Electrical conductivity and local structure of barium manganese iron vanadate glass

Abstract

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 Fe^IIIO₄ tetrahedra. DTA study of these glasses showed a gradual increase of glass transition temperature (T _g) from 329±5 to 411±5°C, showing an improved thermal durability. ‘T _g vs. Δ plot’ yielded a straight line with a large slope of 707°C(K)/mm s^− 1, proving that Fe^III 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 (E _a) from 0.33 ± 0.07 to 0.11±0.01 eV, while Δ of Fe^III decreased from 0.76±0.02 to 0.49±0.02 mm s^− 1. These results suggest that decrease in the distortion of Fe^IIIO₄ tetrahedra involved with the structural relaxation causes an increase in the probability of electron hopping from V^IV or V^III to V^V.