Enhanced thermoelectric power factor led by isovalent substitution in Sr2CrMoO6 double perovskite

Abstract

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 (TE) power factor of double perovskites. Ceramic samples of Ba x Sr2−x CrMoO6 (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 x-ray diffraction (XRD) and field scanning electron microscope (FESEM). Rietveld refinement of XRD data confirms the polycrystalline cubic structure with Pmm space group. Negative values of Seebeck coefficient have been observed for these oxides in the temperature range from room temperature to 1100 K, 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 TE power factor has been attained in Ba x Sr2−x CrMoO6. Charge transport mechanism of these double perovskites is found to be governed by the small polaron hopping conduction model. x-ray photoelectron spectroscopy spectra validate the presence of multivalent cations of Mo5+, Mo6+, Cr3+, and Cr6+ in these double perovskites. Furthermore, the detailed defect chemistry analysis suggests that owing to Ba substitution, Cr is oxidized from Cr3+ to Cr6+ oxidation states, which enhances the electron concentration and reduces the low mobility oxygen vacancies leading to dramatically improved electrical conductivity.