High-temperature transport properties of Ca_0.98 RE _0.02 MnO₃-δ (RE=Sm, Gd, and Dy)

We report measurements of electrical resistivity and thermopower on CaMnO₃-δ and Ca_0.98 RE_0.02 MnO ₃-δ (RE=Sm, Gd, and Dy) prepared by solid state reaction. CaMnO3-δ exhibits nonmetal-like temperature dependence of resistivity while metal-like temperature dependence of thermopower. This inconsistency can be explained by the extrinsic carriers arising from oxygen defects using two-band model. Ca_0.98 RE_0.02 MnO₃-δ exhibits metal-like temperature dependence in both resistivity and thermopower. The transition to metal-like behavior resembles the case in degenerate semiconductors and can be attributed to an impurity band formation with characteristic of hybridized Mn 3d e_g and O 2p states due to electron doping via partial substitution of lanthanides for Ca2+ and oxygen defects.