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

Abstract

Hydrocalumite‐like layered double hydroxides are anionic clays comprising positively charged metal hydroxide layers of the composition [Ca2Al(OH)6(H2O)2]+. The water molecules provide the seventh coordination for the Ca2+ ion. Anions are intercalated in the interlayer space to balance the charge leading to a class of LDHs having the formula [Ca2Al(OH)6(H2O)2]·A (A = Cl, Br, I). Upon dehydration the Ca2+ ion gets coordinatively unsaturated, and crystal chemical considerations dictate that the layered structure should collapse. When A = ClO4–, the LDH dehydrates fully below 80 °C, but the layered structure is retained to well over 180 °C. Structure refinement shows that the ClO4– ion in the hydrated phase has one Cl–O bond parallel to the c‐axis, also the stacking direction (molecular symmetry C3v). 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 Ca2+ ion to provide for coordinative saturation of the Ca2+ ion. In the event, the local symmetry of the ClO4– ion is reduced. The hydrated phase is restored on cooling (temperature 27 °C, ambient humidity ca. 50 %).