Structure of LiAl₂(OH)₇·2H₂O

The room-temperature reaction of lithium hydroxide with aluminum hydroxide (bayerite) in the ratio of 1:2 produces lithium dialuminate, LiAl₂(OH)₇·2H₂O. Rietveld refinement of X-ray and neutron powder diffraction patterns indicate that the structure is composed of [LiAl₂-(OH)₆]⁺ layers separated by water molecules and hydroxide ions. Disorder in the stacking sequence leads to indexing the diffraction pattern on a hexagonal cell (a = 5.0972(3) Å, c = 7.5524(9) Å × 3_n layers); however, the structure is better described by a C-centered monoclinic cell with the constraints the b = a√3 and cos β = −a/3c (a = 5.097 Å, b = 8.829 Å, c = 7.741 Å, and β = 103.0°). The diffraction patterns were refined using a model of up to 54 layers (∼400 Å). The layer stacking vector, which defines the angle between layers, is less than expected based on the position of the diffraction peaks. The structural complexities present in lithium dialuminate have their origins in the conditions of synthesis and the layered nature of the material.