Electrical conductivity and local structure of barium manganese iron vanadate glass

Local structure and electrical conductivity of semiconducting 20BaO·10Fe2O3·x MnO2·(70 − x)V2O5 glass (x = 0 − 30), abbreviated as xBFMV, were investigated by means of 57Fe-Mössbauer spectroscopy, differential thermal analysis (DTA) and DC four-probe method. Mössbauer spectrum of these vanadate glasses consists of a doublet with an identical isomer shift (δ) of 0.38±0.01 mm s − 1, indicating that distorted FeO4 tetrahedra constitute the structural units with distorted VO4 tetrahedra and VO5 pyramids. Quadrupole splitting (Δ) gradually increases from 0.70±0.02 to 0.87±0.02 mm s − 1 with an increase in the MnO2 content, indicating an increased local distortion of FeIIIO4 tetrahedra. DTA study of these glasses showed a gradual increase of glass transition temperature (Tg) from 329±5 to 411±5°C, showing an improved thermal durability. ‘Tg vs. Δ plot’ yielded a straight line with a large slope of 707°C(K)/mm s − 1, proving that FeIII played a role of network former (NWF). An isothermal annealing of 10BFMV glass at 500°C for 1000 min resulted in a marked increase in the electrical conductivity (σ) from (4.5±3.9) × 10 − 7 to (1.4±0.3) × 10 − 2 S cm − 1 and a decrease in the activation energy for the electrical conduction (Ea) from 0.33 ± 0.07 to 0.11±0.01 eV, while Δ of FeIII decreased from 0.76±0.02 to 0.49±0.02 mm s − 1. These results suggest that decrease in the distortion of FeIIIO4 tetrahedra involved with the structural relaxation causes an increase in the probability of electron hopping from VIV or VIII to VV.