Conservation of Order, Disorder, and “Crystallinity” during Anion-Exchange Reactions among Layered Double Hydroxides (LDHs) of Zn with Al

Carbonate and chloride ions mediate an ordered stacking of metal hydroxide slabs to yield ordered layered double hydroxides (LDHs) of Zn with Al, by virtue of their ability to occupy crystallographically well-defined interlayer sites. Other anions such as ClO₄^- (T_d), BrO₃^- (C_3v), and NO₃^- (coordination symmetry C_2v) whose symmetry does not match the symmetry of the interlayer sites (D_3h or O_h) introduce a significant number of stacking faults, leading to turbostratic disorder. SO₄^2- ions (coordination symmetry C_3v) alter the long-range stacking of the metal hydroxide slabs to nucleate a different polytype. The degree of disorder is also affected by the method of synthesis. Anion-exchange reactions yield a solid with a greater degree of order if the incoming ion is a CO₃^2- or Cl^-. Incoming NO₃^- ions yield an interstratified phase, whereas incoming SO₄^2- ions generate turbostratic disorder. Conservation or its converse, elimination, of stacking disorders during anion exchange is the net result of several competing factors such as (i) the orientation of the hydroxyl groups in the interlayer region, (ii) the symmetry of the interlayer sites, (iii) the symmetry of the incoming ion, and (iv) the configuration of the anion. These short-range interactions ultimately affect the long-range stacking order or “crystallinity” of the LDH.