A triangular lattice system CoO2 was synthesized through chemical extraction of Li from LiCoO2 with H2SO4. On the basis of the X-ray powder diffraction and various composition analyses, the structure approximately modelled with space group R3¯m and cell dimensions a = 2.8388 and c = 13.439Å in the hexagonal basis had three O layers with a prismatic O environment (P3) between the layers. It also had stacking faults with shift vectors, s1 = 2a/3 + b/3 and s2 = a/3 + 2b/3 along a shift of c/3. Through measurements of the thermoelectric power and magnetic susceptibility, CoO2 (P3) was suggested to be a weakly correlated metal with a mass enhancement of 2.6. Na0.09CoO2 synthesized through Na extraction from Na0.7CoO2 also shows metallic properties with correlation effects probably stronger than those of CoO2 (P3).

Stacking Faults and Metallic Properties of Triangular Lattice CoO2 with a Three-Layer Structure. M.Onoda, A.Sugawara: Journal of Physics - Condensed Matter, 2008, 20[17], 175207 (6pp)