Impact of sodium oxide, sulfide, and fluoride-doped vanadium phosphate glasses on the thermoelectric power and electrical properties: structure analysis and conduction mechanism

Abstract

Four glasses of the (13.5%X– 32.5%V2O5–54%P2O5) mol%, X = Na2O, Na2S, and NaF, and (13.5% Na2S – 32.5%V2S5–54%P2S5) mol% were prepared. DSC, density, molar volume, XRD, thermoelectric power, and glass transport properties of the prepared glasses were reported. Density was found to increase with decreasing the molar volume, Vm, of glasses containing sulfur. Replacing oxygen with sulfur results in less number of bridging oxygen; this decrease leads to less compactness and molar volume increase of the host network. Thermoelectric power (S) of present samples was reported and the fraction C of reduced transition metal ions (C = V4+/Vtotal or V3+/Vtotal ) was calculated. Moreover, the conductivity data temperature dependence has been studied in two distinct polaronic models. Above θD/2 (high temperature region), the conductivity data are compatible with Mott SPH model between the nearest neighbors, but in the intermediate temperature extent, the appropriate model was Greaves VRH model. Dc conductivity values have a positive relation with C results calculated from thermoelectric power. The dc conductivity of the formed sulfur-containing glasses is enhanced owing to the increase of V4+–V5+ or V3+–V4+ ion pairs as sulfur worked as a reducing agent, which resulted in reduction of the vanadium ions from high valence (V4+ and/or V5+) to low valence states (V3+ and/or V4+). From inspecting the conduction mechanism nature and type, the current work displayed was proved to be in accord with non-adiabatic case of SPH. The hopping carrier mobility is very small in order of 10− 5 cm2 V− 1 s− 1 at 393 K satisfying the localized condition for hopping electrons.