  The pursuit for an appropriate universal scaling factor to satisfy the
time-temperature superposition principle for grain and grain boundary responses
has been explored in the ac conductivity domain for polycrystalline double
perovskite oxides A2HoRuO6 (AHR; A = Ba, Sr, Ca). The samples show different
structural phases ranging from cubic to monoclinic with decreasing ionic radii.
The degree of distortion in the materials is correlated to the strength of
bonding through the bond valence sum (BVS). The conductivity spectra for all
the samples obey the power law behaviour. The contribution of different
microstructural features to the conduction process is established. Thermal
variation of dc resistivity points towards a gradual crossover from nearest
neighbour to variable range hopping. The activation energies obtained from dc
conductivity, hopping frequency and relaxation frequency show close correlation
between the conduction and relaxation mechanisms. The scaled conductivity
curves for AHR showed the presence of two different conduction processes with
dissimilar activation energies in the grain boundary and grain response
regimes. It is thus concluded that a single scaling parameter is insufficient
to satisfy the time temperature superposition principle universally when two
different thermally activated regions are present simultaneously in the
materials.
