engleski

Electrical Conductivity in Iron Phosphate Glasses: Insights into the Role of B2O3 and HfO2 from Model- Free Scaling Analysis of Conductivity Spectra

Iron phosphate-based glasses (IPGs) belong to a family of electronically conducting amorphous materials. Due to the presence of transition metal (TM) ions, e.g. iron, in more than one valence state, small polaron hopping (SPH) conduction mechanism is observed. These materials are of boundless scientific interest due to their potential application as electrode materials for batteries, electronic circuits elements, etc. In this work, we report the effect of the addition of modifiers and network formers on the polaronic transport in binary IPG in which up to 8 mol% of boron and hafnium oxide are added The electrical properties of two glass systems HfO2–B2O3–Fe2O3–P2O5 are studied in detail by Solid State Impedance Spectroscopy (SS-IS) in a wide frequency and temperature range, revealing interesting features behind the polaronic transport. The addition of oxides significantly alters the Fe2+/Fetotal ratio, which directly affects the polaron number density and consequently strongly controlls the DC conductivity trends for both glass-series. Moreover, we found that short-range polaron dynamics are also under the influence of the induced structural changes.Therefore, we have studied them in detail using relatively simple but very informative model-free scaling procedures, Summerfield and Sidebottom scaling. Both scaling procedures of the conductivity spectra confirmed the validity of the time-temperature superposition principle for all glasses. An attempt to construct a super-master curve revealed that in addition to the change in polaron number density, the hopping lengths of the polarons also change, and Sidebottom scaling yields a super-master curve. In the next step, the spatial extent of the localized motion of the polarons is correlated with the polaron number density and two distinct regions containing glasses with low and high polaron concentration are observed. The transition between these regions is not only related to the polaron number density, but also to the combination of glass compositions and parameters that have an impact on the polaron motions. The large increase in the spatial extent of the polaron hopping jump could be related, either to the structural changes caused by the addition of HfO2 and B2O3 and their effects on the formation of polarons, or to the inherent property of polaron transport in IP glasses with low polaron number density. The obtained results provide new valuable information about the resulting mixed glass network and its impact on the polaron formation and its dynamics.

Iron-phosphate glass ; polaronic transport ; impedance spectroscopy ; model-free scaling

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