Synthesis, crystal structure and physical properties of layered cobalt oxide Ca_xCoO₂ (x∼ 0.47)

We have synthesized a single-phase sample of the compound known as CaCo₂O₄ and investigated its crystal structure by means of a single crystal X-ray diffraction technique. The compound crystallizes in a triclinic system (space group P1) with lattice parameters of a = 492.4(2) pm, b = 568.3(3) pm, c = 568.5(3) pm, a = 75.400(9)°, β= 89.974(9)° and γ = 81.261(9)° at 293(2) K. The structure consists of an alternate stack of a CdI₂-type CoO₂ triangular sheet and a layer of Ca atoms, showing great similarity to that of β-Na_xCoO ₂ (x∼ 0.67). However, the Ca atom has four crystallographic sites with different occupation probabilities, leading to the structure formula Ca_1.87(5)Co₄O₈ (Ca_0.47(1)CoO ₂). Due to a low bulk density as well as a small number of hole carriers, equivalent to the nominal valence state of Co ions close to 3.06, a polycrystalline sample of the Ca-phase exhibits much higher electrical resistivity p = 680 μΩ. m, relative to that of the Na-phase ρ = 1.2 μΩ m, with a large positive Seebeck coefficient of S = 145 μV/K at 300 K.