Motion of interlamellar hydrated sodium ions in layered Cd_0.75PS₃Na_0.5(H₂O)₂

Abstract

The motion of hydrated Na ions in the galleries of layered Cd_0.75PS₃Na_0.5(H₂O)₂ have been investigated by proton and ²³Na NMR and impedance measurements as a function of temperature. The material is conducting and the frequency dependent conductivity exhibits a power law characteristic of non-Debye-like electrical conductivity relaxation, arising from Coulomb correlation among charge carriers. The NMR spectra indicates that the interlamellar Na ions are mobile and there is good agreement between the ²³Na spin-lattice relaxation rates and the electrical conductivity relaxation. The proton NMR exhibits a well resolved orientation dependent dipolar splitting which has been rationalized by considering the anisotropic rotational motion of water molecules in the pseudo-octahedral hydration shell of the Na ion. Isotropically tumbling water as well as proton hopping among water molecules is absent in Cd_0.75PS₃Na_0.5(H₂O)₂. Evidence is presented to show that within the galleries the Na ion moves along with its hydration shell, the motion being such that the orientation of the C^→₂ axis of the water molecule with respect to the normal to the layers is always preserved. The motion of the hydrated Na ions are correlated through their mutual Coulombic interaction but the rotational motion of the water molecule in the ions hydration shell may still be described by a single exponential Debye-like relaxation.