Vibrational modes of sodium-tellurite glasses: Local structure and Boson peak changes

Raman spectra of xNa2O−(1−x)TeO2 glasses were measured and analyzed over a broad composition range. The network structure of the glass consists of a mixture of TeO4 trigonal bipyramids and TeO3 trigonal pyramidal units. Increase of alkali content results in conversion of the TeO4 units into TeO3 units with a varying number of non-bridging oxygen atoms. This local structure transformation becomes more evident at x=0.2, where rigidity percolation theory predicts a threshold for the rigid-to-floppy transition. However, other spectral features such as the composition dependence of the energy of the Boson peak do not support the occurrence of the stiffness transition. The controversial dependence of dynamical parameters such as the fragility index as a function of Na2O concentration is also discussed.