Lithium-Ion Mobility in Quaternary Boro–Germano–Phosphate Glasses
2016
Moguš-Milanković, Andrea | Sklepić, Kristina | Mošner, Petr | Koudelka, Ladislav | Kalenda, Petr
Effect of the structural changes, electrical conductivity, and dielectric properties on the addition of a third glass-former, GeO₂, to the borophosphate glasses, 40Li₂O–10B₂O₃–(50 – x)P₂O₅–xGeO₂, x = 0–25 mol %, has been studied. Introduction of GeO₂ causes the structural modifications in the glass network, which results in a continuous increase in electrical conductivity. Glasses with low GeO₂ content, up to 10 mol %, show a rapid increase in dc conductivity as a result of the interlinkage of slightly depolymerized phosphate chains and negatively charged [GeO₄]⁻ units, which enhances the migration of Li⁺ ions. The Li⁺ ions compensate these delocalized charges connecting both phosphate and germanium units, which results in reduction of both bond effectiveness and binding energy of Li⁺ ions and therefore enables their hop to the next charge-compensating site. For higher GeO₂ content, the dc conductivity increases slightly, tending to approach a maximum in Li⁺ ion mobility caused by the incorporation of GeO₂ units into phosphate network combined with conversion of GeO₄ to GeO₆ units. The strong cross-linkage of germanium and phosphate units creates heteroatomic P–O–Ge bonds responsible for more effectively trapped Li⁺ ions. A close correspondence between dielectric and conductivity parameters at high frequencies indicates that the increase in conductivity indeed is controlled by the modification of structure as a function of GeO₂ addition.
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