Misfit-layered cobaltite with an anisotropic giant magnetoresistance:Ca3Co4O9

Abstract

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 \ensuremath{\beta} parameters, but different b parameters: $a=4.8376(7)\AA{},$ $c=10.833(1)\AA{},$ $\ensuremath{\beta}=98.06(1)\ifmmode^\circ\else\textdegree\fi{},$ ${b}_{1}=4.5565(6)\AA{},$ and ${b}_{2}=2.8189(4)\AA{}.$ 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 \ensuremath{\approx}125 \ensuremath{\mu}V ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ at 300 K with a weak temperature dependence in between 100 and 300 K. This result contrasts with the metallic in-plane resistivity.