Variable Temperature PXRD Studies of LiAl2(OH)6X·H2O (X = Cl, Br):  Observation of Disorder → Order Transformation in the Interlayer

The layered double hydroxides of Li with Al containing intercalated chloride and bromide ions (Li−Al−X; X = Cl, Br) crystallize with turbostratic disorder. Turbostraticity in these compounds arises not only on account of disorder in the stacking of the metal hydroxide slabs but also due to positional disorder in the interlayer. On heating to 100−275 °C, as dehydration proceeds, a disorder → order transformation is observed in the interlayer. The emergence of structural order is due to the migration of the X- ion from a position of higher site degeneracy to one of lower site degeneracy. This is facilitated by the decrease in the number density of atoms in the interlayer due to dehydration. The stacking disorders are however retained as the thermal energy is inadequate to bring about a long-range reorganization of the structure.