Electrical relaxation in mixed lithium and sodium iron phosphate glasses

The electrical conductivity of mixed-alkali, lithium and sodium, iron phosphate glasses has been studied in the frequency range from 0.2 to 200 kHz and over a temperature range from 323 to 493 K. The AC conductivity as a function of temperature was divided into two domains, one where the absolute magnitude of the AC conductivity was close to the DC conductivity and another where it was larger than the DC conductivity. The DC conductivity for the mixed-alkali, lithium and sodium, iron phosphate glasses is independent of the Li/Na ratio and there is no evidence of any mixed-alkali effect. The lithium and sodium ions have such a low mobility in both single- and mixed-alkali iron phosphate glasses that they make no detectable contribution to the total conductivity that is of electronic origin. Mössbauer spectral analysis indicates the presence of both Fe2+ and Fe3+ ions.