engleski

Transition from electronic to ionic conduction in Na2O-PbO-Fe2O3-P2O5 glasses

Iron phosphate glasses behave as typical semiconductors, where the electron conduction is thermally activated electron hopping from Fe(II) to Fe(III) ions. On the other hand, electrical conductivity of alkali containing iron phosphate glasses consists of both, electronic and ionic conduction. The ionic conduction should be proportional to the concentration and mobility of alkali ions, whereas, the electronic conduction follows the electron hopping theory. The electrical properties of the xNa2O• (100-x)• [28.3PbO• 28.7Fe2O3• 43.0P2O5], (0 &pound ; ; ; ; ; ; ; ; x &pound ; ; ; ; ; ; ; ; 30), glasses were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and the temperature range from 303 to 473 K. The conductivity for glasses containing &pound ; ; ; ; ; ; ; ; 15 mol% Na2O is predominantly electronic and is controlled by electron hopping between Fe(II) and Fe(III) ions. In these glasses the sodium ions have such a low mobility, caused by ion-polaron interaction, that they make no detectable contribution to the total conductivity. For Na2O contents > 15 mol%, the conductivity increases significantly due to an increase in the sodium ion mobility. The increasing concentration of sodium ions increases the degree of disorder in the glass network, with an increase in the number of non-bridging oxygens. This in turns enhances the pathways suitable for migration of the sodium ions responsible for an increase in the ionic conductivity. The structural changes in these glasses have been investigated by Raman and IR spectroscopy. The Raman spectra show that with increasing Na2O content there is corresponding reduction in number of the Q1 phosphate units and an increase in non-bridging oxygens as more Q0 phosphate units are formed in the glass network. The decrease in glass temperature, Tg and glass density, D, is due to the lower degree of cross-bonding between the sodium and non-bridging oxygens in Q0 phosphate units resulting in a weakening of the glass network.

iron phosphate glasses; Raman spectroscopy; Impedance spectroscopy

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