Effect of sulfur addition and nanocrystallization on the transport properties of lithium–vanadium–phosphate glasses

The glasses defined by the formula 37.5Li₂O–25V₂O₅–37.5P₂O₅ mol% containing different sulfur (0, 10, 50 and 100 mol%) content were studied before and after nanocrystallization. X-ray diffraction and transmission electron micrograph of the heat treated samples indicated nanocrystals embedded in the glass matrix. The average crystallite size was found between 18 and 37 nm. Sulfur (S) behaved as a reducing agent for redox reaction during preparation of glass and affected the conductivity, i.e., the V⁴⁺–V⁵⁺ or V³⁺–V⁴⁺ion pairs increased with increasing S content and led to increasing conductivity of glasses. After creation of the nanocrystalline phase, S-free glass–ceramic nanocomposite exhibited improvement in electrical conductivity around three orders of magnitude than initial glass. This great improvement of electrical conductivity is related to increase in a concentration of V⁴⁺–V⁵⁺or V³⁺–V⁴⁺ ion pairs and also, forming of defective and well-conducting regions along the crystallite/glass interfaces. The decrease in electrical conductivity in the 50S glass–ceramic nanocomposite, which possessed the highest crystallite size, could be related to the increase of grain boundaries scattering because of the increasing crystallite size. The conduction was attributed to non-adiabatic small polaron hopping and mostly determined by hopping carrier mobility.