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

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).