When analyzed with the Rietveld method, the diffraction pattern of Layered Double Hydroxides (LDHs) show quite often some unexplained features (e.g. odd peak broadening or extra reflections). Stacking defects can locally change the structure and introduce a diffuse scattering contribution, incompatible with the hypotheses of a 3D periodic lattice on which the Rietveld method is based. The literature proposes several approaches to the study of those stacking defects, mostly based on simulation or on a rough description of the crystallography of the system. It is here shown that the OD theory (Ferraris et al., 2008) provides the ideal frame to describe this important class of materials and to interpret all features appearing in the diffraction patterns. LDHs represent a wide merotype series in which layer groups and a set of stacking (partial) operations, compatible with the chemical bonding, are sufficient to describe any ordered or disordered variant. The adoption of a stacking probability matrix allows a particular diffraction pattern to be described. It is also shown that the traditional description in terms of a unit cell containing more than one layer is compatible with the description of one of the Maximum Degree of Order (MDO) polytypes and it is just necessary to guarantee the 3D periodicity needed by traditional crystallography. The structures and corresponding microstructure (including the probability matrices) can be refined from X-ray, electron and neutron diffraction data using the DIFFaX+ code (Leoni et al., 2004) based on a recursive description of the stacking (Treacy et al., 1991).

Structure of layered double hydroxides: OD features and modeling of the diffraction pattern

Leoni, Matteo;Koch, Robert Joseph
2013-01-01

Abstract

When analyzed with the Rietveld method, the diffraction pattern of Layered Double Hydroxides (LDHs) show quite often some unexplained features (e.g. odd peak broadening or extra reflections). Stacking defects can locally change the structure and introduce a diffuse scattering contribution, incompatible with the hypotheses of a 3D periodic lattice on which the Rietveld method is based. The literature proposes several approaches to the study of those stacking defects, mostly based on simulation or on a rough description of the crystallography of the system. It is here shown that the OD theory (Ferraris et al., 2008) provides the ideal frame to describe this important class of materials and to interpret all features appearing in the diffraction patterns. LDHs represent a wide merotype series in which layer groups and a set of stacking (partial) operations, compatible with the chemical bonding, are sufficient to describe any ordered or disordered variant. The adoption of a stacking probability matrix allows a particular diffraction pattern to be described. It is also shown that the traditional description in terms of a unit cell containing more than one layer is compatible with the description of one of the Maximum Degree of Order (MDO) polytypes and it is just necessary to guarantee the 3D periodicity needed by traditional crystallography. The structures and corresponding microstructure (including the probability matrices) can be refined from X-ray, electron and neutron diffraction data using the DIFFaX+ code (Leoni et al., 2004) based on a recursive description of the stacking (Treacy et al., 1991).
2013
The XV International Clay Conference
Rio de Janeiro
AIPEA (Association Internationale pour l'Etude des Argiles)
Leoni, Matteo; Koch, Robert Joseph
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11572/35294
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