Incommensurately modulated crystal structure of '(O'3)-type sodium cobalt oxide NaCoO ( 0.78)

Miyazaki, Yuzuru*; Igawa, Naoki   ; Yubuta, Kunio*

Crystal structure of '(O'3)-type layered sodium cobalt oxide NaCoO ( 0.78) was analyzed using the (3+1)-dimensional superspace approach to the neutron diffraction data. The crystal structure is described based on the superspace group 2 = (0)00, wherein the positions of Na atoms are most significantly modulated in the monoclinic -direction to form an ordered arrangement. Such a positional modulation causes a quasi-periodic shift of Na atoms from the centers of the NaO polyhedra between undulated CoO sheets, changing the form of the NaO polyhedron from an octahedral coordination (O) to a trigonal prismatic one (P). Where the Na atoms are most significantly shifted, the neighboring Na atoms are located at almost touching distances and yielding Na-deficient sites. As a result, a typical sequence of NaO octahedra with -O-O-O-P--P- along the a-axis is realized.