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Correlation between structural changes and electrical properties of crystallized iron phosphate glass

The emergence of nanocrystals in glass leads to changes in the properties thus opening new fields of the application and research of glass-ceramics. Iron phosphate glass of the composition 40Fe2O3- 60P2O5 (mol%) was crystallized at various temperatures for various times (1, 6, 24 and 48 h). The structural changes and evolution of crystalline phases formed in glass-ceramics were studied by means of X-ray diffraction, Raman and Mossbauer spectroscopy. Depending on crystallization temperature and time three crystalline phases are found to be present in crystallized samples. Impedance spectroscopy was used for investigation of electrical conductivity over vide temperature and frequency range. In the early stage of crystallization at about first crystallization temperature, TC1, the electrical conductivity decreases as a results of formation of randomly dispersed Fe3(P2O7)2 grains and a decrease of Fe2+-Fe3+ pairs concentration in glassy phase. The temperature at which the minimum in electrical conductivity appeared strongly depends on the amount of crystallinity. Above 15% of crystallinity the electrical conductivity increases as the grains of Fe3(P2O7)2 are better defined. At second crystallization temperature, TC2, the formation of second and third crystalline phase causes further increase in the electrical conductivity. In order to distinguish between crystalline grains/residual glassy phase and grain boundary contributions the impedance spectra were fitted to equivalent circuits. It was found that although the total electrical conductivity is higher above TC2, the grain boundary contribution is low due to a large grain boundary resistance. It seems that the imperfect crystalline grain contacts in these polycrystalline samples have an effect on the grain boundary transport.

Iron phosphate glass; glass-ceramics; structural changes; electrical properties

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