spectrum of Li2Ni(WO4)2 measured at room temperature presented 17 phonon modes.
With decreasing temperature, the onset of magnetic ordering of SrFeO3-d did not influence the phonon parameters. By contrast, the stretching vibrations of CuO6 octahedra located at 679 cm−1 in Ba2CuTeO6 had the largest spin-phonon coupling constant (1.67 mRy/Å2). The stretching vibrations of WO6 octahedra located at 914 cm−1 in Li2Ni(WO4)2 exhibited the spin-phonon coupling constant (0.94 mRy/Å2).
In this study, we demonstrated the direct optical band gap at low photon energy and charge-transfer bands at higher photon energy in the perovskite oxides of SrFeO3-d and double perovskite oxides of Ba2CuTeO6 and Li2Ni(WO4)2. Their room-temperature Raman scattering spectra showed rich phonon modes. The symmetric stretching vibrations of CuO6 and WO6 octahedra connected the magnetic ordering and the spin-phonon coupling constants were estimated.

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