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Download fileConservation of Order, Disorder, and “Crystallinity” during Anion-Exchange Reactions among Layered Double Hydroxides (LDHs) of Zn with Al
journal contribution
posted on 2007-04-05, 00:00 authored by A. V. Radha, P. Vishnu Kamath, C. ShivakumaraCarbonate 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- (Td), BrO3- (C3v), and NO3- (coordination symmetry
C2v) whose symmetry does not match the symmetry of the interlayer sites (D3h or Oh) introduce a significant
number of stacking faults, leading to turbostratic disorder. SO42- ions (coordination symmetry C3v) 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 CO32- or Cl-. Incoming NO3- ions yield an interstratified phase, whereas
incoming SO42- 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.