Mobile ion crossover effects in the system LiFPbF₂Al(PO₃)₃ from electrical, mechanical and T_g studies

Fluorophosphate glasses in the system LiFPbF₂Al(PO₃)₃ have been synthesized and the glass transition temperatures, de conductivities, and mechanical and electrical relaxations have been studied. The dc electrical conductivity shows a sharp minimum as a function of composition similar to that observed in mixed alkali glasses. It is likewise here attributed to the presence of two mobile ions (Li⁺ and F^-) but primarily through their effect on the liquid structural relaxation rather than on their individual mobilities. It may therefore not imply a crossover in Li⁺ and F^- mobilities. The electrical and mechanical relaxation behavior have been compared in the isofrequency-inverse temperature representation. Phenomena characteristic of the mixed effect are found; the most probable relaxation times for the two responses show a crossover. The electrical relaxations in some of these mixed conducting glasses are among the broadest yet observed, yet the maximum width is not found at the conductivity minimum. It is suggested that the halfwidth maximum rather than the conductivity minimum may signal the mobility crossover.