Structural relaxation and electrical conductivity of molybdovanadate glass

Abstract

57Fe Mossbauer spectrum of conductive barium iron vanadate glass with a composition of 20BaO·10Fe2O3·70V2O5 (in mol%) showed paramagnetic doublet peak due to distorted FeIIIO4 tetrahedra with isomer shift (δ) value of 0.37 (± 0.01) mm s−1. Mossbauer spectra of 20BaO·10Fe2O3·xMoO3·(70 − x)V2O5 glasses (x = 20–50) showed paramagnetic doublet peaks due to distorted FeIIIO6 octahedra with δ’s of 0.40–0.41 (± 0.01) mm s−1. These results evidently show a composition-dependent change of the 3D-skeleton structure from “vanadate glass” phase, composed of distorted VO4 tetrahedra and VO5 pyramids, to “molybdate glass” composed of distorted MoO6 octahedra. After isothermal annealing at 500 °C for 60 min, Mossbauer spectra also showed a marked decrease in the quadrupole splitting (Δ) of FeIII from 0.70 to 0.77 to 0.58–0.62 (± 0.02) mm s−1, which proved “structural relaxation” of distorted VO4 tetrahedra which were randomly connected to FeO4, VO5, MoO6, FeO6 and MoO4 units by sharing corner oxygen atoms or edges. DC-conductivity (σ) of barium iron vanadate glass (x = 0) measured at room temperature was 3.2 × 10−6 S cm−1, which increased to 3.4 × 10−1 S cm−1 after the annealing at 500 °C for 60 min. The σ’s of as-cast molybdovanadate glasses with x’s of 20–50 were ca. 1.1 × 10−7 or 1.2 × 10−7S cm−1, which increased to 2.1 × 10−2 (x = 20), 6.7 × 10−3 (x = 35) and 1.9 × 10−4 S cm−1 (x = 50) after the annealing at 500 °C for 60 min. It was concluded that the structural relaxation of distorted VO4 tetrahedra was directly related to the marked increase in the σ, as generally observed in several vanadate glasses.