57Fe-Mössbauer study of electrically conductive alkaline iron vanadate glasses

Abstract

A relationship between local structure, thermal stability and electrical conductivity (σ) of xR2O·10Fe2O3·(90 − x)V2O5 glasses (abbreviated as xRFV glasses, where R = Li, Na, K; x = 20 and 40 in mol %) was investigated by 57Fe-Mossbauer spectroscopy, X-ray diffractometry, differential thermal analysis (DTA) and DC two- and four-probe method. From DTA study, thermal stability of 20RFV glasses is lower than that of 40RFV glasses by evaluating Hruby parameter (K gl). Constant activation energy for crystallization (E a) of 2.5 eV obtained from both 20RFV and 40RFV glasses indicate that the crystallization proceeds with the cleavage of Fe–O bond having the energy of 2.6 eV. Isochronally annealed 20RFV glass at 400–450 °C resulted in the increase in electrical conductivity (σ) from the order of 10−3 to 10−1 S cm−1, whereas slight decrease in σ was observed for 20RFV glass annealed above 460 °C. A paramagnetic doublet with an identical isomer shift (δ) of 0.39 mm s−1 was observed in the 57Fe-Mossbauer spectra of 20RFV glass after isothermal annealing conducted at 400–450 °C for 100 min, which caused a decrease of quadruple splitting (Δ) from 0.67 to 0.52 mm s−1 for 20LiFV glass and from 0.66 to 0.53 mm s−1 for 20NaFV glass. On the other hand, three paramagnetic doublets with δ and Δ of 0.40 and 0.25, 0.38 and 0.60, and 0.31 and 1.11 mm s−1 respectively were observed for 20RFV glass annealed at 460–550 °C, reflecting precipitation of semiconducting FeVO4 phase having σ of 6.0 × 10−7 S cm−1. It can be concluded that isochronal annealing of 20RFV glass below 450 °C resulted in increase in σ due to the structural relaxation, while annealing above 500 °C resulted in the decrease of σ due to the precipitation of FeVO4 phase.