Synthesis and characterization of (68-x) CuO – xV<inf>2</inf>O<inf>5</inf> – 32TeO<inf>2</inf> (x = 0–68 mol%) and (35-x) CuO – xV<inf>2</inf>O<inf>5</inf> – 65TeO<inf>2</inf> (x = 0–35 mol%) glasses: Conduction mechanism, structure and EPR study

Abstract

In this work, two series of glasses, i.e. (68-x) CuO – xV2O5 – 32TeO2 (x = 0–68 mol%, Te32 series) and (35-x) CuO – xV2O5 – 65TeO2 (x = 0–35 mol%, Te65 series), were synthesized by the melt-quenching method and subjected to physical, thermal and electrical characterization. Their vitreous nature was confirmed by X-Ray diffraction and differential scanning calorimetry, while their structural units were determined by Raman spectroscopy. CuO substitution by V2O5 led to a decrease in density and glass-transition temperature, together with a conductivity increase. Conduction mechanism was interpreted as mainly due to small polaron hopping from the lower (V4+) to the higher (V5+) vanadium valence states. Te32 glasses, possessing the highest electronic conductivities (ranging from 2 E−4 to 5 E−7 Ω−1 cm−1), were investigated by the Electron Paramagnetic Resonance technique, in order to more deeply analyze their structure-conductivity correlation. Particularly, the observed signals were determined to consist in a superposition of a first line due to paramagnetic Cu2+ ions and a second line due to exchange-coupled CuO clusters. Differences in the spectra were determined between samples with higher (i.e. 20-30 mol%) Cu2+ concentrations and samples with lower Cu2+ concentrations, suggesting they are located in different local environments. Finally, it was found that the Cu2+ ions are not involved in the process of electron transfer