First-Principles Studies of Paramagnetic Vivianite Fe3(PO4)2·8H2O Surfaces

Using density-functional theory, we have computed the structural and electronic properties of paramagnetic vivianite crystal Fe3(PO4)2·8H2O and its (010)-(1 × 1) and (100)-(1 × 1) surfaces. The properties of bulk vivianite are studied with a set of functionals: HSE06, PBE, AM05, PBEsol, and PBE with on-site Coulomb repulsions corrections (PBE+U). The appropriate U parameter is estimated by considering the HSE06 results, and it is used to study the vivianite surfaces. The computed surface energy predicts the (010) surface to be the most stable. The less stable (100) surface is observed to have important reconstructions with the spontaneous formation of a water molecule at the surface and two hydroxide hydrate anions per unit cell. Using thermodynamical considerations within DFT, we have calculated the phase diagram of the (010) surface in equilibrium with hydrogen gas. The results suggest that under ultralow hydrogen pressure, the (010) surface with two hydrogen vacancies is stable. The electronic structure calculations for the surfaces are complemented with the computed scanning tunneling microscopy (STM) images for constant-current mode. The topology is dominated by the surface Fe-3d states that protrude into the vacuum.