Combining electron diffraction, x-ray diffraction, and high-resolution electron microscopy techniques, a structural model for the cobaltite $“{\mathrm{Ca}}_3{\mathrm{Co}}_4{\mathrm{O}}_9”$ 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: $a=4.8376\mathrm{}\left(7\right)\mathrm{}\AA ,$ $c=10.833\mathrm{}\left(1\right)\mathrm{}\AA ,$ $\beta =98.06\mathrm{}\left(1\right)\mathrm{}°,$ $b_1=4.5565\mathrm{}\left(6\right)\mathrm{}\AA ,$ and $b_2=2.8189\mathrm{}\left(4\right)\mathrm{}\AA .$ The structure is built up from the stacking along c of triple rock salt-type layers ${\mathrm{Ca}}_2{\mathrm{CoO}}_3$ (first subsystem) with single ${\mathrm{CdI}}_2$-type ${\mathrm{CoO}}_2$ 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 ${\mathrm{CoO}}_2$ 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 ${\mathrm{K}}^{\mathrm{-}1}$ at 300 K with a weak temperature dependence in between 100 and 300 K. This result contrasts with the metallic in-plane resistivity.

• Received 1 February 2000

DOI:https://doi.org/10.1103/PhysRevB.62.166