In this investigation, the ionic conduction mechanism in mixed ionic electronic conductors composites of Sr2TiCoO6/YSZ has been studied with the help of universal dynamic response. 3 mol% and 8 mol% yttria stabilized ZrO2 have been mixed with Sr2TiCoO6 (STC) double perovskite in 1:1 ratio to prepare STC/3YSZ and STC/8YSZ composites via solid-state reaction route. AC Impedance spectroscopy has been carried out to examine the charge transport mechanism, which has been modeled using the microstructural networks of resistors and capacitors. Grain boundaries are more resistive and capacitive compared to the bulk. Modulus spectroscopy analysis demonstrates the non-Debye character of conductivity relaxation with frequency. Complex frequency-dependent AC conductivity is found to obey Almond West power law and reveals that ion migration occurs through the correlated hopping mechanism. Further, the DC conductivity and relaxation time have been found to follow the Barton Nakajima and Namikawa relation, which is correlated with AC to DC conduction. The time-temperature superposition principle has been used to explain the conductivity scaling in the intermediate frequency range. At low temperatures, the ions are localized in the asymmetric potential well, while at high temperatures, hopping behavior starts dominating. Further Kramers–Kronig transformation connects the dielectric strength with conductivity relaxation and verifies the impedance data.

In this work, the conduction mechanism in B a x S r 2 − x CrMo O 6 (0 ≤ x ≤ 0.3) double perovskite has been investigated using different techniques, such as universal dynamic response (UDR) and modulus spectroscopy. AC impedance spectroscopy has been studied in the frequency range of 0.3 Hz–5 MHz in a wide range of temperatures. It has been found that the conduction mechanism is thermally activated and frequency-dependent. The impedance fitting response to the microstructure reveals that grain boundaries are more resistive and capacitive than grains. Analysis of the complex modulus indicates that both short- and long-range charge carrier transport is responsible for conduction with non-Debye-type response in these oxides. The permittivity analysis indicates the existence of both universalities, near constant loss and UDR. Furthermore, from the Almond West power law, hopping frequency ( ω c) and activation energy E a c have been calculated. The relaxation time and DC conductivity are found to obey Barton Nakajima and Namikawa's relation. In addition, the Kramers–Kronig relation and conductivity scaling are discussed to validate the impedance data and provide insight into the conduction processes in this mixed ionic electronic conductor.

The strategy of using aliovalent substitution in A2B2O6 double perovskites remained the popular choice to enhance the charge carrier concentration in order to increase their electrical conductivity. In the present investigation, we have shown that the isovalent substitution in A-site can facilitate in manipulating the oxidation states of B-site transition metal cations in double perovskites, which in turn helps in increasing the carrier concentration. Further, using the strategy of manipulating valence states of B-site cations, we could enhance the thermoelectric power factor of double perovskites. Ceramic samples of BaxSr2-xCrMoO6 (x=0.0, 0.1, 0.2, and 0.3) double perovskites have been synthesized via solid-state reaction route. The phase constitution and morphological study have been carried out via XRD and FESEM. Rietveld refinement of XRD data confirms the polycrystalline cubic structure with Pm3-m space group. Negative values of Seebeck coefficient have been observed for these oxides in the temperature range from room temperature to 1100K, confirming electrons as the majority charge carriers. The electrical conductivity of Sr2CrMoO6 double perovskite is found to be increased by more than an order of magnitude due to isovalent Ba2+ doping in place of Sr2+. As a result, 5 times enhancement of thermoelectric power factor has been attained in BaxSr2-xCrMoO6. Charge transport mechanism of these double perovskites is found to be governed by the small polaron hopping conduction model. XPS spectra validates the presence of multivalent cations of Mo5+, Mo6+, Cr3+, and Cr6+ in theses double perovskites. Furthermore, the detailed defect chemistry analysis suggests that owing to Ba substitution, Cr is oxidized from Cr3+ to Cr6+ oxidationstates, which enhances the electron concentration and reduces the low mobility oxygen vacancies leading to dramatically improved electrical conductivity.

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