Misfit-layered cobaltite with an anisotropic giant magnetoresistance: Ca3Co4O9

Combining electron diffraction, x-ray diffraction, and high-resolution electron microscopy techniques, a structural model for the cobaltite ''Ca3Co4O9'' has been found. This compound is a misfit-layered oxide consisting in two monoclinic subsystems with identical a, c, and β parameters, but different b parameters: a=4.8376(7) A, c=10.833(1) A, β=98.06(1) deg., b1=4.5565(6) A, and b2=2.8189(4) A. The structure is built up from the stacking along c of triple rock salt-type layers Ca2CoO3 (first subsystem) with single CdI2-type CoO2 layers (second subsystem). Two different sets of Co-O distances are involved which are interpreted as the existence of cobalt with three different oxidation states 2+, 3+, and 4+, in agreement with x-ray appearance near-edge structure spectra at the Co K edge. At about 420 K, both resistivity and susceptibility show an anomaly which results from a spin-state transition of cobalt at this temperature. Below 300 K, the resistivity measured along the CoO2 layers shows a metal-insulator transition as T decreases, whereas the much larger out-of-plane resistivity values show the anisotropic behavior of this phase. The application of a magnetic field induces a negative magnetoresistance which reaches -35% for 7 T. Moreover, thermoelectric power measurements yield a high positive value of ≅125 μV K-1 at 300 K with a weak temperature dependence in between 100 and 300 K. This result contrasts with the metallic in-plane resistivity