Thermoelectric and Magnetic Properties of Ca0.98RE0.02MnO3−δ (RE = Sm, Gd, and Dy)
Polycrystalline samples of Ca0.98RE0.02MnO3−δ (RE = Sm, Gd, and Dy) have been prepared by conventional solid-state reactions and their properties measured at 300 K to 700 K. All samples were single phase with orthorhombic structure. The average valence and oxygen content of Ca0.98RE0.02MnO3−δ were determined by iodometric titration. Doping at the Ca site by rare-earth metals causes a strong decrease of electrical resistivity due to the creation of charge carrier content by Mn3+ in the Mn4+ matrix, as evidenced by iodometric titration results. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers are electrons over the entire temperature range. Among the doped samples, Ca0.98Dy0.02MnO3−δ had the highest dimensionless figure of merit of 0.073 at 612 K, representing an improvement of about 115% with respect to the undoped CaMnO3−δ sample at the same temperature. All the samples exhibited an antiferromagnetic transition with Néel temperature of around 120 K. Magnetization measurements indicated that Ca0.98RE0.02 MnO3−δ samples exhibited a high-spin state of Mn3+.