Thermal stability and ionic conduction characteristics of lithium germanate glasses

A report is given of the systematic investigation on the glass stability and conduction characteristics of binary lithium germanate glasses in the series, mol% xLi₂O-(100-x)GeO₂, 5 ≤ x ≤ 30. The glass stability (against devitrification) parameters along with density and glass transition temperature reveal an extremum around 15 mol% of Li₂O, a signature of the germanate anomaly. The analysis of impedance data in conductivity and modulus formalisms reflect different aspects of the structural changes as the amount of Li₂O increases. The lithium ion conductivity increases with the increase in the Li₂O concentration while the activation energy for ionic motion does not change much from mol% 5 to 10 Li₂O and decreases monotonically from mol% 10 to 30 Li₂O indicating the absence of germanate anomaly. However, the data in modulus formalism does not scale to a master curve for all the glasses suggesting the germanate anomaly. Modeling of the modulus data using the Kohlrausch-Williams-Watts (KWW) function depicts the germanate anomaly in the stretched exponential parameter (β) at 15 mol% Li₂O. Raman spectra of the glasses reveal the preferential formation of Q² units at low alkali concentration along with the formation of ring structures of several Ge tetrahedra and/or the formation of higher coordinated Ge species around the anomalous composition region. A correlation of these structural changes with the conduction characteristics of the glasses is also discussed.