Towards solving the crystal structure of polytype 3R2 Mg–Al layered double hydroxides

An analysis of the crystal structure of polytype 3R2 layered double hydroxides has been carried out based on XRPD diffractograms. Measurement of the transmission-XRPD pattern carried out in a spinning and tilting capillary using synchrotron radiation has been shown to diminish the preferred orientation effect of the sample. In order to approach the structure of polytype 3R2, structural models, based on possible interlayer anion arrangement as revealed by FT-IR, 27-Al solid state NMR and XRPD, were constructed. A structural model is proposed that is consistent with the by XRPD measured interlayer distance, in which tetrahedral aluminate ions are grafted with their apical oxygen ions onto the octahedral sheet. The presence of a measured extra reflection at 2θ value of 6.3, which is not predicted by this structure, can be obtained however by enlargement of the unit cell to a′ = a√3 = 0.5279 nm and c′ = c = 2.1985 nm, which simulates the aluminate anions ordering in the interlayer.

► The interlayer of polytype 3R2 Mg-Al LDH is occupied by tetrahedral aluminates.
► The apical oxygen of those aluminates is grafted onto an octahedral metal layer.
► Only 2/3 charge compensation is given by aluminates and the rest by hydroxides.
► Anion ordering could simulate the experimentally observed extra PXRD reflection.