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Frequency shift and vibronic relaxation of Raman modes in barium nitrate molecular ionic crystal

In this paper the results on high resolution spontaneous Raman spectroscopy of barium nitrate molecular ionic crystal at temperature from 8 to 600 K are presented. The temperature dependencies of the linewidth broadening and frequency shifts of internal Raman modes are investigated. The proposed mechanisms of vibronic relaxation of internal Raman modes exhibit low degree of phonon-phonon coupling in the medium. The phonon frequency shifts are found to be due to lattice expansion and are discussed in terms of Gruneisen parameter theory.