Thermodynamics of Al-substitution in Fe-oxyhydroxides

Ab initio simulation results are presented for dilute Al-substitution in the common Fe-(oxyhydr)oxide materials hematite, goethite, lepidocrocite and ferrihydrite. Experimental evidence from the literature suggests that Al-substitution in these materials may influence particle stability and phase transformation behavior, typically stabilizing the oxyhydroxide phases relative to the oxide, hematite. We find that all the alloyed phases studied are unstable with respect to phase separation into their unalloyed Fe/Al-(oxyhydr)oxide end members. Among the phases studied, ferrihydrite is predicted to allow Al-substitution with the lowest energy cost, while hematite appears to have the strongest tendency for phase separation. Considering the effect on thermodynamic stability relative to the stable Fe-oxide hematite, the three Fe-oxyhydroxide materials (goethite, lepidocrocite, ferrihydrite) are shown to be stabilized relative to doped Al-hematite as Al content is increased. Interactions between Al dopant atoms on neighboring Fe sites are simulated and are shown to have minimal influence on Fe-Al alloying thermodynamics in each of the materials simulated within the range of Al-dopant concentrations calculated. Simulations of Al-substitutions at the goethite (1 0 1) surface indicate that surface segregation of Al dopants is energetically favored for low Al concentrations, however this tendency diminishes when Al dopants form a full monolayer at the mineral surface.