On the Thermal Stability and Structures of Layered Double Hydroxides Mg sub(1-x)Al sub(x)(OH) sub(2)(NO sub(3)) sub(x).mH sub() O (0.18 less than or equal to x less than or equal to 0.38)

• Published in: European journal of inorganic chemistry Vol. 2015; no. 10; pp. 1775 - 1788
• Main Authors: Sjastad, Anja Olafsen, Andersen, Niels Hoejmark, Vajeeston, Ponniah, Karthikeyan, Jayakumar, Arstad, Bjoernar, Karlsson, Arne, Fjellvag, Helmer
• Format: Journal Article
• Language: English
• Published: 01.04.2015
• Subjects: Aluminium; Aluminum; Dehydration; Diffraction; Hydroxides; Nitrates; Orientation; Oxides
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201403232

Nitrate-based layered double hydroxides, Mg sub(1-x)Al sub(x)(OH) sub(2)(NO sub(3)) sub(x).mH sub(2)O, have been investigated to settle issues relating to stability towards humidity, dehydration and staging, nitrate group orientation, Al migration and crystal structure. Raman and IR spectroscopy together with modelling methods proved that flat-lying water and nitrate become tilted towards [001] at higher nitrate concentrations, accompanied by expansion along the c-axis. The distribution of the molecular tilts is greatest for x = 0.25, which explains the extraordinary broad diffraction peaks. The unit cell volume was modified for all samples when subjected to moist air, most strongly for x = 0.25. On heating, in situ synchrotron diffraction data showed continuous changes along the c-axis during dehydration of samples with x = 0.20 and 0.33. For x = 0.25, an intermediate phase appeared. Dehydroxylation was initiated at the OH groups bonded to the cations MgAl sub(2), Mg sub(2)Al and Mg sub(3) (in said order), whereas complete nitrate loss occurred at higher temperatures for higher Al contents. Some aluminium simultaneously moved to tetrahedral sites, which, according to super(1)H- super(27)A l CP NMR spectroscopy, lack protons in their near neighbourhood. In contrast, octahedral aluminium remains surrounded by protons during decomposition. For the metastable oxide product, neutron diffraction indicated that 2/3 of aluminium reside in tetrahedral interstices. This oxide is defective, probably mimicking wuestite rather than a regular rock salt structure. Structural aspects of nitrate-based layered double hydroxides Mg sub(1-x)Al sub(x)(OH) sub(2)(NO sub(3)) sub(x).mH sub(2)O have been investigated by various experimental and theoretical methods. The main focus was on the orientation of the NO sub(3) super(-) groups in the interlayer gallery, the reversible hydration/dehydration process and the transport of Al from octahedral to tetrahedral sites during the formation of a metastable rock salt oxide.