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The aurivillius family of compounds SrBi 4 Ti 4 O 15 (SBTi) and SrBi 4 Ti 3.8 Nb 0.2 O 15 has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang-Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher's universal power law. The frequency exponent (n), pre-exponential factor (A), and bulk dc conductivity (s dc ) are determined from the fitting curves of Jonscher's universal power law. From the frequency exponent (n) versus temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300-475 C) to small-polaron tunneling (475-550 C), and in that of the niobium doped it is small-polaron tunneling (300-375 C) to correlated band hopping (375-550 C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti 4+ /Ti 3+ ions.

Oxide thermoelectric ceramics attract extensive research interest due to their excellent thermal stability, non-toxicity, cost-effectiveness, and earth-abundant constituent elements. In the present work, the authors study the effect of lanthanum (La) and samarium (Sm) co-doping on the structural, microstructural, and thermoelectric properties of CaMnO 3 ceramics. The X-ray diffraction (XRD) studies show an increase in unit cell volume with an increase in Sm concentration. A shift in Raman modes is observed, confirming distortions in MnO 6 octahedra with the substitution of rare-earth elements. The co-doping strategy leads to a two-order increment in electrical conductivity and reduction in thermopower as expected due to the increase in Mn 3+ and charge carrier concentration. The highest electrical conductivity value attained is ≈200 S cm −1 at around 570 K for La 0.025 Sm 0.1 Ca 0.875 MnO 3 . The maximum value of thermoelectric power factor achieved is 194 𝝁W m -1 K -2 , which is two orders higher than that of the undoped CaMnO 3 .

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