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

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