The narrow conduction band(s) of Ca2FeReO6 detected by thermopower

Abstract We report on the thermopower ( S ( T )) of Ca 2 FeReO 6 (CFRO) up to 400 K. The resistivity ( ρ ( T )) measurements carried out on our samples reproduced with high accuracy previous results showing a negative d ρ /d T at all T . At very low- T , S is proportional to T and d S /d T is large; a “metallic-like” behavior of S in a material with d ρ /d T k B T and a large value of d S /d T —with a shallow E F . With increasing T , S ( T ) has a maximum followed by a steep drop; above ∼200 K it levels off, indicating transport in a very narrow band. The high- T S ( T ) calculated in the atomic limit of a narrow band, including correlations, is in reasonable agreement with the measured value. The interpretation of our results at all temperatures, is consistent with the scheme of band crossing induced by the structural transition, as previously proposed. Our results suggest that the change in the electronic transport in CFRO is subtler than the previously assumed metal–insulator transition.