Investigation of anharmonicity in rotational phonon mode for BaWO4 crystal

Spontaneous Raman spectra in the BaWO4 were measured in the temperature range from 4 K to 280 K, and the temperature dependence of the linewidth of the Ag (191 cm−1) Raman mode was analyzed using the lattice dynamical perturbative approach and one-phonon density of states (PDOS). The linewidth slope for the 191 cm−1 peak for an external mode is 7.2 times larger than that for the 926 cm−1 peak for a breathing mode. The different behaviors of these two modes in the case of temperature broadening could be attributed to the large energy band gap in the one-phonon density of states (PDOS) resulting in different anharmonic interactions. The origin may be that the ratio of up-conversion TDOS to down-conversion TDOS for Eg mode (191 cm−1) is more than that for Ag (926 cm−1). The peak of the Eg mode (191 cm−1) is attributed to the coupling mode both a rotation of the Barium and an out-of-phase rotation of the oxygen in x–y plane as a librational mode.