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

Layered double hydroxides (LDHs) occur naturally and are synthesized 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 influences the reactivity and complicates structural characterization. We have used pair distribution function (PDF) analysis to provide detailed information about local and medium range order (<= 9 nm) and 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 similar to 6 nm for synthetic nikischerite. Thus, the length scale of atomic ordering decreased 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 the formation of the Al-rich LDH.