Structure and properties of the CaFe2O4-type cobalt oxide CaCo2O4

The calcium cobalt oxide CaCo2O4 was synthesized for the first time and characterized from a powder X-ray diffraction study, measuring magnetic susceptibility, specific heat, electrical resistivity, and thermoelectric power. CaCo2O4 crystallizes in the CaFe2O4 (calcium ferrite)-type structure, consisting of an edge- and corner-shared CoO6 octahedral network. The structure of CaCo2O4 belongs to an orthorhombic system (space group: Pnma) with lattice parameters, a=8.789(2) Å, b=2.9006(7) Å and c=10.282(3) Å. Curie–Weiss-like behavior in magnetic susceptibility with the nearly trivalent cobalt low-spin state (Co3+, 3d  t2g6, S=0), semiconductor-like temperature dependence of resistivity (ρ=3×10−1 Ω cm at 380 K) with dominant hopping conduction at low temperature, metallic-temperature-dependent large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K), and Schottky-type specific heat with a small Sommerfeld constant (γ=4.48(7) mJ/Co mol K2), were observed. These results suggest that the compound possesses a metallic electronic state with a small density of states at the Fermi level. The doped holes are localized at low temperatures due to disorder in the crystal. The carriers probably originate from slight off-stoichiometry of the phase. It was also found that S tends to increase even more beyond 380 K. The large S is possibly attributed to residual spin entropy and orbital degeneracy coupled with charges by strong electron correlation in the cobalt oxides.

Graphical abstractA new calcium cobalt oxide CaCo2O4 phase, which crystallizes in the calcium-ferrite-type structure (space group: Pnma; a=8.789(2) Å, b=2.9006(7) Å, and c=10.282(3) Å), has been synthesized for the first time under high temperature and high pressure (1500 °C, 6 GPa). This compound exhibits large thermoelectric power (Seebeck coefficient: S=+147 μV/K at 380 K) and an unsaturated temperature dependence of S around 380 K.Figure optionsDownload full-size imageDownload as PowerPoint slide