Dehydration-Rehydration Studies on Polytypes of Chloride and Nitrate Layered Double Hydroxides of Nordstrandite and Bayerite: a Comparative Study.

Abstract

In this work, we report for the first time, the dehydration-rehydration studies of nordstrandite-derived layered double hydroxides (LDHs) of Li and Al, n-[Li–Al–X] (X = Cl– and NO3–) (n-nordstrandite derived). n-[Li–Al–NO3], an orthorhombic phase, dehydrated at 180 °C to a monoclinic phase. Refinement placed the NO3– ions parallel to the hydroxide layers. The dehydration showed no change in basal spacing. The monoclinic n-[Li–Al–Cl] dehydrated at 160 °C with a 0.49 Å compression in basal spacing to an orthorhombic polytype. We compared our results with the published results of their bayerite counterparts b-[Li–Al-X] (b-bayerite derived) and observed that though n-[Li–Al–X] and b-[Li–Al–X] LDHs have similar structures, their dehydrated phases are structurally different. We also report the refinement of b-[Li–Al–Cl] (DH). Previous studies attribute the basal spacing values to the (i) degree of hydration and (ii) orientation of anions in the interlayer. We observe that basal spacing is a manifestation of the symmetry of the crystal. Dehydration of nitrate intercalated LDH, which proceeds from an orthorhombic symmetry to a monoclinic symmetry with no decrease in the interlayer spacing, is attributed to sliding of the hydroxyl layers in the ab-plane due to the increase in the β value. This sliding stabilizes the interlayer through weak long-range electrostatic forces that mainly contribute to the stabilization of the layered structure at separations much larger than the effective radius of hydrogen bonds. Such stabilization would negate the need for the layers to compress, thus conserving the basal spacing in n-[Li–Al–NO3] (DH).