Interlayer Structure and Dynamics of ClO₄^-Layered Double Hydroxides

This paper presents a Cl-35 NMR and XRD study of the structure and dynamical behavior of ClO4- intercalated into the interlayers of Mg,Al and Li,Al layered double hydroxides (LDHs). The variable temperature and variable relative humidity (R.H.) Cl-35 NMR data show that for these phases ClO4- is rigidly held at low R.H.'s and temperatures, but that at high R.H.s and in pastes at room temperature it undergoes isotropic reorientation at frequencies > 10(3) Hz. Surface and interlayer ClO4- cannot be distinguished. The dynamical behavior is similar for both phases, even though the Li,Al phase swells to a two-water-layer structure but the Mg,Al phase does not. The Cl-35 static NMR peak shape is dominated by uniaxial chemical shift anisotropy (CSA) due to the layer structure. These results contrast with previously published Se-77 NMR data for interlayer SeO42- in comparable Mg,Al and Li,Al LDHs, which show that SeO42- does not undergo isotropic reorientation at any R.H. SeO42- and ClO4- have similar tetrahedral structures and ionic radii, and we attribute the difference in their dynamical behavior to the difference in ionic charge and the corresponding differences in electrostatic and hydrogen bonding between the anions and the hydroxide layer and interlayer water molecules.