Effect of A- or B-site doping of perovskite calcium manganite on structure, resistivity, and thermoelectric properties

Bismuth-, lanthanum-, and molybdenum-doped calcium manganite (CaMnO3, abbreviated Mn113) are synthesized by solid-state synthesis route from their respective oxide precursors at a same doping level (x= 0.05). Depending on the ionic sizes, trivalent dopants (Bi3+ and La3+) replace Ca2+(A site), while penta/ hexavalent dopant Mo5+/Mo6+ replaces Mn4+ (B site) in the Mn113 structure. XRD of all three doped samples confirm formation of single phase. In all three samples, doping causes unit cell volume to expand, while volume expansion is maximum for the Mo-Mn113. The transport behavior of the doped samples follows small polaron hopping mechanism. Resistivity of the doped samples depends not only on the carrier concentration but also on the effective bandwidth determined by the structural distortion introduced by the dopant ions. Bi-Mn113 has highest resistivity at the both temperature end, while La-Mn113 has the lowest. Thermopower is determined by the carrier concentration only and does not depend on dopant type, having value similar to 260 mu V/K at 1000 K. At high (> 800 K), S reaches a saturation value and becomes independent of T. La-Mn113 is having highest figure of merit (zT) 0.19 at 1000 K.