Neutron Diffraction Study of Heavy Water Intercalation in Superconducting Deuterated Sodium Cobaltate Na_0.35CoO₂1.4D₂O

Author

Cinzia Metallo, University of Tennessee - Knoxville

Date of Award

12-2006

Degree Type

Thesis

Degree Name

Master of Science

Major

Physics

Major Professor

Takeshi Egami

Committee Members

Elbio R. A. Dagotto, James R. Thompson

Abstract

When Na-deficient Na_xCoO₂ is intercalated with water¹ or heavy water², it becomes a superconductor. The maximum critical temperature of 4.5 K is found for the composition Na_xCoO₂yD₂O (x= 0.3 and y=1.4). In spite of its low transition temperature, several similarities with high temperature superconducting cuprates have raised interest in this compound. Nevertheless, up to now, a clear understanding of the role of water has not been achieved.

Since superconductivity appears only when water is inserted in the parent compound, the goal of this research work was to understand what kind of effect water intercalation has in terms of electron conduction and superconductivity. Neutron scattering played a crucial role in this study because of its ability to determine the accurate positions of light elements such as hydrogen or deuterium.

We have focused our attention on the intra-molecular range of D₂O, studying the structural changes that take place within the (heavy) water molecules themselves. In order to do this the distance correlations D-D, D-O, and O-O have been studied.

Powder neutron diffraction data of the deuterated sodium cobaltate have been analyzed using the Pair Density Function (PDF) technique, which gives information about the local structure of the water molecules. The peaks of the PDF of the neutron diffraction data, in fact, give directly in real space the distances between pair of atoms, in this case the distances D-D, D-O, and O-O. If a peak shifts to a lower (or higher) value of r (Å) it means that the bond between that particular pair of atoms has become shorter (or longer). In this way it was possible to determine any change in the geometry of the water molecules.

The results obtained show that the D-D distance and the D-O-D angle in Na_0.35CoO₂1.4D₂O are significantly different from those of ordinary water (D₂O). Two 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. In this case, water may be embedded in an electronically active environment and indirectly participate in electronic conduction.

Recommended Citation

Metallo, Cinzia, "Neutron Diffraction Study of Heavy Water Intercalation in Superconducting Deuterated Sodium Cobaltate Na_0.35CoO₂1.4D₂O. " Master's Thesis, University of Tennessee, 2006.
https://trace.tennessee.edu/utk_gradthes/1739