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 $A_1\left(\mathrm{TO}\right)$ and $A_1\left(\mathrm{LO}\right)$ spectra. The splitting of the unstable 47 ${\mathrm{cm}}^{-1\mathrm{}}$ room-temperature $A_1\left(\mathrm{TO}\right)$ line into a doublet at 44.5 and 51.5 ${\mathrm{cm}}^{-1\mathrm{}}$, observed previously only in ir absorption, is also observed in these spectra. On heating, the 44.5-${\mathrm{cm}}^{-1\mathrm{}}$ line remains fixed in frequency whereas the 51.5-${\mathrm{cm}}^{-1\mathrm{}}$ line broadens and shifts toward lower frequency until, above 210°K, the 44.5-${\mathrm{cm}}^{-1\mathrm{}}$ line is obscured. Another $A_1\left(\mathrm{TO}\right)$ line, peaking at 83 ${\mathrm{cm}}^{-1\mathrm{}}$ at 80°K, shifts toward lower frequency on heating, reaching 75 ${\mathrm{cm}}^{-1\mathrm{}}$ 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-${\mathrm{cm}}^{-1\mathrm{}}$ line in the $A_1\left(\mathrm{LO}\right)$ 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 ${\mathrm{cm}}^{-1\mathrm{}}$ and a long-wave acoustic phonon.