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Nitrate-Intercalated Layered Double Hydroxides – Structure Model, Order, and Disorder

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Abstract

Layered double hydroxides with a high layer charge (+0.33 per empirical formula unit) intercalate nitrate ions with the molecular plane of the NO3 ion inclined at ca. 70° to the metal hydroxide layer, which results in a basal spacing of 8.8 Å. Three different N–O bond lengths are observed and yield Cs coordination symmetry. At lower charge (0.165 ≤ x ≤ 0.2), a basal spacing of 8.0 Å is observed, which indicates that the nitrate ion is intercalated with its molecular plane parallel to the metal hydroxide layer (coordination symmetry D3h) in a manner isostructural with carbonate-intercalated layered double hydroxides. Consequently, crystal chemical considerations predict the layer charge corresponding to the idealized composition of the nitrate-intercalated phase to be +0.165 per empirical formula unit. The [Mg–Al–NO3]0.165 phase is highly ordered with robust crystal growth along [001] and is stable to hydrothermal treatment, in contrast to the carbonate analogue of the same layer charge. Phases obtained with intermediate layer charge (+0.20 to 0.25 per empirical formula unit) exhibit structural disorder arising out of (i) planar faults and (ii) random interstratification of layers comprising nitrate ions in different orientations.

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