Role of free volume in governing electrical transport in V₂O₅-Cu₂O-P₂O₅ glass ceramic nanocomposites

Glass-ceramic nanocomposites of xV₂O₅-(40-x)Cu₂O-60P₂O₅ (x = 10-40 mol%) were prepared by melt quenching followed by heat treatment at 873 K. XRD analysis confirmed the formation of Cu₂O and V₂O₅ nanocrystalline phases with crystallite sizes in the range of 16-27 nm embedded within a partially amorphous matrix. DC electrical conductivity increased by nearly four orders of magnitude with increasing V₂O₅ concentration, accompanied by a systematic decrease in activation energy, indicating enhanced thermally activated small-polaron hopping conduction. Positron annihilation lifetime (PAL) spectroscopy revealed a gradual reduction in vacancy-type defect size and free-volume parameters with increasing V₂O₅ content, suggesting improved local structural ordering and reduced carrier localization. In contrast, the oxygen molar volume increased slightly with composition, reflecting macroscopic structural rearrangement and modification of the phosphate network. These observations indicate that the macroscopic network expansion and microscopic reduction of localized free-volume cavities occur simultaneously at different structural scales. The enhancement in electrical conductivity is primarily governed by increased vanadium ion concentration and reduced hopping distance, while defect reduction contributes indirectly through improved structural compactness and orbital overlap between neighboring vanadium ions. The present results provide insight into the correlation between structural evolution, free-volume defects, and electrical transport in transition-metal oxide glass-ceramic nanocomposites.