57Fe-Moessbauer study of electrically conducting barium iron vanadate glass after heat treatment

Local structure and thermal durability of semiconducting xBaO·(90 - x)V2O5 · 10Fe2O3 glasses (x = 20, 30 and 40), NTA glassTM, before and after isothermal annealing were investigated by 57Fe-Moessbauer spectroscopy and differential thermal analysis (DTA). An identical isomer shift (δ) of 0.39 ± 0.01 mm s-1 and a systematic increase in the quadrupole splitting (Δ) were observed from 0.70 ± 0.02 to 0.80 ± 0.02 mm s-1 with an increasing BaO content, showing an increase in the local distortion of FeIIIO4 tetrahedra. From the slope of the straight line in the Tg-Δ plot of NTA glassTM, it proved that FeIII plays a role of network former. Large Debye temperature (ΘD) values of 1000 and 486 K were respectively obtained for 20BaO · 70V2O5 · 10Fe2O3 glass before and after isothermal annealing at 400 deg. C for 60 min, respectively. This result also suggests that FeIII atoms constitute the glass network composed of tetrahedral FeO4, tetrahedral VO4 and pyramidal VO5 units. The electric conductivity of 20BaO · 70V2O5 · 10Fe2O3 glass increased from 1.6 x 10-5 to 5.8 x 10-2 S cm-1 after isothermal annealing at 450 deg. C for 2,000 min. These results suggest that the drastic increase in the electric conductivity caused by heat treatment is closely related to the structural relaxation of the glass network structure.