We present an investigation of the influence of structural distortions in charge-carrier doped La1−xMxCoO3 by substituting La 3+ with alkaline earth metals of strongly different ionic sizes, that is M = Ca 2+ , Sr 2+ , and Ba 2+ , respectively. We find that both, the magnetic properties and the resistivity change non-monotonously as a function of the ionic size of M. Doping La1−xMxCoO3 with M = Sr 2+ yields higher transition temperatures to the ferromagnetically ordered states and lower resistivities than doping with either Ca 2+ or Ba 2+ having a smaller or larger ionic size than Sr 2+ , respectively. From this observation we conclude that the different transition temperatures and resistivities of La1−xMxCoO3 for different M (of the same concentration x) do not only depend on the varying chemical pressures. The local disorder due to the different ionic sizes of La 3+ and M 2+ play an important role, too.

Using soft-x-ray absorption spectroscopy at the Co L(2,3) and O K edges, we demonstrate that the Co3+ ions with the CoO5 pyramidal coordination in the layered Sr2CoO3Cl compound are unambiguously in the high spin state. Our result questions the reliability of the spin state assignments made so far for the recently synthesized layered cobalt perovskites and calls for a reexamination of the modeling for the complex and fascinating properties of these new materials.

We present a study of the structure, the electric resistivity, the magnetic susceptibility, and the thermal expansion of La1−xEuxCoO3. LaCoO3 shows a temperature-induced spin-state transition around 100 K and a metal-insulator transition around 500 K. Partial substitution of La 3+ by the smaller Eu 3+ causes chemical pressure and leads to a drastic increase of the spin gap from about 190 K in LaCoO3 to about 2000 K in EuCoO3, so that the spin-state transition is shifted to much higher temperatures. A combined analysis of thermal expansion and susceptibility gives evidence that the spin-state transition has to be attributed to a population of an intermediate-spin state without orbital degeneracy for x < 0.5 and with orbital degeneracy for larger x. In contrast to the spin-state transition, the metal-insulator transition is shifted only moderately to higher temperatures with increasing Eu content, showing that the metal-insulator transition occurs independently from the spin-state distribution of the Co 3+ ions. Around the metal-insulator transition the magnetic susceptibility shows a similar increase for all x and approaches a doping-independent value around 1000 K indicating that well above the metal-insulator transition the same spin state is approached for all x.

We present a study of the thermal conductivity κ and the thermopower S of single crystals of La1−xSrxCoO3 with 0 ≤ x ≤ 0.3. For all Sr concentrations La1−xSrxCoO3 has rather low κ values. For the insulators (x < 0.18) this arises from a suppression of the phonon thermal conductivity by lattice disorder due to temperature-and/or doping-induced spin-state transitions of the Co ions. For larger x, the heat transport by phonons remains low, but an additional contribution from mobile charge carriers causes a moderate increase of κ. The thermopower of the low-doped crystals is positive and shows a pronounced maximum as a function of temperature. With increasing x, this maximum strongly broadens and its magnitude decreases. For the highest Sr content (x = 0.3) S becomes even negative in the intermediate temperature range. From S, κ, and the electrical resistivity ρ we derive the thermoelectric figure of merit Z = S 2 / κρ. For intermediate Sr concentrations we find notably large values of Z indicating that Co-based materials could be promising candidates for thermoelectric cooling.