Modeling the iron oxides and oxyhydroxides for the prediction of environmentally sensitive phase transformations

Abstract

Iron oxides and oxyhydroxides are challenging to model computationally as competing phases may differ in formation energies by only several kJ mol −1 , they undergo magnetization transitions with temperature, their structures may contain partially occupied sites or long-range ordering of vacancies, and some loose structures require proper description of weak interactions such as hydrogen bonding and dispersive forces. If structures and transformations are to be reliably predicted under different chemical conditions, each of these challenges must be overcome simultaneously, while preserving a high level of numerical accuracy and physical sophistication. Here we present comparative studies of structure, magnetization, and elasticity properties of iron oxides and oxyhydroxides using density functional theory calculations with plane-wave and locally-confined-atomic-orbital basis sets, which are implemented in VASP and SIESTA packages, respectively. We have selected hematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and magnetite (Fe 3 O 4 ) as model systems from a total of 13 known iron oxides and oxyhydroxides; and use same convergence criteria and almost equivalent settings in order to make consistent comparisons. Our results show both basis sets can reproduce the energetic stability and magnetic ordering, and are in agreement with experimental observations. There are advantages to choosing one basis set over the other, depending on the intended focus. In our case, we find the method using PW basis set most appropriate, and combine our results to construct the first phase diagram of iron oxides and oxyhydroxides in the space of competing chemical potentials, generated entirely from first principles.keywords: iron oxides and oxyhydroxides; phase diagram; density functional theory; modeling and simulation.

Aluminum coprecipitates with Fe (hydr)oxides: Does isomorphous substitution of Al3+ for Fe3+ in goethite occur?

“…where Dl 0 (T r ) is the connection energy at standard pressure (1 atm) and temperature (300 K), and C p is the molar heat capacity (Guo and Barnard, 2011). As reported in Table 2, the results of these calculations showed that the relative stabilities of Al-goethites were not affected by temperature corrections.…”

“…The difference for the six Al case is even greater, so the ZPE approximation would have an even smaller effect for these models. Lastly, we tested the effect temperature differences might have on the relative energies of our systems (third assumption) by comparison of DTOTEN VASP rxn (at 0 K) and connection energy at standard state conditions (Guo and Barnard, 2011). The connection energy, which represents energy variations between TOTEN at 0 K and at standard state, was calculated by:…”

Environmentally dependent stability of low-index hematite surfaces

“…Experimental studies encounter difficulties of reproducibility in controlling environmental conditions and characterizing hydrated surfaces [17][18][19]. Computational modeling is confronted with strongly-correlated electronic structures of hematite (and other iron oxides) [20][21][22] and an enormous number of possible surface hydroxylation configurations [23,24].…”

First-principles study of neutral defects in Fe-doped cubic barium titanate

“…Substitution energies were calculated for each mode at the vertices of the polygon (points A to E, which run from maximum oxidizing to maximum reducing conditions) for three different iron environments: FeO (case 1), α-Fe 2 O 3 (case 2) and bcc bulk Fe (case 3). Table 2 compares the values for the cohesive energies, lattice parameters, energy gaps and magnetic moment per site calculated in this work with those taken from the literature [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] -no data were found for the gaps of tetragonal BaO 2 and TiO. In the context of this work, these data are relevant, since the defect energies are defined in terms of the respective chemical potentials, which depend on the cohesive energies.…”