Structure and Composition of the Layered Double Hydroxides of Mg and Fe: Implications for Anion-Exchange Reactions

Partial cation exchange of Mg2+ ions with Fe3+ ions employing solid Mg(OH)(2) as precursor yielded an ordered layered double hydroxide of Mg and Fe in the presence of carbonate anions. Structure refinement revealed that the compound adopts the polytype structure 3R(1) (space group R (3) over barm, a = 3.108 angstrom, c = 23.08 angstrom) and does not show any signs of cation order. It crystallizes with a unique cation ratio of [Mg]/[Fe] = 4. At this ratio, the compound shows a single sharp absorption in its electronic spectrum at 280 nm. Attempts to prepare the LDH with a higher Fe content resulted in the phase separation of excess Fe into X-ray amorphous binary compounds, the existence of which can be discerned only by the appearance of absorptions at lambda > 350 nm, a characteristic of oxide-hydroxides of Fe3+. The nitrate-containing compound also forms with a similarly low Fe content. At this composition, the compound does not exhibit any anion-exchange properties as the nitrate ions intercalated in layered hydroxides of low layer charge are not labile. This explains the paucity of information on anion-exchange reactions of layered double hydroxides of Mg and Fe.