Order and Disorder in Layered Double Hydroxides: Lessons Learned from the Green Rust Sulphate - Nikischerite Series.

<div>Layered double hydroxides (LDHs) occur naturally and are synthesised for catalysis, drug</div><div>delivery and contaminant remediation. They consist of Me(II)-Me(III) hydroxide sheets</div><div>separated by hydrated interlayers and weakly held anions. Often, LDHs are nanocrystalline and</div><div>sheet stacking and Me(II)-Me(III) arrangement can be disordered, which influence reactivity and</div><div>complicate structural characterisation. We have used pair distribution function (PDF) analysis, to</div><div>provide detailed information about local and medium range order (< 9 nm), to determine the</div><div>structure of synthetic Fe(II)-Fe(III)/Al(III) LDH. The data are consistent with ordered Me(II)</div><div>and Me(III) in hydroxide sheets, where structural coherence along the c axis decreases with </div><div>increasing Al content. The PDF for Fe(II)-Al(III) LDH (nikischerite) is best matched by a</div><div>pattern for a single metal hydroxide sheet. Parallel to decreased structural coherence between</div><div>layers, coherence within layers decreased to ~6 nm for synthetic nikischerite. Thus, disorder</div><div>developed within and between the sheets, resulting in mosaic crystals with coherent scattering</div><div>domains decreasing in all directions. The high density of grain boundary terminations would</div><div>affect reactivity. Based on classical nucleation theory and the Kossel crystal growth model, we</div><div>propose that loss of structural coherence stems from increased supersaturation and the presence</div><div>of Al-hydroxides during formation of the Al-rich LDH</div>