Monoclinic phase of the misfit-layered cobalt oxide (Ca 0.85OH) 1.16CoO 2

Abstract

A monoclinic phase of the misfit-layered cobalt oxide (Ca 0.85OH) 1.16CoO 2 was successfully synthesized and characterized. It was found that this new material is a poly-type phase of the orthorhombic form of (CaOH) 1.14CoO 2, recently discovered by the present authors. Both the compounds consist of two interpenetrating subsystems: CdI 2-type CoO 2 layers and rock-salt-type double-atomic-layer CaOH blocks. However, these two phases exhibit a different stacking structure. By powder X-ray and electron diffraction (ED) studies, it was found that the two subsystems of (Ca 0.85OH) 1.16CoO 2 have c-centered monoclinic Bravais lattices with common a=4.898 Å, c=8.810 Å and β=95.8° lattice parameters, and different b parameters: b 1=2.820 Å and b 2=4.870 Å. Chemical analyses revealed that the monoclinic phase has a cobalt valence of +3.1–3.2. Resistivity of the monoclinic phase is approximately 10 1–10 5 times lower than that of the orthorhombic phase. This suggests that the monoclinic phase is a hole-doped phase of the insulating orthorhombic phase. Furthermore, large positive Seebeck coefficients (∼100 μV/K) were observed near room temperature.