Infrared Spectra of Oriented and Nonoriented Layered Double Hydroxides in the Range from 4000 to 250 cm-1, with Evidence for Regular Short-Range Order in a Synthetic Magnesium−Aluminum LDH with Mg:Al = 2:1 but Not with Mg:Al = 3:1

We have extended the experimental examination of layered double hydroxides of the types Mg 2 Al(OH) 6 X and Mg 3 Al(OH) 8 X, both as isotropic pellets and as oriented supported specimens, to cover the entire range from 4000 to 250 cm -1 by the use of CsI pellets and CdTe and polyethylene supports and compared our results with theoretical expectations. Close inspection of the group theoretically predicted vibrational modes for these materials shows that formal allowedness, while necessary, is not sufficient for spectroscopic intensity. The OH z -translational mode occurs around 700 cm -1, while the x, y -translations and the OH rotation (MOH bend) cover a broad region from around 1000 cm -1 down. The hydroxide O−H stretching mode, which is suppressed by orientation, is concentrated at the higher end of the total O−H stretching region. Aged material of the type Mg 2 Al(OH) 6 X shows reproducible structure around 400 cm -1 both when X is chloride and when X is 1 / 4 ferrocyanide. This structure is weak or absent in freshly prepared Mg 2 Al(OH) 6 Cl and in both fresh and aged Mg 3 Al(OH) 8 X, strongly suggesting that local superlattice formation, required for charge avoidance when M(II):M(III) = 2:1 but not when M(II):M(III) = 3:1, is specific to the 2:1 material. We regard the inferred structural differences between fresh and aged Mg 2 Al(OH) 6 Cl as evidence for ripening by a solution−reprecipitation mechanism.