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 beta parameters, but different b parameters: a = 4.8376(7) Angstrom, c = 10.833(1) Angstrom, beta = 98.06(1)degrees, b(1) = 4.5565(6) Angstrom, and b(2) = 2.8189(4) Angstrom. 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 approximate to 125 mu V 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.