Deutsche Tagung f ¨ur Forschung mit ... - SNI-Portal

Struktur und Dynamik Poster: Mi., 14:00–16:30 M-P122
Understanding the role of water in the superconductor Na0.28CoO2·1.3H2O
using inelastic neutron scatering
H. N. Bordallo 1,2 , N. Jalarvo 1 , N. Aliouane 1 , J. Pieper 1,3 , M.A. Adams 4 ,
D.N. Argyriou 1
1 Hahn-Meitner-Institut, SF6 Glienicker Straße, 100, D-14109 Berlin, Germany –
2 Institut Laue-Langevin, BP 156 - 38042 Grenoble Cedex 9, France – 3 Technische
Universität Berlin, Institut für Chemie Max-Volmer-Laboratorium für Biophysikalische
Chemie, Straße des 17. Juni 135 10623 Berlin, Germany – 4 4ISIS Pulsed Neutron
Scattering Facility, Rutherford Appleton Laboratory,Chilton Didcot, Oxon, OX11
0QX, U.K
Sodium cobalt oxyhydrate NaxCoO2·yH2O shows superconductivity with Tc=5 K for
x=0.35 and y=1/3. In the same way that the two-dimensional nature of the copper
oxide layers is thought to be important in this class of superconductors, the large
separation of the cobalt oxide layers seems to be crucial in the superconducting behavior
of NaxCoO2·yH2O. Unlike the square lattice found in cuprates, cobalt oxide forms a
triangular lattice. In order to understand the mechanisms of how the dynamics of the
H-bond network is modified in the crystalline lattice as well as to get insight on the
role of water in this new class of compounds a series of incoherent neutron scattering
experiments were performed in Na0.7CoO2 and NaxCoO2·yH2O (x∼0.28, y∼1.3).
Using quasi-elastic neutron scattering (QENS) we were able to distinguish the water
chemically bound into the sodium cobalt oxyhydrate structure during the hydration
process, from the physically bound or “glassy water”. Physically bound water is defined
as the water that can hydrogen bond less strongly in the hydration shells around the
Na ions. At room temperature the elastic incoherent structure factor (EISF), extracted
from the neutron time-of-flight data, rules out single rotational-jump models, as well
as isotropic rotational motions for the physically bound water molecule.
Besides, high resolution QENS experiments, carried out on Na0.28CoO2·1.3H2O, using
the high resolution neutron spectrometer IRIS at ISIS, show that at higher temperatures
the water dynamics is well described by the random jump diffusion model with
a mean residence time of 22 ps. Furthermore we show that the sodium ions have
no influence on the rotational dynamics of the bulk-water molecules, however they
introduce the disorder necessary to prevent total crystallization of the water below
240 K. This is reflected by the existence of a liquid-liquid transition involving water in
NaxCoO2·yH2O.