Structure and stability of the Al(OH)3 polymorphs doyleite and nordstrandite: a quantum mechanical ab initio study with the CRYSTAL06 code.

The crystal structures and relative energies of doyleite and nordstrandite, two of the four aluminum trihydroxide polymorphs, were investigated at the periodic ab initio quantum-mechanical level with the CRYSTAL06 computer program, by using an all-electron Gaussian-type basis set and the hybrid B3LYP Hamiltonian. By least-energy optimizations of different starting arrangements of H atoms, a noncentrosymmetrical P1 structure model was proved to be slightly more stable than a P1̅ one in doyleite. The primitive P1̅ unit cell of nordstrandite was confirmed to contain four formula units, unlike doyleite (Z = 2). The layered structures of nordstrandite and doyleite were shown to be closely related to that of bayerite, differing from one another by the interlayer shift vectors only. From the optimized positions of H atoms, the hydrogen bonding schemes and geometries were fully determined for both polymorphs. The computed Gibbs free energies at 298 K of bayerite, doyleite, and nordstrandite, referred to that of gibbsite, are 3.9, 4.4, and 15.2 kJ mol−1 per formula unit, respectively. Nordstrandite was then predicted to be largely the less stable of all four Al(OH)3 polymorphs.