Evidence for site memory effects in the ionic relaxation of (Li₂O)_x(Na₂O)_y(GeO₂)_1-x-y glasses

We report measurements of the dielectric response due to ionic relaxation in alkali germanate glasses (Li₂O)_x(Na₂O)_y(GeO₂)_1-x-y. The relaxation is investigated in both electric modulus and ac conductivity formalisms, between which important discrepancies are observed. In single alkali glasses, the width of the electric modulus decreases with decreasing alkali concentration. However, the power law dispersion of the ac conductivity scales regardless of the alkali concentration. This scaling property of the ac conductivity implies that inter-ionic interactions are not responsible for the non-Debye relaxation. In mixed alkali glasses, the exponent of the power law dispersion of the ac conductivity decreases and the dielectric relaxation strength increases relative to the single alkali glasses. With increasing total alkali concentration in the mixed alkali glasses, the frequency of maximum dielectric loss decreases, approaching frequencies comparable to that of the mixed alkali mechanical loss peak. These findings indicate a transition of the ionic relaxation toward a slower polarization process which is consistent with site memory mechanisms recently proposed for the mixed alkali effect.