Vibrational study of lithium borotellurite glasses

Ion-conducting lithium borotellurite glasses of composition (Li2O)(y)-[(2TeO(2))(x)-(B2O3)(1-x)](1-y) (y = 0.33 and 0.40 and 0 <= x <= 1) were investigated by Raman and infrared (IR) techniques. The Raman spectra demonstrated the transition from TeO4 trigonal bipyramid (tbp's) units to TeO3 trigonal pyramids (tp's) with terminal oxygens upon increasing B2O3 content, but gave minimal information for the borate species due to their weak scattering efficiency relative to the tellurite species. In contrast, IR spectroscopy was very effective in probing the vibrational response of borate tetrahedra and triangles. Upon increase of tellurite content, the borate tetrahedra showed enhanced absorption relative to borate triangles, this result being in line with an increased fraction of borate tetrahedra seen in previous NMR studies. Raman and IR results are interpreted in the context of weak Te center dot center dot center dot O-B interactions, through which tellurium oxide acts as a second modifier next to Li2O and leads to an enhanced formation of borate tetrahedral units.