Size dependence of optical phonons in yttria nanoparticles by means of Raman light scattering

Nanocrystalline Y2O3 was prepared by a sol-gel type hydrolysis in the inverse micelles of w/o-microemulsions with controlled particle sizes between 7 and 20 nm and with very narrow particle size distributions. The material exhibits the cubic rare earth c-structure (S. g. Ia3) and was studied by means of X-ray diffraction and micro-Raman scattering. With decreasing crystallite sizes the characteristic Raman line at 378 cm-1 shifts to lower frequencies up to 373 cm-1 and exhibits an increasing line broadening, in qualitative agreement with the phonon confinement model of Richter et al. The parallel down shift of all lines, however makes an quantitative explanation by this model questionable. Instead, using the mode Grüneisen parameter of Y2O3, we propose to explain the common shift of the Raman lines by structural gradients in the subsurface regions of the nanoparticles ; these regions comprise appreciably increasing volume fractions for decreasing particle sizes. Rietveld analysis of the full powder X-ray diffraction pattern as well as Warren-Averbach analysis of the nanocrystalline material show a significant increase of the lattice constant from 10.602 Å to 10.680 Å with decreasing crystallite sizes. Evidently this phenomenon contributes to the softening of the characteristic vibrations and can be described by the mode Grüneisen Parameter