TY - JOUR

T1 - Order and Disorder in Layered Double Hydroxides

T2 - Lessons Learned from the Green Rust Sulfate-Nikischerite Series

AU - Dideriksen, Knud

AU - Voigt, Laura

AU - Mangayayam, Marco C.

AU - Eiby, Simon H. J.

AU - van Genuchten, Case M.

AU - Frandsen, Cathrine

AU - Jensen, Kirsten M. O.

AU - Stipp, S. L. S.

AU - Tobler, Dominique J.

PY - 2022

Y1 - 2022

N2 - 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 ∼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.

AB - 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 ∼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.

KW - structural coherence

KW - stacking disorder

KW - crystal size

KW - pair distribution function analysis

KW - Mossbauer spectroscopy

U2 - 10.1021/acsearthspacechem.1c00293

DO - 10.1021/acsearthspacechem.1c00293

M3 - Journal article

VL - 6

SP - 322

EP - 332

JO - ACS Earth and Space Chemistry

JF - ACS Earth and Space Chemistry

SN - 2472-3452

IS - 2

ER -