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The structural, magnetic and transport properties of the LaMn 1−x Co x O 3 system were investigated over a wide temperature range, 10-900 K. Two structural types, depending on x, were detected-orthorhombic Pbnm (x 0.5) and rhombohedral R 3c (x > 0.6), separated by the bi-phasic sample x = 0.6. At both ends of the LaMn 1−x Co x O 3 system, the respective substituents are unambiguously characterized as Co 2+ (x 0.1) and Mn 4+ (x 0.9). On the Mn-rich side, up to x < 0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of the Co 2+ valence towards the prevailing Co 3+ , while for the Co-rich samples (0.6 x 0.8), the Co 3+ states tend to disproportionate at high temperatures to Co 2+ + Co 4+ , which probably controls both the transport and magnetic properties. At low temperatures, the long range ferromagnetic order was confirmed by means of neutron diffraction for the x = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up to x = 0.8 and was associated with FM clusters that are formed below ∼220 K and coagulate below ∼150 K.

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The layered cobaltites La 2−x Sr x CoO 4 have been complexly studied over the range of mixed Co 3+/4+ valency and their properties discussed in comparison with cubic perovskites La 1−x Sr x CoO 3 . The sample x = 1.0 exhibits a paramagnetic ground state based on the intermediate spin state of Co 3+ and undergoes a broad resistivity transition at 400-900 K, associated with diffusive changes of magnetic susceptibility and volume expansion. The Co 4+ doping in samples with x Ͼ 1 induces gradually ferromagnetism and leads to a dramatic decrease of resistivity and thermopower, though the metallicity is not achieved. The neutron diffraction on the limit composition x = 1.4 suggests that the long-range ordered ferromagnetic regions coexist with paramagnetic ones, and their amount changes little with an application of high pressures up to 4.3 GPa.

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