Topotactic routes to layered calcium cobalt oxides

Four CaxCoC2 (0.26 less than or equal to x less than or equal to 0.50) oxides have been synthesized by the low-temperature ion exchange of layered NaxCoO2 oxides, The CaxCoO2 structures all consist of [CoO2](n) sheets of edge-sharing octahedra with Ca2+ ions between layers in trigonal prismatic coordination. Different packing sequences of the close-packed oxygen layers can be obtained. Ca0.50CoO2 (a = 4.881(2) Angstrom, b = 2.826(1) Angstrom, c = 5.654(3) Angstrom, beta = 105.86(5)degrees) and Ca0.26CoO2 (a = 4.961(7) Angstrom, b = 2.805(3) Angstrom, c = 5.671(6) Angstrom, beta = 106.7(1)degrees) assume the monoclinic P'3 structure, while Ca0.35CoO2 (a = 2.812(3) Angstrom, c = 10.872(2) Angstrom) and Ca0.27CoO2 (a = 2.812(1) Angstrom, c = 10.874(5) Angstrom) take the hexagonal P2 structure. The structural relationships between the CaxCoO2 products and their NaxCoO2 precursors are discussed, and the relationship between composition and [CoO2](n) layer-spacing in various A(x)CoO(2) systems is illustrated. (C) 1998 Academic Press.