Abstract
Magnetic and thermoelectric properties of Nd1−xCaxBaCo2O5.5 and Nd1−xCaxCoO3 have been studied. Ca doping in Nd1−xCaxBaCo2O5.5 (x ≤ 0.2) preserves the metal to insulator transition (MIT) at 340–360 K. While the antiferromagnetic state disappears upon doping, the Curie temperature is increasing and becomes close to MIT for x > 0.12. The magnetic susceptibility of Nd1−xCaxCoO3 is paramagnetic for x up to 0.2, similar to the parent compound, with some indication of cluster-glass-like behavior at temperatures below 30 K. The increasing effective paramagnetic moments with doping suggest a low spin state of Co3+ and a high spin state of Co4+. Maximum observed ZT reaches a value close to 0.2 for x = 0.15 at 800 K, which is one of the highest values for perovskite cobaltites.
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