Mixed Polaron and Bipolaron Transport in (xV2O5–(65–x) Sb2O3–35P2O5) Glasses

This study presents the electrical and optical properties of 35P2O5–xV2O5–(65–x) Sb2O3 glasses for 0 ≤ x ≤ 65 mol%. The direct current (DC) resistivity was measured by the Van der Pauw method and optical absorption spectra were taken in the Ultraviolet–Visible-Near-Infrared (UV–VIS–NIR) range. Electrical transport is attributed to simultaneous hopping of small polarons (SPs) between V4+ and V5+ (vanadium ion) sites and small bipolarons (SBPs) between the Sb3+ and Sb5+ (antimony ion) sites. The resistivity exhibits a non-linear dependence on the ionic fraction of vanadium (nv), whereas the resistivity exhibits a minimum in the composition range 0 ≤ nV ≤ 0.3, and a resistivity maximum was observed in the range 0.3 ≤ nV ≤ 0.5. On further increasing nv, the resistivity exhibits a monotonic decline. In the composition range 0 ≤ nV ≤ 0.3, where the hopping distance between V ions decreases, while that between the Sb ions increases, the resistivity minimum has been shown to be the consequence of decreasing tunneling distance of SPs between the V4+ and V5+ ion sites. In the composition range 0.3 ≤ nV ≤ 0.5, the resistivity, activation energy for DC conduction, glass transition temperature, and density exhibit their respective maxima even though the separation between the V4+ and V5+ sites continues to decrease. This feature is explained by enhanced localization of electrons on account of increased disorder (entropy) among the SPs and SBPs, like that of Anderson localization. This argument is further supported by a shift in the polaronic optical absorption bands associated with the SPs and SBPs toward higher energies. The transport behavior of all the glasses except the x = 0 composition has been explained by adiabatic transport, principally, by the SPs on V ions while the Sb ions contribute little to the total transport process. The results provide a clear relation between composition, polaron/bipolaron contributions, and conduction in these glasses.