Raman Scattering Study on Vibrational Modes and Structure of Lanthanum Tellurite Glasses

Raman spectra of xLa(2)O(3)-(1-x)TeO2 (x=0, 0.05, 0.10, 0.15, 0.20, and 0.25) lanthanum tellurite glasses were measured and analyzed over the entire glass-forming region in an effort to quantitatively follow the structural changes caused by lanthanum oxide variation. For the first time, systematic intensity measurements have been performed to elucidate the composition induced structural changes in the high-frequency stretching vibration region and a possible mechanisrn was proposed. The network structure of the glasses is formed by mixing TeO4 trigonal bipyramid and TeO3 trigonal pyramid units. The change of the lanthanum oxide content tesults in conversion of the Teal units to TeO3 units with a varying number of non-bridging oxygen atoms. Analysis of the Raman band contours in terms of vibrations due to different oxygen bridged trigonal bipyramid and trigonal pyramid tellurite structural units, allowed to calculate the relative amounts of the species involved in the structural changes with composition. The fraction of the terminal oxygen atoms has been estimated from the Raman intensities with the aid of a structural model concerning the structure of tellurite network systems. The simulation of the experimental density of lanthanum tellurite glasses with modifier content up to 25% revealed that the short range order building units assumed here are sufficient to account for the overall structure in these glasses.