TRANSPORT-PROPERTIES OF (SM(0.7)A(0.3))MNO3 (A=CA2+,SR2+,BA2+,PB2+)

Polycrystalline samples of the mixed-valence perovskite-type manganite s (Sm(0.7)A(0.3))MnO3 (A=Ca2+,Sr2+,Ba2+,Pb2+) have been synthesized an d their transport properties studied. The electrical conductivity of t he series involves a variable-range hopping mechanism in the paramagne tic state, which extends well below T-C 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 T-P, just below the Curie point, T -C=70 K with a transition from thermally activated type to metallic co nductivity at low temperature with a large residual resistivity, rho(0 )=6x10(-4) Omega m. By comparison with the (Ln(0.7)A(0.3))MnO3 compoun ds, Ln=La, Pr, Nd the samarium compounds are more localized. The decre ase of both T-P and T-C with decreasing lanthanide radius and increasi ng 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. (C) 1997 American Institute of Phys ics.