engleski

Fundamental Insights Gained on Polaronic Conduction in Pure Iron Phosphate Glasses with varying Fe2+ content from a Comprehensive Analysis of Conductivity and Permittivity Spectra using MIGRATION Concept

It is well known that binary iron phosphate (IP) glasses exhibit electronic conduction via a polaronic conduction mechanism. Since iron has two valence states, conduction takes place by thermally activated small polaron hopping from Fe2+ to Fe3+ sites. It is also well-established in literature that polaron transport strongly depends both on the overall iron oxide content and on the Fe2+/Fetot ratio. While well-established theories of polaron transport in binary IP glasses are concerned with DC conductivity, deeper insights on polaron dynamics can be gained by analysing various features of the conductivity and permittivity spectra measured over wide range of temperatures and frequencies. Using MIGRATION Concept, a general and simplistic model used to study charge transport in a wide class of materials, we modelled the conductivity spectra of four binary IP glasses: 40FeO2 – 60Fe2O3 (mol%) with Fe2+/Fetot ratio of 15, 18, 23 & 25%, and present and discuss new results that have emerged. One significant result is that in this system the increase of DC conductivity from 15 to 18 % is steeper compared to that for 18% and above, and this correlates directly to a change in the value of the shape parameter of the model spectra that reproduces experimental spectra for the two regimes. Thus, Fe2+/Fetot ratio of about 18% seems to mark a threshold in the ease of polaron conduction through the glass. This effect, as well as the trends shown by the model estimate of the spatial extent of the local excursion of the polaron, confirms the influence of the changing local structure due to varying Fe2+ fraction in these four glasses.

Iron phoshate glasses ; Electrical conductivity ; Polaronic conductivity ; MIGRATION concept

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