The LaCo1−xFexO3 compounds have been investigated by means of neutron powder diffraction (NPD), x-ray powder diffraction (XPD) and magnetization measurements. The NPD and XPD patterns were successfully refined as rhombohedral (x≤0.5) and orthorhombic (x≥0.6). The temperature-induced transition from the rhombohedral phase into the orthorhombic one is characterized by a two-phase crystal structure state. Magnetization and neutron powder measurements have revealed that compounds with x<0.4 exhibit a paramagnetic-like behaviour, whereas for x≥0.4 samples a weak ferromagnetic component was observed. The NPD patterns were successfully refined by admitting a Gz spatial orientation of the antiferromagnetic vector. The magnetic properties of the LaCo1−xFexO3 samples can be explained assuming a low spin state of the Co3+ ions, whereas antiferromagnetism is caused by magnetic interactions between the Fe3+ ions. Based on the obtained data the combined crystal and magnetic phase diagram has been constructed.

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The crystal structure and magnetoresistance of are investigated. La1-xNaxMnO3 crystallizes in a rhombohedrally distorted perovskite structure and exhibits a sharp ferromagnetic transition as well as a negative magnetoresistance at around room temperature. On the basis of alternating-current susceptibility and resistivity measurements as well as a comparison with La1-xNaxMnO3 compounds, it is proposed that Na doping tends to drive the system from a regime characterized by strong Hund coupling and strong electron-phonon coupling to one characterized by weak Hund coupling and weak electron-phonon coupling.

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For the perovskites that show colossal-magnetoresistance (CMR) behaviour, we use the global instability index, R1, as a measure of the influence of the static lattice effects on the magnetic and electrical properties. These effects arise from the size mismatch between the ions at A and Mn sites as well as the size distribution of ions at A sites. A magnetic and electronic phase diagram as a function of R1 for (R = trivalent rare-earth ions, alkaline-earth ions) reveals four well-defined regions: paramagnetic insulator (PMI), ferromagnetic metal (FMM), spin glass or ferromagnetic insulator (FMI), and a transition region (TMI), in which the compounds exhibit a variety of behaviours.