Abstract
Crystal structures of two kinds of cobalt oxide superconductors were investigated by neutron powder diffraction. Comparison between the structures of the two phases, one derived from γ-Na0.7CoO2 and the other from α-NaCoO2, indicated that the guest arrangement is common to the two phases in spite of the difference in stacking made of CoO2 host layers. Structure analysis on the basis of composite crystal models with stacking faults revealed that the hydrated sodium ions in the galleries are ordered to form a 2/ a × 2/ a trigonal lattice, where a is a lattice constant of the fundamental unit cell. The guest layers were found to be stacked along the c-direction with a short-range interlayer ordering: when a guest layer is stacked onto another over a CoO2 layer interposed between them, the stacking must be accompanied by a lateral shift with shift vectors, 1/2a2, 1/2b2, and 1/2a2 +1/2 b2, where a2 and b2 are lattice vectors of the 2/ a × 2/ a lattice with the same probability.
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