engleski

Electronic relaxation in zinc iron phosphate glasses

Zinc iron phosphate glasses have a chemical durability comparable to soda lime and borosilicate what makes them potential matrix host for vitrification of radioactive vaste. The purpose of the present study was to investigate conduction mechanism of different glass composition and the effect of ZnO on electrical properties of zinc iron phosphate glasses. Zinc iron phosphate glasses with composition (40-x)ZnO-xFe2O3-60P2O5, x=10, 20 and 30 (mol%) were prepare by melting homogeneous mixtures of reagent grade ZnO, Fe2O3, and NH4H2PO4 in dense alumina crucibles at 1100 &ordm ; ; ; ; C in air for 2 h. The melt was quenched in air by pouring it into a steel mold to form bars of. The bars were immediately transferred to a furnace and annealed at 450 &ordm ; ; ; ; C for 3 h. The structural changes in glasses with different molar compositions have been investigated by Raman spectroscopy. Electrical properties were obtained by measuring complex impedance using an impedance analyzer over wide frequency range from 0, 01 Hz to 4 MHz and in temperature range from 303 to 473 K. The complex impedance data, Z*(ω ), were plotted in the Nyquist representation form, a typical complex plane plot represented by imaginary Z’ ’ (ω ) vs. real part Z’ (ω ) for each temperature. The experimental impedance spectra were fitted using complex non-linear square (CNLSQ) fitting procedure. The band assignments in Raman spectra are characteristic of the pyrophosphate structure with most prominent bands at 1074 and 748 cm-1 related to the symmetric stretching mode of non-bridging (PO2)sym and bridging (P-O-P)sym oxygen atoms. Bands at 1215 and 620 cm-1 are attributed to the non-bridging, (PO2)sym, and bridging oxygen atoms (P-O-P)sym in metaphosphate units. An increase of Fe2O3 content and decrease of ZnO content cause the depolymerization of phosphate network from metaphosphate chains to pyrophosphate dimmers of P2O7. Frequency dependence of the real part of conductivity, σ ’ (ω ), for glass containing 10 mol% ZnO at different temperature shows at low frequency that conductivity is independent of frequency and identical to dc conductivity, σ dc, of glass, whereas, at high frequency the ac conductivity, σ ac, exhibits power-law frequency dependence. By applying Summerfield scaling on frequency dependent conductivity we have obtained one single master curve. The analysis indicates that the dc and ac conductivity processes are due to same polaron hopping mechanism between iron ions. The dependence of dc conductivity at 473 K and activation energy for dc conductivity upon the Fe2O3 content for all glasses show that the lowest value of σ dc has glass with 10 mol% Fe2O3. The σ dc increases with increasing Fe2O3 content up to 30 mol%. Similarly, Edc decreases with increasing Fe2O3 content. Since the dc conductivity is associated with the migration of polaron between Fe(II) and Fe(III) ions with distribution in hopping distance it seems that increase in Fe2O3 content from 10 to 30 mol% decreases the distance between Fe ions, RFe-Fe. Such decrease in RFe-Fe enhances the probability of electron hopping and possibility of polaron formation.

phosphate glasses; electrical conductivity; Raman spectroscopy

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