engleski

Electrical Transport in Iron (Boro)Phosphate Glasses Containing HfO2 and CeO2

Phosphate glasses containing transition metal oxide such as Fe2O3 exhibit polaronic conductivity as a result of electron hopping from Fe2+ to Fe3+ ions, which is described by small polaron hopping model. The electrical and dielectric properties of three series of glasses (11 samples), HfO2-Fe2O3-P2O5, CeO2-Fe2O3-P2O5, and HfO2-Fe2O3-B2O3-P2O5, were investigated using impedance spectroscopy over a wide frequency (0.01 Hz-1 MHz) and temperature range (303-513 K), whereas fraction of ferrous ions was determined by Mössbauer spectroscopy. Detailed analysis of conductivity and permittivity spectra was done to gain a better insight into factors that possibly influence polaron transport. In all three glass systems with wide variation of Fe2+ content a maximum in conductivity is observed at slightly below Fe2+/Fetot≈0.5 which confirms that electrical transport is strongly controlled by polaron concentration via fraction of ferrous ions, Fe2+/Fetotal, and overall Fe2O3 content. Conductivity spectra for all glasses were analyzed in the model-free approach with various scaling procedures and in addition were also modelled with MIGRATION concept. It is found that in each series of glasses the shape parameter describing conductivity dispersion remains the same indicating that the mechanism of conductivity is unchanged. Sidebottom scaling of conductivity spectra which takes into account experimental dielectric strength parameter showed that it is possible to obtain super-master curve, whereas with Summerfield scaling additional shift is required on the frequency axis. Observed feature implies that for all our systems not only does the charge carrier concentration change with composition, but also the typical length of a polaron hop. Furthermore, characteristic spatial extent of localized hopping of polarons is calculated from scaled experimental permittivity spectra and it shows a decreasing trend with increasing polaron concentration. Obtained values lie in the range of values for polaron radius calculated from equation proposed by Bogomolov. This study revealed that electrical transport in these polaronic glasses is, in addition to polaron concentration, also influenced by polaronic hopping distance which is changing with composition due to an alteration in distribution of iron ions in the glass matrix.

iron phosphate glasses ; polaronic conductivity ; scaling properties

nije evidentirano