Raman spectrum of gadolinium molybdate at 80°K

Lines that overlap from thermal broadening in the room-temperature Raman spectrum of gadolinium molybdate are resolved in the 80^∘{}K spectrum. A total of 126 of the possible 201 lines have been resolved. Symmetry assignments are derived from considerations of both point symmetry and polariton dispersion, supported by separate measurements of A₁(TO) and A₁(LO) spectra. The splitting of the unstable 47 cm^-1 room-temperature A₁(TO) line into a doublet at 44.5 and 51.5 cm^-1, observed previously only in ir absorption, is also observed in these spectra. On heating, the 44.5-cm^-1 line remains fixed in frequency whereas the 51.5-cm^-1 line broadens and shifts toward lower frequency until, above 210^∘{}K, the 44.5-cm^-1 line is obscured. Another A₁(TO) line, peaking at 83 cm^-1 at 80^∘{}K, shifts toward lower frequency on heating, reaching 75 cm^-1 at room temperature and remaining at that value on further heating up to the transition at 159^∘{}C where it becomes Raman inactive. The broadening of the 51.5-cm^-1 line in the A₁(LO) spectrum is less pronounced so its correlation with the modes of the high-temperature phase can be determined unambiguously. These three lines are suggested to be associated with a system of three coupled modes, whose behavior is discussed, qualitatively, in terms of energy transfer between modes and damping via a two-phonon decay involving an optic phonon of about 44.5 cm^-1 and a long-wave acoustic phonon.