Monoclinic phase of the misfit-layered cobalt oxide (Ca0.85OH)1.16CoO2

A monoclinic phase of the misfit-layered cobalt oxide (Ca0.85OH)1.16CoO2 was successfully synthesized and characterized. It was found that this new material is a poly-type phase of the orthorhombic form of (CaOH)1.14CoO2, recently discovered by the present authors. Both the compounds consist of two interpenetrating subsystems: CdI2-type CoO2 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 (Ca0.85OH)1.16CoO2 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: b1=2.820 Å and b2=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 101–105 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.

Graphical abstractCrystal structure of two kinds of poly-type phases of the misfit-layered cobalt oxides (Ca1−δOH)xCoO2: (a) projection along the a-axis of the orthorhombic structure; (b) projection along the b-axis of the monoclinic structure; and (c) projection along the [−1 0 0] direction of the monoclinic structure. In each case, the rectangle is the unit cell.Figure optionsDownload full-size imageDownload as PowerPoint slide