The formation mechanism of a textured ceramic of thermoelectric [Ca2CoO3]0.62[CoO2] on β-Co(OH)2 templates through in situ topotactic conversion

We investigated the formation mechanism of thermoelectric [Ca2CoO3](0.62)[CoO2] (CCO) on beta-Co(OH)(2) templates with maintained orientations by identifying the intermediate phases and specifying the relationship between their crystallographic orientations. We mixed beta-Co(OH)(2) templates with the complementary reactant CaCO3 and prepared a compact by tape casting, with the developed (001) plane of the templates aligned along the casting plane. High-temperature XRD of the compact revealed that beta-Co(OH)(2) decomposed into Co3O4 by 873 K, and Co3O4 reacted with CaO to form CCO by 1193 K via the formation of the newly detected intermediate phase beta-NaxCoO2-type CaxCoO2 at 913-973 K. Pole figure measurements and SEM and TEM observations revealed that the relationship between the crystallographic planes was (001) beta-Co(OH)(2)//{111} Co3O4//(001) CaxCoO2//(001) CCO. The crystal structures of the four materials possess the common CoO2 layer (or similar), which is composed of edge-sharing CoO6 octahedra, parallel to the planes. The cross-sectional HRTEM analysis of an incompletely reacted specimen showed transient lattice images from CaxCoO2 into CCO, in which every other CoO2 layer of CaxCoO2 was preserved. Thus, it was demonstrated that a textured CCO ceramic is produced through a series of in situ topotactic conversion reactions with a preserved CoO2 layer of its template.