119Sn and 57Fe Mӧssbauer study of highly conductive vanadate glass
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57Fe Mössbauer spectra of 20BaO·3SnO2·7Fe2O3·70V2O5 glass measured at room temperature shows a marked decrease in quadrupole splitting (Δ) of iron (III) from 0.71 to 0.57 and 0.58 (± 0.02) mm s−1 after isothermal annealing at 500 °C for 30 and 60 min, respectively. The Δ value also shows a decrease from 0.71 to 0.65 and 0.60 (± 0.02) mm s−1 after the annealing at 450 °C for 30 and 60 min, respectively. These results reflect decreased distortion of FeO4 and VO4 tetrahedra or structural relaxation of the 3D-network (skeleton). 119Sn Mössbauer spectra of 20BaO·3SnO2·7Fe2O3·70V2O5 glass are comprised of a broad singlet due to distorted SnIVO6 octahedra, which have an identical Δ of 0.51 (± 0.02) mm s−1 irrespective of the annealing at 450 and 500 °C. These Mössbauer studies reveal that FeIII atoms occupy network former (NWF) sites by sharing corner oxygen atoms with VIV and VV atoms to constitute the 3D-network, while SnIV atoms are ionically bonded to the oxygen atoms at “interstitial” sites to play a role of network modifier (NWM). A marked decrease in the DC-resistivity (ρ) from 1.7 MΩ cm to 5.6 and 5.0 Ω cm was observed at room temperature after isothermal annealing at 500 °C for 15 and 30 min, respectively. This is attributed to an increase in the carrier mobility associated with the wide isotropic 5s orbitals of SnIV atoms occupying NWM sites in addition to the increased carrier density caused by the structural relaxation of the 3D-network.
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