Thermal stability and ionic conduction characteristics of lithium germanate glasses

Abstract

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.