Motion of interlamellar hydrated sodium ions in layered Cd0.75PS3Na0.5(H2O)2

Abstract

The motion of hydrated Na ions in the galleries of layered Cd 0.75 PS 3 Na 0.5 ͑H 2 O͒ 2 have been investigated by proton and 23 Na 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 23 Na 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 0.75 PS 3 Na 0.5 ͑H 2 O͒ 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 ជ 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.

An extended revision of the interlayer structures of one- and two- layer hydrates of Na- vermiculite

“…(2) The X-ray diffraction (XRD) data and NMR spectra of structures analogous to Na-vermiculite which can be used as a reference, are known from hydrated chalcogenides (Bos-Alberink et al, 1979;Röder et al, 1979Röder et al, , 1981 and from layered Cd 0.75 PS 3 Na 0.5 (H 2 O) 2 (Arun et al, 1999).…”

Effect of the Extent of Hydration on the Motion of Interlamellar Water: A 1H NMR Study of the Ion-Exchanged Layered Hydrates Cd0.75PS3Na0.5(H2O)y, y = 1, 2

“…Assuming that r H-H of the intercalated water molecules is 1.58 Å, i.e., the water molecules maintain their equilibrium geometry even after intercalation, the observed dipolar splitting can be interpreted based on the motion of water molecules. The proton NMR spectrum of Cd 0.75 PS 3 -Na 0.5 (H 2 O) y (y ) 2) has been previously analyzed 6 and the interlamellar water molecules have been shown to undergo fast rotational motion about the interlamellar normal, C B*, as well as about their C B 2 symmetry axis. Corresponding to this motion the dipolar splitting may be expressed as where ψ is the angle between the C B 2 axis of the water molecule and the C B* axis, δ is the angle between the C B* axis and H B o , and γ is the angle between the H-H vector and C B 2 axis of the water molecule (the definition of angles is indicated in Figure 5a).…”

“…Alternatively these clusters may be viewed as being formed by edge sharing Na(H 2 O) 6 octahedra. This model accounts not only for the density of the interlamellar water but also the average Na to H 2 O stoichiometric ratio of 1:4, which is between that of an isolated Na(H 2 O) 6 and the infinite Na(H 2 O) 2 sheet. Cd 0.75 PS 3 Na 0.5 (H 2 O) 2 is conducting with a dc conductivity of 10 -6 S/cm at 300 K and Arrhenius activation energy 15.6 kcal/mol.…”

“…5 In the y ) 2 bilayer phase NMR studies had shown that the Na ions are mobile and consequently the material is conducting whereas in the insulating monolayer phase they are immobile. 5 NMR studies 6 had also shown that in the bilayer y ) 2 phase conductivity was not due to hopping of Na ions between octahedral vacancies of the H 2 O bilayer but by an anisotropic sliding motion of the Na ion along with its hydration shell. The motion is such that the orientation of the C B 2 symmetry axis of the water molecules of the hydration shell with respect to the interlamellar normal, C B*, is always preserved.…”

“…In such situations the NMR spectra can exhibit characteristic features which can provide information on the dynamics and type of motion of the guest molecules. In a number of hydrated layered compounds ,− the proton NMR spectra display a near classical Pake line shape characteristic of the proton−proton dipole−dipole interaction which has been assigned to water molecules undergoing anisotropic motion in the interlamellar space. In Cd 0.75 PS 3 Na 0.5 (H 2 O) y we find that the conversion of y = 2 to y = 1 phase is accompanied by loss in the rotational degrees of freedom of the interlamellar water.…”