Perchlorate‐Intercalated Layered Double Hydroxide of Ca and Al: Synthesis, Structure Refinement, and Reversible Dehydration

Hydrocalumite‐like layered double hydroxides are anionic clays comprising positively charged metal hydroxide layers of the composition [Ca 2 Al(OH) 6 (H 2 O) 2 ] + . The water molecules provide the seventh coordination for the Ca 2+ ion. Anions are intercalated in the interlayer space to balance the charge leading to a class of LDHs having the formula [Ca 2 Al(OH) 6 (H 2 O) 2 ] · A (A = Cl, Br, I). Upon dehydration the Ca 2+ ion gets coordinatively unsaturated, and crystal chemical considerations dictate that the layered structure should collapse. When A = ClO 4 – , the LDH dehydrates fully below 80 °C, but the layered structure is retained to well over 180 °C. Structure refinement shows that the ClO 4 – ion in the hydrated phase has one Cl–O bond parallel to the c ‐axis, also the stacking direction (molecular symmetry C 3 v ). Upon loss of water, the perchlorate ion not only diffuses to a different position, but also reorients in such way that one of its O atoms directly coordinates with the Ca 2+ ion to provide for coordinative saturation of the Ca 2+ ion. In the event, the local symmetry of the ClO 4 – ion is reduced. The hydrated phase is restored on cooling (temperature 27 °C, ambient humidity ca. 50 %).