Structures and energies of AlOOH and FeOOH polymorphs from plane wave pseudopotential calculations

Plane wave pseudopotential methods are used to investigate the structures a nd total energies of AlOOH and FeOOH in the five canonical oxyhydroxide str uctures: diaspore (goethite), boehmite (lepidocrocite), akaganeite, guyanai te, and grimaldiite. The local density approximation was used in conjunctio n with ultrasoft pseudopotentials in full optimizations of both AlOOH and F eOOH in each of these structures. Structures are in reasonably good agreeme nt with experiment, with lattice parameters and bond lengths within 3% of e xperimental values. Neither AlOOH nor FeOOH have been identified in the gri maldiite or guyanaite structures, however we find that total energies for A lOOH and FeOOH in these structures are comparable to or lower than the tota l energies of the commonly observed polymorphs (with the exception of FeOOH in the grimaldiite structure, which is anomalously high energy). Estimated zero-point energy corrections do not alter this result. For diaspore and b oehmite, we also provide calculations using the generalized gradient approx imation and norm-conserving pseudopotentials to assess the extent to which the results depend on the particular level of theory used. We find that dia spore is predicted to have a lower energy, consistent with experimental obs ervation, using all but one methodological combination where the generalize d gradient approximation is combined with ultrasoft pseudopotentials. Thus, although one may reasonably conclude that the differences in total energie s of the various (Al,Fe)oxyhydroxide polymorphs are small, current electron ic structure methods do not appear to be fully capable of accurately resolv ing these small differences. These findings provide further confirmation th at the structures of oxyhydroxide polymorphs and surface precipitates are m ore likely to be a function of kinetics than of intrinsic lattice stability .