Pressure dependent conductivities and activation volumes in Li_xNa_(1-x)PO₃ glasses: evidence for a new matrix-mediated coupling mechanism in mixed-cation glasses?

Ionic conductivities in a range of LixNa(1-x)PO3 glasses are reported as a function of both temperature and pressure. This series of mixed-cation glasses characteristically exhibits minima in glass transition temperatures and in ionic conductivities, and a maximum in the activation energy for ionic conductivity. There is also a maximum in the corresponding activation volume (DeltaV*), where DeltaV*(max) congruent to DeltaV*(Li glass) + DeltaV*(Na glass). It is suggested that this approximate equality is evidence for direct coupling between Li+ and Na+ ion motions, which is mediated through the glass matrix. This coupling process directly influences the mechanism of ion transport in mixed-cation glasses and could be responsible for many features of the mixed-alkali effect.