Understanding alkali oxide induced structural modification at different length-scales in tellurite glasses for improved optical properties

Highlights: • Elastic properties degrade with modifier cation M = Li to Cs indicating depolymerization of network with bigger and massive M. • Distortion of TeO₄ units decreases with M, leading to lesser conversion of TeO₄.→TeO₃ units • Increase in size of M creates more number of terminal TeO₄ units and reduction in number of bridging units. • Medium range correlations increase with increasing size of alkali cation M. -- Abstract: Investigating the length-scales of order in glasses is important to understand glass properties and hence, the underlying glass structure. Addition of alkali oxides (M₂O) in tellurite glass is well-known to modify its network and alter properties by conversion of its basic structural units of TeO₄ → TeO₃ and increasing the conversion upon further addition of M₂O. In this study, we report results of our Raman and Brillouin scattering investigations on 0.12M₂O:0.88TeO₂ (M = Li, Na, K, Rb and Cs) glasses with an aim to study the effect of the specific alkali cation (M) at fixed composition, on the resulting network structure that determines their interesting optical properties. The network-polyhedral (TeO₃) stretching vibrations exhibits red-shift while the Te–O–Te bending mode blue-shifts as M varies from Li to Cs. The fraction of terminal TeO₄ units increase steeply with respect to TeO₃ and also at the cost of TeO₄ units within the network chains, with increasing size of M, contrary to earlier predictions that at fixed concentration of M₂O, only the interactions between alkali cation (M) and anions of structural units change, keeping the network structure unaltered. A reduction in frequency with rising mass of M was observed for the Brillouin modes and the Boson peak. Combining results from these techniques, probing structure at different length-scales, depicts that even though increasing M size lowers the conversion of TeO₄ to TeO₃ units, still the network disintegrates further since the fraction of TeO₄ with non-bridging oxygens (NBOs) enhance. This causes reduction in elastic modulus at a larger length-scale, even though specific intra-molecular interactions become stronger within the glass network. The increasing size of the cation from Li⁺ to Cs⁺, while preserving the charge of the alkali ion, helps distribute the same electric charge on larger number of anionic tellurite units of the network, and thus, enlarges the correlation length with M, while simultaneously decreasing the network strength. These observations provide evidence for the structural changes leading to the enhanced densities, increased band gaps, reduced refractive indices and the earlier-reported reduction in non-linear susceptibilities with increasing size and mass of M.