Neutron powder diffraction patterns of 2H- and 3R-Na
xCoO
2·
yD
2O (
x~0.35,
y~1.3), which show superconductivity with Tc of about 4.7 K, show conspicuous diffuse maxima presumed to originate from the Na/D
2O part in addition to sharp reflections due to the CoO
2 part. The structure is treated as an interpenetration of two layered structures, the CoO
2 part (
a1=
b1=2.82 Å) and the Na/D
2O part (
a2=
b2=3.26 Å). A model of the short-range-order stacking of the Na/D
2O part is expressed and diffuse scattering intensities simulated using the matrix method satisfactorily explain the neutron diffraction patterns. Information about atomic coordinates was obtained based on superstructure expression with
A=
B=2
a1=
a2√3=5.64 Å including short-range order stacking.
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