A-site Pb doping effect on structural, microstructural and magnetotransport properties of La0.5Sm0.2Ca0.3-xPbxMnO3 (x=0, 0.05, 0.10) manganite

Structural, microstructural and magneto-transport properties of polycrystalline La_0.5Sm_0.2Ca_0.3-x Pb_xMnO₃ (x = 0, 0.05 and 0.10) samples, prepared by conventional solid state reaction, were studied. Refinement of the Powder X-ray diffractograms revealed that all the samples crystallize into orthorhombic structure with Pnma space group and the lattice parameters increase with increasing Pb content. The scanning electron microscope (SEM) micrographs show a granular character and the estimated grain sizes ranges between 10 μm and 20 μm. The experimental electrical resistivity and the magnetoresistance in the temperature range 20 K–250 K under magnetic field of zero and 1 Ta, have been recorded using four probe technique. The resistivity curves without magnetic field, exhibit a ferromagnetic-metallic (FM) to paramagnetic-insulating (PI) transition at T_MI = 125 K, 91 K and 37 K for x = 0, 0.05 and 0.10, respectively. The magnetoresistance reaches 58% for undoped sample and decrease with Pb doping under 1T. The resistivity values increases drastically with Pb doping. These two results were explained mainly by the increase of the disorder ${\sigma }^2$ and $<r_A>$. In the low temperature range (T < T_MI) and for x = 0 and 0.05 samples, the resistivity curves is well fitted by a combination of the residual resistivity, weak localization and electron-electron scatterings; in addition to small polaron contribution or electron-phonon interaction respectively. In the high temperature regime T > T_MI, all our resistivity curves has been analyzed using adiabatic small polaron hopping model (ASPH) above θ_D/2 and variable range hopping model (3D-VRH) (T_MI < T < θ_D/2). An attempt to use the percolation model to describe the resistivity data in the entire temperature range for both x = 0 and 0.05 samples, was established. Finally, density of state, mean hopping distance Rh and mean hopping energy Eh were calculated and discussed.