Novel structural properties of Ni_1-x Al_x layer double hydroxides

A series of nickel-aluminum layer double hydroxides (LDHs) with the composition [Ni_1-xAl_x (OH)₂][CO₃)_x/2 · H₂O] have been synthesized where x ranges from 0 to 0.4. Ordering of the host Ni_1-xAl_x layer and of the guest carbonate layer have been studied using composition-dependent X-ray diffraction measurements of powder samples. The basal spacings of these LDHs along with steric considerations indicate that the guest layer carbonate ions have their three-fold axes perpendicular to the basal plane. Powder patterns of these Ni_1-xAl_x LDHs near x = 0.30 exhibit diffraction peaks which can clearly be associated with a (√13x√13)R13.90° commensurate ordering of the guest carbonate ions. Despite the predominant ABCABC... stacking regularity of the host layers and the commensurate nature of the carbonate ordering, the diffraction peak line shapes indicate that the guest layer stacking is random. This unusual observation is attributed to the high degree of degeneracy of possible carbonate sites. X-ray diffraction measurements also provide evidence for a (√13x√3)R30° honeycomb superlattice ordering of the host Ni_1-xAl_x layer near x = 0.33.