Structural, magnetic, and transport properties of the single-layered perovskites La2-xSrxCoO4 (x=1.0-1.4)

The layered cobaltites La2-xSrxCoO4 have been complexly studied over the range of mixed Co3+/4+ valency and their properties discussed in comparison with cubic perovskites La1-xSrxCoO3. The sample x=1.0 exhibits a paramagnetic ground state based on the intermediate spin state of Co3+ and undergoes a broad resistivity transition at 400-900 K, associated with diffusive changes of magnetic susceptibility and volume expansion. The Co4+ doping in samples with x > 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.