Enhancement of electrical conductivity and chemical durability of 20R2O•10Fe2O3•xWO3•(70 − x)V2O5 glass (R = Na, K) caused by structural relaxation

A relationship between local structure, thermal endurance, chemical durability and electrical conductivity of alkaline iron tungsten vanadate glass, 20R2O•10Fe2O3•xWO3•(70 − x)V2O5, R = Na and K, x = 0–50 mol.%, abbreviated as RFWV was investigated by 57Fe-Mössbauer spectroscopy, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA), leaching test using 20 vol.% HCl and dc-probe method. Mössbauer spectra of 20Na2O•10Fe2O3•xWO3•(70 − x)V2O5 glass (NFWV) consisted of one paramagnetic doublet with a stable isomer shift (δ) of 0.40 mm s− 1 and an increasing quadrupole splitting (Δ) from 0.68 to 0.82 mm s− 1 with the increase of “x” from 0 to 50 mol.%, indicating that local distortion of FeIIIO4 tetrahedra increased with WO3 content. After isothermal annealing at 500 °C for 100 min, Δ decreased to 0.62, 0.59, 0.67 and 0.67 mm s− 1 for NFWV glass with “x” of 0, 10, 20 and 30, showing that local distortion of FeIIIO4 tetrahedra was reduced due to the structural relaxation. Original NFWV glass exhibited a slightly decreasing electrical conductivity (σ) from 2.9 × 10− 6 to 5.5 × 10− 8 S cm− 1 with the increase of “x” from 0 to 50. The isothermal annealing of NFWV glass caused a remarkable increase in σ of 4.5 × 10− 3, 6.2 × 10− 4 and 9.8 × 10− 4 S cm− 1 with “x” of 0, 10 and 20, all of which showed a small activation energy for electron hopping (WH) of 0.21, 0.20 and 0.15 eV, respectively. On the other hand, increases in glass transition temperature (Tg), crystallization temperature (Tc) and activation energy for crystallization (Ea) were observed for NFWV glass with WO3 content, proving that the tungsten ion contributes to enhancement of chemical bond strength of NFWV glass. Similar results of an increase in σ and a decrease in dissolution rate due to annealing were observed for KFWV glass. It is concluded that structure, physical and chemical properties of RFWV glass can be controlled by introduction of WO3.