Size mismatch, grain boundary and bandwidth effects on structural, magnetic and electrical properties of Pr0.67Ba0.33MnO3 and Pr0.67Sr0.33MnO3 perovskites

We have studied the effect of size mismatch on the structural, magnetic and electrical properties of Pr_0.67Ba_0.33MnO₃ (PBMO) and Pr_0.67Sr_0.33MnO₃ (PSMO) perovskites with considerable difference in their variance σ² values. Samples were prepared by ceramic route at 1200 °C. Morphological study, microstructure and chemical composition were examined by scanning electron microscopy, EDX analyses and Rietveld structure refinement. Both compounds exhibit single orthorhombic Pnma crystalline phase and with strongly connected and larger grains for PSMO than for PBMO that gives a beginning of single crystal growth in PSMO case. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour, and with higher transition temperatures than those usually reported in the literature. Extrinsic effects, however, are not observed in the lower σ² PSMO sample. Both compounds exhibit Curie temperature (T_C) values significantly higher than those reported in the literature, and with higher T_C for PSMO due to its larger bandwidth W. The experimental paramagnetic effective moment for PSMO is very close to the theoretical one, whereas there is a significant difference between these moments for PBMO sample, probably due to the considerable size mismatch effect between Pr and Ba, and to the ferromagnetic correlations in the paramagnetic state. Ferromagnetic-metallic regime in the two compounds seems to emanate from the electron-(phonon, magnon) scattering processes with a larger effect for PBMO than that for PSMO, due to the prominent role of the grain boundary in PBMO. Above paramagnetic-insulating transition temperature the data were well fitted by both variable range hopping (VRH) and small polaron hopping (SPH) models giving higher density state, and lower activation energy and Mott temperature T₀ for PSMO than those for PBMO, essentially due to their considerable difference in their variance values.