Raman spectrum of gadolinium molybdate at 80°K

Abstract

Lines that overlap from thermal broadening in the room-temperature Raman spectrum of gadolinium molybdate are resolved in the 80\ifmmode^\circ\else\textdegree\fi{}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 ${A}_{1}(\mathrm{TO})$ and ${A}_{1}(\mathrm{LO})$ spectra. The splitting of the unstable 47 ${\mathrm{cm}}^{\ensuremath{-}1}$ room-temperature ${A}_{1}(\mathrm{TO})$ line into a doublet at 44.5 and 51.5 ${\mathrm{cm}}^{\ensuremath{-}1}$, observed previously only in ir absorption, is also observed in these spectra. On heating, the 44.5-${\mathrm{cm}}^{\ensuremath{-}1}$ line remains fixed in frequency whereas the 51.5-${\mathrm{cm}}^{\ensuremath{-}1}$ line broadens and shifts toward lower frequency until, above 210\ifmmode^\circ\else\textdegree\fi{}K, the 44.5-${\mathrm{cm}}^{\ensuremath{-}1}$ line is obscured. Another ${A}_{1}(\mathrm{TO})$ line, peaking at 83 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 80\ifmmode^\circ\else\textdegree\fi{}K, shifts toward lower frequency on heating, reaching 75 ${\mathrm{cm}}^{\ensuremath{-}1}$ at room temperature and remaining at that value on further heating up to the transition at 159\ifmmode^\circ\else\textdegree\fi{}C where it becomes Raman inactive. The broadening of the 51.5-${\mathrm{cm}}^{\ensuremath{-}1}$ line in the ${A}_{1}(\mathrm{LO})$ 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 ${\mathrm{cm}}^{\ensuremath{-}1}$ and a long-wave acoustic phonon.