The enthalpies of formation of hydrotalcite-like phases containing Mg and Al and intercalated with NO3 -, Cl-, I-, ReO4 -, or CO3 2- were determined using high-temperature oxide melt and room-temperature acid solution calorimetry. The relative stability of phases bearing the various anions was gauged by comparing the enthalpy of formation from the single-cation components (Δf H scc). Trends relating Δf H scc to the nature of intercalating anions (halides, NO3 -, and CO3 2-) show small stabilization from the mechanical mixtures of single-cation components. The aim of this study was to relate the enthalpy of formation to the nature of interlayer bonding in hydrotalcite-like compounds (HTLCs) bearing various anions, to uncover trends in the relative aqueous solubilities of these phases. The entropy of formation of these compounds was estimated using an approximation based on third-law entropy measurements for the compound Mg0.74Al0.26(OH)2(CO3)0.13·0.39H2O which were performed in a previous study. This approximation for the third-law entropy was combined with the enthalpy data from our calorimetric measurements performed in this work in order to calculate the standard-state free energy of formation for the HTLCs. The solubility products for the compounds investigated in this study were calculated from these free energies of formation and were used in geochemical calculations. The results of these calculations support our previous hypothesis that carbonate-intercalated HTLCs are less soluble than phases bearing other anions such as nitrates and halides. We suspect that the solubilities of HTLCs bearing anions other than carbonate may correspond to the solubilities of single-cation phases bearing the same anions.

Magnesium orthovanadate crystals (< Φ 5 mm × L 55 mm) with a well-defined crystallographic orientation have been grown successfully in a four-mirror optical floating zone furnace. The crystals grow in the [010] direction. The transparent orange-brown crystals develop a predominant facet along the [20-1] during the crystal growth and cleave along the [20-1] and [100] directions.

The role of occluded water in ion exchange selectivity is examined in a class of molecular sieves named Sandia Octahedral Molecular Sieves (SOMS:  Na2Nb2- x M x O6- x (OH) x ·H2O (i.e., M = Ti; 0 < x < 0.4)). SOMS exhibit a high selectivity for divalent cations only when the framework Nb(5+) are substituted by M(4+) atoms. Vibrational dynamics of the water molecules with varying charge balancing cations and M atoms are studied by inelastic neutron scattering (INS) measurements and 1H MAS NMR and correlated to density functional theory and molecular dynamics data. The experimental INS spectra were compared with those of ice Ih to characterize the changes induced by confinement on the occluded H2O and the resulting hydrogen-bonding network. Data indicates that, with increasing M(4+) content and divalent ion exchange, the trend of occluded water molecules is to change from extended rigid ice-like networks to restored bulk-like arrangements with increased solvation effects on the channel charge-balancing cations.