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, drugdelivery and contaminant remediation. They consist of Me(II)-Me(III) hydroxide sheetsseparated by hydrated interlayers and weakly held anions. Often, LDHs are nanocrystalline andsheet stacking and Me(II)-Me(III) arrangement can be disordered, which influence reactivity andcomplicate structural characterisation. We have used pair distribution function (PDF) analysis, toprovide detailed information about local and medium range order (< 9 nm), to determine thestructure 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 withincreasing Al content. The PDF for Fe(II)-Al(III) LDH (nikischerite) is best matched by apattern for a single metal hydroxide sheet. Parallel to decreased structural coherence betweenlayers, coherence within layers decreased to ~6 nm for synthetic nikischerite. Thus, disorderdeveloped within and between the sheets, resulting in mosaic crystals with coherent scatteringdomains decreasing in all directions. The high density of grain boundary terminations wouldaffect reactivity. Based on classical nucleation theory and the Kossel crystal growth model, wepropose that loss of structural coherence stems from increased supersaturation and the presenceof Al-hydroxides during formation of the Al-rich LDH