Synthesis, polytypism, and dehydration behaviour of nitrate-intercalated layered double hydroxides of Ca and Al

Abstract

Rapid precipitation of the hydroxide phase from a mixed metal nitrate solution comprising Ca²⁺ and Al³⁺ ions leads to the formation of a one-layered hexagonal polytype of [Ca–Al] layered double hydroxide. In contrast, slow precipitation results in a three-layered polytype of rhombohedral symmetry. Both polytypes comprise a stacking of positively charged metal hydroxide layers having the composition [Ca₂Al(OH)₆(H₂O)₂]⁺. The Ca²⁺ ions are seven coordinated with water molecules providing the seventh coordination. In the absence of any prior knowledge of the structure of the 1H polytype, translationengleiche and klassengleiche graphs were used to arrive at the space group and atom positions of the 1H polytype. Rietveld refinements of the structures of the two polytypes show that nitrate ions are intercalated in the interlayer gallery with its molecular plane inclined to the metal hydroxide layer. The angle of inclination in the 1H polytype (∼61°) is greater than that in the 3R polytype (∼30°). When the 1H polytype is completely dehydrated, the metal hydroxide layers undergo a rigid translation relative to one another resulting in (i) a 1H → 3R interpolytype transition, and (ii) the grafting of the nitrate ion to the metal hydroxide layer to satisfy the seventh coordination of the Ca²⁺ ion. The as-prepared 3R polytype has a mixed anion interlayer comprising nitrate and hydroxyl ions. On heating, this phase progressively loses crystallinity before decomposition.