Abstract
Polycrystalline samples of the mixed-valence perovskite-type manganites (Sm0.7A0.3)MnO3 (A=Ca2+,Sr2+,Ba2+,Pb2+) have been synthesized and their transport properties studied. The electrical conductivity of the series involves a variable-range hopping mechanism in the paramagnetic state, which extends well below TC for three of the samples. Only (Sm0.7Sr0.3)MnO3 shows a metal-insulator transition. There is a peak in the resistivity at a temperature Tp, just below the Curie point, TC=70 K with a transition from thermally activated type to metallic conductivity at low temperature with a large residual resistivity, ρ0=6×10−4 Ω m. By comparison with the (Ln0.7A0.3)MnO3 compounds, Ln=La, Pr, Nd the samarium compounds are more localized. The decrease of both Tp and TC with decreasing lanthanide radius and increasing mass depends on the nature of the substituted divalent cation, but the data cannot be scaled on a universal curve with respect to average ionic radius or tolerance factor.
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