Network former mixing effects in alkali germanotellurite glasses: A vibrational spectroscopic study

Highlights: • Glasses 0.3M₂O–0.7[(1-x)GeO₂–xTeO₂], M=Li, Na and 0 ≤ x ≤ 1, were synthesized and studied. • The glass transition temperature decreases non-linearly with increasing TeO₂ content. • The glass structure was probed by Raman and infrared spectroscopy. • A mixed glass (0 < x < 1) is not the weighted average of the x = 0 and x = 1 glasses. • Structure changes by mixing cancel out changes in the activation energy for ion transport. -- Abstract: Alkali germanotellurite glasses of composition 0.3M₂O–0.7[(1-x)GeO₂–xTeO₂], M=Li, Na and 0 ≤ x ≤ 1, were investigated by Raman and infrared vibrational spectroscopic techniques to search for the origins of the alkali ion-dependent network former mixing (NFM) effect in these ion-conducting glasses. The vibrational spectra measured on mixed network-former glasses, and the spectral comparison between equimolar-mixed glasses (x = 0.5) and pellet-mixtures of the endmember glasses, 0.3M₂O–0.7GeO₂ and 0.3M₂O–0.7TeO₂, provided evidence for the formation of hetero-atomic Ge–O–Te linkages and structural rearrangements in the germanate and tellurite components of the glass. The mixing-induced structural rearrangements were expressed in terms of chemical equilibria between the network-building units and were used to make qualitative predictions for changes in the network cross-linking density and the related network-strain energy, as well as in the binding energy part of the activation energy for ion conduction. Thus, it is proposed that the mixing-induced structural modifications in the germanate and tellurite parts of glass cause the cancelation of changes in the binding energy and the network-strain energy contributions to the activation energy for ion transport. These qualitative predictions were discussed in the context of the previously found absence of an NFM effect in ionic conductivity for M=Na and the presence of a weak positive NFM effect for M=Li.