engleski

Ionic conductivity in mixed-alkali aluminophosphate glasses

The mixed-alkali effect (MAE) is nonlinear deviation of the ionic conductivity observed when one type of alkali cation is replaced by another at their constant total cation content. Various models try to describe this effect but a comprehensive understanding is still lacking. In this work, three series of mixed-alkali aluminophosphate glasses, xLi2O-(40-x)Na2O- 10Al2O3-50P2O5, xLi2O-(40-x)K2O- 10Al2O3-50P2O5 and xNa2O-(40-x)K2O-10Al2O3-50P2O5, x=0-40 mol%, are examined to clarify the influence of cation size mismatches on the extent of MAE. Electrical properties of glasses are studied by impedance spectroscopy (IS) in a wide frequency (0.01 Hz – 106 Hz) and temperature (303 K – 523 K) range, whereas Raman spectroscopy is used to obtain information about glass network and different cation environment. Raman spectra show two most intensive bands corresponding to vs(P-O-P) and vs(P-O2) stretching vibrations of bridging and non- bridging oxygen atom in metaphosphate glasses, respectively. Both of the vibrational frequencies are sensitive to the cation environment and are shifted to the lowest frequency with addition of smaller cations. In all three series of glasses the results of IS measurements revealed strong MAE with DC conductivity minimum at 0.5 of cation substitution. The change in extent of MAE corresponds to the difference in cation size mismatches ; that is a deepest minimum obtained for the Li-K series. A possible explanation of the effect assumes that ion hopping probability is reduced because of the mismatched energy sites adopted for more dissimilar cations.

Mixed-alkali effect ; electrical properties ; Raman spectroscopy

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