Physical properties of fixed-charge layer double hydroxides

Abstract

The physical properties of a series of layer double hydroxides (LDH) of the form $[({\mathrm{CO}}_{3}{)}_{0.195(1\ensuremath{-}x)}{\mathrm{Cl}}_{0.39x}({\mathrm{H}}_{2}\mathrm{O}{)}_{y}]:[{\mathrm{Zn}}_{0.61}{\mathrm{Al}}_{0.39}(\mathrm{OH}{)}_{2}],$ $0<~x<~1,$ $0<~y<~(0.4+0.2x)$ have been studied. The hydration dynamics of these materials indicate that the guest layer water molecules form a hydration ring which defines the height of the solvated, nested Cl anion. The water molecules can tilt around their ${C}_{2v}$ axis such that the height of the solvated Cl ion is a function of the number of molecules forming the hydration ring. The composition dependence of the basal spacing, determined from x-ray-diffraction powder patterns measured as a function of humidity and temperature for these materials, is a function of both the Cl concentration $(x)$ and the number of guest layer water molecules $(y).$ Distinct basal spacing curves are observed for fully hydrated, partially hydrated, and dehydrated materials. At $x=1$ the Cl end-member material exhibits a change in stacking sequence from a $3R$ polytype to a $2H$ polytype upon dehydration. The dehydrated form of this material also exhibits a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ superlattice ordering of the Cl ions. Due to the nesting of the Cl ion and the active nature of the water molecules, the basal spacing vs x curve for the dehydrated materials is the only curve that can be fit by the discrete finite layer rigidity model. The interlayer rigidity parameter for LDH materials has been determined to be $p=4.84\ifmmode\pm\else\textpm\fi{}0.06$ indicating that these materials are stiffer than class-II layered solids but not as stiff as class-III layered solids.