Neutron diffraction study of water intercalation in superconducting sodium cobaltate

Neutron powder diffraction has been used to investigate the role of heavy water in deuterated sodium cobaltate Na$_{0.35}$CoO$_{2}$1.4D$_{2}$O. In spite of the fact that superconductivity appears exclusively when water is intercalated in the (non superconducting) Na-deficient Na $_{x}$CoO$_{2}$, a clear understanding of the role of water has not been achieved. Neutron diffraction data at two different temperatures (T=15K, 100K) were analyzed using the Pair Density Function (PDF) technique, which gives information about local ordering in real space. The measured and calculated PDFs of Na $_{0.7}$CoO$_{2}$, Na$_{0.35}$CoO$_{2}$1.4D$_{2}$O and D$_{2}$O were compared. At both temperatures the D-D distance and the D-O-D angle in Na$_{0.35}$CoO$_{2}$1.4D$_{2}$O are significantly different from those of ordinary water. Two wide coexisting distributions of possible D-O-D bond angles are observed. We speculate that the altered geometry of the intercalated water molecules is due to a modification of the dynamics of the hydrogen bond. The possible implications are discussed in terms of electron conduction and superconductivity.