Electrical properties of semiconducting barium-vanadate glasses doped with Fe 2O 3

Abstract

Semiconducting barium vanadate glasses doped with Fe 2O 3 ranging from 0.1 to 10 wt% were studied. Electrical DC and AC conductivity measurements, Mössbauer and EPR investigations in the Q-band were made. In addition to this, X-ray diffractograms were obtained with the heat-treated samples. The concentration dependence of DC-conductivity, σ −, exhibits a minimum at 5 wt% Fe 2O 3; the energy activation, ΔE, increases with increasing Fe 2O 3 content up to 6–7 wt% and remains unchanged at higher concentrations of Fe 2O 3; the pre-exponential term, σ 0, increases weakly in the range from 0 to 5 wt% Fe 2O 3 and then increases very rapidly. Inspection of the temperature dependence of the DC-conductivity shows that the effect of Fe 2O 3 additives on it is considerable at low temperature for small concentrations of Fe 2O 3 (⩽ 3 wt%) and remains almost unchanged at higher contents of Fe 2O 3. Above 300 K the plots of log σ − versus 1000/ T are linear for all the Fe 2O 3-doped glasses. The frequency dependence of the polarization component of conductivity, σ P, for base and Fe 2O 3-doped glasses has a region of a rapid variation of σ P. At T > 306−310 K the latter is in the low-frequency range and is shifted to higher frequencies when the temperature increases. Addition of Fe 2O 3 to the base glass leads to the very weak decrease in the σ P value. The frequency dependence of the relative dielectric constant, ϵ′, has only a single region of dispersion for each temperature for the base and for all doped glasses. Cole-Cole plots are exhibited as a circular arc. The ϵ s and ϵ ∞ values were calculated using Cole-Cole plots. Parallel measurements of magnetic susceptibility and Mössbauer spectra indicate the presence of fine particles of α-Fe 2O 3 and superparamagnetic particles for all glasses with Fe 2O 3 ⩾ 5. The average size of these particles was about 160 Å at 10 wt% Fe 2O 3 and decreased to about 100 Å in glasses with (5–7) wt% Fe 2O 3. EPR measurements at the Q-band suggest that there is a weak exchange between V 4+ and Fe 3+ ions with J ≅ 0.01 cm −1. The AC electrical measurements of barium-vanadate glasses doped with Fe 2O 3 indicate that the transport of carriers occurs through vanadium sites. The data of the DC investigation were analysed in terms of small polaron theory. The increase in the pre-exponential term was explained by the increase of the concentration of V 4+ ions with increasing Fe 2O 3 content. The increase of ΔE with increasing Fe 2O 3 concentration from 0 to 6 wt% was interpreted in terms of the increase of the distance between vanadium sites. The effect of the clusterization of Fe 3+ ions on the electrical parameters results in the reduction of the increase of the distance between vanadium sites which may be influenced by the rapid increase of the pre-exponential term σ 0 at high concentrations of Fe 2O 3.