Improved electrical transport properties of polycrystalline La_0.8(Ca_0.12Sr_0.08)MnO₃ ceramics by Ag₂O doping

Polycrystalline La_0.8(Ca_0.12Sr_0.08)MnO₃:mol%Ag (LCSMO:Ag , x = 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 Pnma. With the increase in Ag doping (x), 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 (a, b and c) and slight expansion of cell volume (V). For the LCSMO:Ag sample with x = 0.3, the optimal values of temperature coefficient of resistivity (TCR) and magnetoresistance (MR) reached 16.22% K^-1 (265.1 K) and 42.07% K^-1 (270.48 K), respectively. In addition, the fitting analysis of ρ-T curves showed that the experimental data were consistent with the theoretical calculation data. In the T < T _MI (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 T > T _MI 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 Mn⁴⁺ ion concentration and the GBs connectivity in the LCSMO:Ag polycrystalline ceramics.