Synthon Approach to Structure Models for the Bayerite-Derived Layered Double Hydroxides of Li and Al.

The Br- ion intercalated layered double hydroxide (LDH) of Li and Al obtained from the bayerite-Al(OH)3 precursor crystallizes in a structure different from that of the gibbsite-Al(OH)3 derived counterpart. Additionally, it undergoes temperature- and humidity-induced reversible interpolytype transformations. The dehydrated LDH (T ≈ 120 °C) adopts a structure of hexagonal symmetry (space group P3̅1m) and comprises a stacking of the metal hydroxide layers arranged one above another. On cooling and rehydration, the LDH adopts a structure of monoclinic symmetry with a stepwise increase in the stacking angle, β. Using the structural synthon approach, based on the systematic elimination of the principal symmetry elements of the hexagonal crystal, structure models were generated for each of the two hydration steps (relative humidity ∼50%, >70%) and the structures refined (space group C2/m). The refined structures show that the interpolytype transitions are a result of rigid translations of successive metal hydroxide layers relative to one another by translation vectors (1/6, 0, 1) and (1/3, 0, 1), respectively.