Optical Spectroscopic Investigations of Li2Ni(WO4)2
Yun-Chen Chung1*, Sunil K. Karna2, Fan-Cheng Chou2, Hsiang-Lin Liu1
1Department of Physics, National Taiwan Normal University, Taipei, Taiwan
2Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
* presenting author:Yun-Chen Chung, email:coldlizem@gmail.com
Spectroscopic ellipsometry and Raman scattering measurements of polycrystalline Li2Ni(WO4)2 are reported. This material is interesting on account of its complex magnetic phase transitions. The room temperature optical absorption spectrum of Li2Ni(WO4)2 exhibits two bands at approximately 5.2 and 6.0 eV. The first optical absorption was assigned to the charge transfer transitions between O 2p and Ni 3d states. The second absorption peak was associated with transitions between O 2p and W 5p states. The room temperature Raman scattering spectrum of Li2Ni(WO4)2 shows 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. The phonon peaks of 112 and 143 cm-1 are due to the local vibrations of Li+ and Ni2+ ions . The phonon modes from 193 to 352 cm-1 involve symmetric bending vibrations of WO6 octahedra, whereas the phonon modes of 387 and 418 cm-1 result from asymmetric bending vibrations of WO6 octahedra. The phonon modes from 451 to 914 cm-1 are associated with asymmetric and symmetric stretching vibrations of WO6 octahedra. Upon cooling across magnetic phase transition temperatures, all phonon modes show a shift of the peak position to higher frequencies and a narrowing of the resonance linewidth. Notably, anomalies of the phonon intensities have been detected, indicating strong spin-phonon coupling in this material.

Keywords: raman scattering , spin-phonon coupling