Vibrational modes and the boson peak of Y2O3-Al2O3 glasses

Six glasses in the Y2O3-Al2O3 binary system with compositions ranging between 24 and 41 mol% Y2O3 have been prepared by containerless levitation and CO2 laser melting techniques. Polarized and depolarized Raman spectra measured at ambient conditions for all glasses have a high intensity boson peak at similar to 100 cm(-1) and three polarized bands at frequencies v(A)=780, v(B)=650 and v(C)=450 cm(-1). The relative intensities of the v(A), v(B) and v(C) bands are dependent on glass composition and are assigned to stretching of Al-O terminal bonds in Q(2)-AlO4 type species, Al-O-Al and Y-O stretching vibrations respectively. Raman spectral measurements at temperatures of up to 1100 K do not show any significant change in the relative intensities of the v(A), v(B) and v(C) bands. The boson peak is found to be independent of both composition and temperature and its frequency is used to estimate correlation lengths of structural order in these glasses. It appears that the Y-O and Al-O polyhedra in the glass network are linked via Al-O-Al and Al-O-Y bridges while the presence of Al-O terminal bonds offers a partial ionic character to the glass structure. Micro-Raman spectra measured for the low and high density amorphous phases formed in these glasses are found to be indistinguishable indicating common structural characteristics for both phases. It appears that the stability of the high density phase against crystallization is higher than that of the low density phase. Finally extensive polarization measurements suggest the Raman depolarization ratio is a sensitive indicator for monitoring phase separation in glasses at micron to nanometre length scales.