Investigating the role of GeO2 in enhancing the thermal stability and proton mobility of proton-conducting phosphate glasses

Abstract

In this study, the effect of GeO₂ on the thermal stability and proton mobility (μ_H) of proton-conducting phosphate glasses was experimentally investigated using 22HO_1/2–3NaO_1/2–(12 − x)LaO_3/2–xGeO₂–63PO_5/2 glasses. Increasing glass transition temperature (T_g) and μ_H at T_g were experimentally found out, when LaO_3/2 was substituted with GeO₂, and this behavior coincides well with the prediction obtained from a linear regression model previously proposed. Based on the glass structure and nature of a P–O bond, both enhanced T_g and μ_H at T_g were strongly related to the formation of heteroatomic P–O–Ge linkages. The Ge atoms are six-fold coordinated with O atoms, and they tightly bridge the (PO₃⁻)_n phosphate chains through the heteroatomic P–O–Ge linkages, resulting in rigid phosphate frameworks with higher T_g. The formation of the heteroatomic P–O–Ge linkage results in increased covalency in the P–O bond. This increases the ionicity of the O–H bond owing to the balanced electron transfer to prevent excessive decrease in electron density on the O atom in the P–O–H bond. Thus, μ_H at T_g increases because the increasing ionicity weakens the O–H bond. Herein, the increasing covalency in the P–O bond is discussed according to the difference in the electronegativity and atomic orbitals that contribute to the bond with an O atom between La⋯O and Ge–O bonds.