Improved electrical transport properties of polycrystalline La(CaSr)MnO ceramics by AgO doping.

Polycrystalline La(CaSr)MnO:mol%Ag (LCSMO:Ag , = 0, 0.1, 0.2, 0.3 and 0.4) ceramics were synthesized by the sol-gel technique. Structural, electrical and magnetic properties of the LCSMO:Ag ceramics were investigated in detail. X-ray diffraction (XRD) data analyses revealed that all the samples were crystalized in the orthorhombic structure with space group of . With the increase in Ag doping (), the grain sizes of the LCSMO:Ag samples increased and the amount of grain boundaries (GBs) decreased accordingly. At the same time, the Mn-O bond distance and the Mn-O-Mn bond angles changed correspondingly, leading to the slight increase in the lattice constants (, and ) and slight expansion of cell volume (). For the LCSMO:Ag sample with = 0.3, the optimal values of temperature coefficient of resistivity (TCR) and magnetoresistance (MR) reached 16.22% K (265.1 K) and 42.07% K (270.48 K), respectively. In addition, the fitting analysis of - curves showed that the experimental data were consistent with the theoretical calculation data. In the < (metal-insulator transition temperature) region, the electrical conduction mechanism of LCSMO:Ag was clarified by electron-magnon, electron-electron and electron-phonon scattering. In the > region, the resistivity data were interpreted by using the adiabatic small-polaron hopping model. Furthermore, in the entire temperature range, the phenomenological equation called the percolation model was used to explain the resistivity data and the phase-separation mechanism of ferromagnetic metallic (FM) and paramagnetic insulating (PI) phases. All the obtained results indicated that the improvement in the electrical properties of the LCSMO:Ag samples was attributed to the doping of Ag, which changed the A-site (La, Ca and Sr ions) average ion radius, the Mn-O-Mn bond angles and the Mn-O bond distance. In addition, the grain size increased, which led to improvement in the Mn ion concentration and the GBs connectivity in the LCSMO:Ag polycrystalline ceramics.