Crystal Structure of the Superconducting Layered Cobaltate Na X Coo 2 ·yd 2 O Crystal Structure of the Superconducting Layered Cobaltate Na X Coo 2 ·yd 2 O

We have used electron diffraction and neutron powder diffraction to elucidate the structural properties of superconducting Na x CoO 2 ·yD 2 O over a wide compositional range. Our measurements show that superconducting samples exhibit a number of supercells ranging from 1 3 a * to 1 15 a * , but the predominant modulation, observed also in the neutron data, is a double hexagonal cell with dimensions 2a × 2a × c. Rietveld analysis reveals that D 2 O is inserted between CoO 2 sheets to form a layered network of NaO 6 triangular prisms. Our model removes the need to invoke a 5 K superconducting point compound and suggests that a solid solution of Na is possible within a relatively constant amount of water y. (Some figures in this article are in colour only in the electronic version)