Electrical transport in rare earth ortho-chromites, -manganites and -ferrites

Rare earth ortho-chromites, -manganites and -ferrites are p-type semiconductors with conductivities in the range 10−4–10−1 ohm−1 cm−1. The conductivity in each series of perovskites decreases with the increasing atomic number of the rare earth. The ionic contribution to conductivity is small in all the three series of solids. None of these solids exhibits intrinsic behavior up to ∼1000°C. The conductivity behaviors of these rare earth compounds reflect the known crystallographic, dielectric and magnetic transitions in these materials. Seebeck coefficients in these compounds are large, typical of narrow-band materials; the Seebeck coefficients show marked changes at temperatures where magnetic and dielectric transitions occur. The electrical transport properties of all the three series of rare earth compounds are essentially controlled by the d-electrons of the transition elements which show localized behavior. This conclusion is in agreement with the results from optical spectra and the predictions of Goodenough. In all these compounds small polarons seem to be responsible for the conduction.