Raman Spectrum of Gadolinium Molybdate above and below the Ferroelectric Transition

Abstract

90\ifmmode^\circ\else\textdegree\fi{}-scattered Raman spectra of single-domain gadolinium molybdate have been obtained at room temperature and above the 159 \ifmmode^\circ\else\textdegree\fi{}C ferroelectric transition on several crystal orientations. Lines in the 0-200-${\mathrm{cm}}^{\ensuremath{-}1}$ range can all be associated with Raman-active phonons and are identified by symmetry considerations, scattering-efficiency calculations, and comparison of spectra for different crystal orientations. Lines in the 200-1000-${\mathrm{cm}}^{\ensuremath{-}1}$ range can be identified with coupled vibrations of tetrahedral molybdate ions. Spectra for frequencies above 1000 ${\mathrm{cm}}^{\ensuremath{-}1}$ originate from impurity flourescence. The effect of the presence of multiple domains on the spectra was determined; the observed differences between single-domain and multiple-domain crystals result from ${B}_{1}\ensuremath{-}{B}_{2}$ symmetry mixing from the twinning of adjacent domains. No new soft modes, in addition to the known soft mode at about 43 ${\mathrm{cm}}^{\ensuremath{-}1}$, were observed. A few of the lines that are present in only the low-temperature phase cannot be identified from symmetry analysis and are probably derived from zone-boundary modes of the high-temperature phase.