High temperature thermoelectric power study on calcium substituted lanthanum manganites

Abstract

Compositional and temperature dependent thermoelectric power measurements have been performed on the polycrystalline manganite perovskite system: La1−xCaxMnO3+δ (x = 0.00, 0.25, 0.33, 0.50 and 1.00), in the paramagnetic state (T > Tc) (300–673 K). The Seebeck coefficient (α) has been used to calculate the Fermi energy, the activation energy (Eα), the Mn4+-ion concentration and the oxygen content (δ), for different compositions. The observed decrease in Eα with increase in Ca-concentration and a large value of Eα as compared with a relatively high temperature sintered material of the same compositions have been explained in the light of the decrease in the interatomic distances and grain growth with Ca-substitution. It is shown that [Mn4+][Mn3+] ion concentration is not the only factor that governs conduction in the manganite perovskites. The significantly small values of α (−22 µV K−1) at high temperature can be explained on the basis of (Mn3+–Mn3+) charge disproportionation into stable Mn2+–Mn4+pairs in the system rather than on the basis of nominal Mn3+/4+ valence arguments. Finally, nearly temperature independent variation of α, charge-carrier concentration at high temperature and low value of mobility are hallmarks of small polaron hopping.