Article pubs.acs.org/JPCC
First-Principles Studies of Paramagnetic Vivianite Fe3(PO4)2·8H2O Surfaces Henry P. Pinto,* Andrea Michalkova, and Jerzy Leszczynski Interdisciplinary Center for Nanotoxicity, Department of Chemistry, Jackson State University, Jackson, Mississippi 39217, United States ABSTRACT: 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.
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INTRODUCTION The vivianite Fe3(PO4)2·8H2O mineral has been experimentally well studied, but there are sparse theoretical models that support those experiments. The surfaces of vivianite have been experimentally less studied, and no reliable atomic-scale model exists for the surface structure and their properties. The reason might be associated with experimental difficulties in preparing clean surfaces free of impurities. Developing and understanding structural models for the surfaces of paramagnetic vivianite Fe3(PO4)2·8H2O is the aim of this work. The vivianite group of minerals are hydrated iron phosphates having the A32+(XO4)2· 8H2O general formula. A2+ can be any of the following elements: Co, Fe, Mg, Ni, and Zn. The variable X is either As or P. They can be found in coatings of water pipes, soils, morasses, and sediments, which makes them photosensitive.1,2 Vivianite is a typical member of this mineral group with the Fe3(PO4)2·8H2O chemical formula. The vivianite crystal structure has a monoclinic lattice with C2/m symmetry and with cell parameters a = 10.021 Å, b = 13.441 Å, c = 4.721 Å, and β = 102.84°. 3 The vivianite crystal is also an antiferromagnet with a Neél temperature TN ∼ 10 K, above this temperature, vivianite has paramagnetic properties.4 Hydrogen bonding between the H2O ligands holds together sheets consisting of linked Fe and PO4 polyhedra.5 Vivianite can be oxidized through auto-oxidation or by the air when Fe2+ is oxidized to Fe3+.1,6 It is typical for anoxic environments and indicative for geochemical conditions where ferric iron oxides usually dissolve.7 It has great chemical and thermal stability. Vivianite can disintegrate into strongly magnetic magnetite and weakly magnetic hematite upon heating in air.8−10 © 2014 American Chemical Society
Several experimental studies on vivianite have been published. The early qualitative structure of vivianite11 has been redetermined by X-ray12 as well as by neutron diffraction.3 The vibrational and rotational atomic properties of bulk vivianite have been carefully studied by optical and near-IR spectroscopies.13−15 The magnetic properties of vivianite have been investigated using different techniques such as NMR,16 specific heat,17 static susceptibility measurements,18 neutron diffraction,19 and Mössbauer spectroscopy.20 According to our best knowledge, only one theoretical study of the electronic structure of vivianite has been published.21 The authors have investigated the electronic and magnetic structure of vivianite using the cluster molecular orbital calculations in the local spin density approach. They have assigned unambiguously the optical and Mössbauer spectra for ferrous iron. However, the assignment for ferric iron was not conclusive due to uncertainties in the geometrical changes accompanying the oxidation.21 Experiments on vivianite surfaces are scarce, and we are only aware of the work of Pratt.22 In that study, a X-ray photoelectron spectroscopy was performed on vivianite (010) surfaces cleaved in a N2 gas atmosphere. The main result points an autoreduction−oxidation process triggered by the rupture of hydrogen bonds leading to the formation of the hydroxyl groups and ferric sites; this process was originally suggested by Moore et al.23 and experimentally confirmed by Pratt.22 Received: May 18, 2013 Revised: February 24, 2014 Published: March 2, 2014 6110
dx.doi.org/10.1021/jp404896q | J. Phys. Chem. C 2014, 118, 6110−6121
The Journal of Physical Chemistry C
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COMPUTATIONAL DETAILS AND METHODS Density-functional theory (DFT) calculations have been performed using the plane wave basis Vienna ab initio simulation package (VASP).24,25 We describe the Fe-[Ar], O1s2 and P-[Ne] core electrons with projector augmented wave (PAW) potentials.26 Using a cutoff kinetic energy of 650 eV and a Γ-centered Monkhorst−Pack grid with 0.04 Å−1 spacing between k points (e.g., this is equivalent to 3 × 3 × 5, 6 × 3 × 1, and 2 × 5 × 1 grids of the corresponding primitive cell of bulk vivianite, Fe3(PO4)2·8H2O (010)-(1 × 1) slab and Fe3(PO4)2·8H2O (100)-(1 × 1) slab, respectively), we converge the total energy to
