  Electrical modulus analysis of the frequency dependent conductivity data over
the range from 100 Hz to 10 MHz and at temperature range of 140 oC to 300 oC
were carried out on the mixed alkali vanadate glass samples having lithium and
rubidium as alkali elements. The analysis of the data is done using
Kohlrausch-Williams-Watts (KWW) equation which describes the relaxation
behavior of the mobile species. We have used the Bergman equation (R. Bergman,
J. Apl. Phys. 88 (2000) 1356.) for fitting the imaginary part of the electrical
modulus. The stretching parameter beta derived from this fit shows a nonlinear
variation with respect to alkali content which is attributed to mixed alkali
effect. The variations of beta with respect to alkali element concentration is
analyzed on the basis of alkali ion distance and molar volume. It is found that
the coupling between the mobile ions is better at 50 mol percentage of the
alkali mixture when compared to single alkali concentrations. It is observed
that coupling between the ions are weak in case of all Rubidium glass than all
Lithium or Lithium-Rubidium mixture, even though the alkali concentration is on
the higher (30 mol percentage) side. The contributions from fast and slow types
of relaxation mechanisms were determined. It again shows that at low
concentrations of Rubidium ions, there exists a good coupling between alkali
ions and contribute to a macroscopic relaxation time responsible for the
stretching, whereas at high concentrations of Rubidium ions the relaxing sites
decouple and relax with a microscopic relaxation time. A scaling of modulus
spectra with respect to frequency shows that stretching parameter beta is
slightly dependent on temperature.
