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

Abstract