Electronic structure and lattice dynamics of Li₂Ni(WO₄)₂

We combined spectroscopic ellipsometry and Raman scattering measurements to explore the electronic structure and lattice dynamics in Li₂Ni(WO₄)₂. The optical absorption spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents a direct optical band gap at 2.25 eV and two bands near 5.2 and 6.0 eV, which are attributed to charge-transfer transitions from oxygen 2p states to nickel 3d or tungsten 5p states. The Raman scattering spectrum of Li₂Ni(WO₄)₂ measured at room temperature presents seventeen phonon modes at approximately 112, 143, 193, 222, 267, 283, 312, 352, 387, 418, 451, 476, 554, 617, 754, 792, and 914 cm⁻¹. When the temperature is decreased to 20 K, the frequency, linewidth, and normalized intensity of all phonon modes exhibited almost no temperature dependence. Upon cooling across 13 K, which is the antiferromagnetic phase transition temperature, the oxygen octahedra stretching mode at 914 cm⁻¹ exhibited a softening and an increase in intensity, thus suggesting a coupling between the magnetic and lattice degrees of freedom. The spin-phonon coupling constant was estimated to be 0.94 mRy/Ų, indicating a weak spin-phonon interaction in Li₂Ni(WO₄)₂.