Solid-state 77Se NMR and XRD study of the structure and dynamics of seleno-oxyanions in hydrotalcite-like compounds

We report here the first Se-77 nuclear magnetic resonance (NMR) and XRD study of the dynamical and structural behavior of selenate and selenite in hydrotalcite-like compounds (HT-SeO4 and HT-SeO3). The swelling properties and dynamical behavior of the anions in HT-SeO4 and HT-SeO3 are very different, and the results provide significant new insight into the structure and dynamical behavior of the interlayer and surface anions of this important group of compounds. HT-SeO4 expands from a one water layer to two water layer structure in response to increasing relative humidity, and the structural transition is captured by both the Se-77 chemical shift anisotropy (CSA) and the basal lattice spacing. Although selenate is a tetrahedral anion, the observed Se-77 CSA is uniaxial, probably due to the combined effects of hydrogen bonding to the hydroxyls of octahedral layer and the II atoms of interlayer waters, and Coulombic interaction with the positive charge of the main layer. The selenate reorientation frequency follows an Arrhenius relationship. The reorientation frequencies are higher and the apparent activation energy lower for unrecrystallized samples with relatively small particle size than for recrystallized samples with relatively large particle size. These observations and the lack of a well-defined CSA pattern for the unrecrystallized sample suggest that the spectra of the recrystallized samples are dominated by signals from interlayer selenate, whereas those of the unrecrystallized samples are dominated by signals from surface selenate and selenate in disordered interlayers. In contrast, HT-SeO3 does not expand with increasing relative humidity, and spectroscopically resolvable surface-absorbed and interlayer selenite have dramatically different dynamical characteristics. The interlayer selenite shows a well-defined, relative humidity (RH)-independent uniaxial chemical shift anisotropy (CSA) powder pattern, indicating that it is rigidly held or perhaps rotating on its S-fold axis at all hydration conditions at room temperature, consistent with the observed lack of expansion. For surface selenite, however, the reorientation frequency increases substantially with increasing relative humidity. Because selenium and sulfur have many similar properties and HT-SO4 and HT-SeO4 both expand with increasing RH, the Se-77 NMR data reported here are likely to be relevant to understanding the behavior of sulfate and sulfite HTs.