Vibrational spectra of sodium gadolinium tungstate NaGd(WO4)2 single crystals: Observation of spatial dispersion

Abstract Room temperature polarized Raman scattering and infrared reflectance spectra of a NaGd(WO 4 ) 2 single crystal have been measured. The IR spectra interpretation was aided by a Kramers-Kronig analysis, and fitted to the independent oscillator model. All 13 theoretically expected Raman-active bands have been identified and assigned, as well as 7 out of 8 expected IR active bands. Splitting of bands in both Raman and IR clearly indicates a lowering of the crystal symmetry due to occupation disorder in the 4 a site, that randomly accommodates either an Na + or a Gd 3+ ion. The reflectance IR spectra reveal a spatial dispersion, namely a dependence of the transverse optical (TO) polariton frequencies, on the propagation direction in the crystal. The crystal vibrational modes are correlated to the internal modes of the tungstate group WO 4 2− , and to the internal modes of the molecular skeleton. A detailed correlation map of the symmetry analysis is presented.