Crystallization and structural relaxation of xBaO · (90-x)V 2O5 · 10Fe2O3 glasses accompanying an enhancement of the elctric conductivity

Abstract

Structure of electrically conductive xBaO (90 - x) V2O 5 10Fe2O3 glasses (x = 20, 30 and 40), NTA glass™, was investigated by 57Fe-Mössbauer spectroseopy, Fourier-transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), differential thermal analysis (DTA) and scanning electric micrography (SEM). A remarkable increase in the electric conductivity (σ) was observed from 1.6 × 10-5 to 1.1 × 10° S cm-1 after annealing NTA glass™ with 'x' of 20 at 500°C for 1000 min. Precipitation of a small amount of BaFe2O4 and BaV 2O6 phases was confirmed by XRD, indicating that the sample still has a glassy phase even after prolonged isothermal annealing. An activation energy for the crystallization (Ea) was estimated to be 2.3 ±0.2 eV, indicating that the crystallization of the vanadate glasses is involved in a cleavage of Fe-O bond having binding energy of 2.6 eV, not of the V-O bond of which binding energy is known to be 3.9-4.9 eV. A consistent result was obtained in the FT-IR spectra, showing that the absorption band attributed to a stretching vibration of Fe-O bond of FeO6 octahedra appeared after the annealing. By applying 'Tg-δ rule', i.e., a linear relationship between glass transition temperature and quadrupole splitting of the Mossbauer spectra, it proved that FeIII plays a role of a network former (NWF) like VIV and V4 in NTA glass™. A constant isomer shift of 0.40 ± 0.01 mm s-1 and a decrease of quadrupole splitting from 0.70 ± 0.02 to 0.53 ± 0.02 mm s-1 were observed in the Mössbauer spectra after the isothermal annealing. These results suggest that σ of NTA glass™ is enhanced as a result of structural relaxation, i.e., reduced local distortion of the network composed of FeIIIO4, VO4 and VO5 units.