Comment Cite This: ACS Earth Space Chem. XXXX, XXX, XXX−XXX
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Reply to “Comment on ‘Roles of Hydration and Magnetism on the Structure of Ferrihydrite from First Principles’” Michel Sassi* and Kevin M. Rosso Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
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excessively hydrous5 have also been acknowledged, we reiterate that the study performed by Sassi and Rosso2 essentially focuses on developing a better understanding of structure− composition relationships within the ferrihydrite core; surfaces along with their variable amount of adsorbed water molecules are not explicitly taken into account in the composition. With a specific focus on the composition of the core of ferrihydrite, the stoichiometry range explored spans from Fe5O8H to FeOOH (as given by Fe5O8H + 2H2O), which is within the low-H range of the composition space (i.e., OH/Fe ≤ 1) of the mineral core, generally assumed to be close to FeOOH, as stated by Manceau.1 Before the study of Sassi and Rosso,2 one could hypothesize based on the literature that tetrahedral iron, if it were to be present, might arise preferentially within drier structures (i.e., low OH/Fe ratio) rather than in more wet structures. This follows logically from a simple survey of the most commonly cited models for ferrihydrite, among which only the Michel model,3 being the driest proposed, shows tetrahedral iron, while more wet models, such as Drits,6 Jansen,7 and Manceau,8 involve only octahedral iron. However, the new insights brought by the results of the structural search reveal a more complex relationship between the likelihood of tetrahedral iron and water content, because, for example, in the search with a FeOOH stoichiometry (OH/Fe = 1), as given by Fe5O8H + 2H2O, tetrahedral iron could be identified in the structure. Sassi and Rosso2 have therefore noted at the end of the results that the presence of tetrahedral iron “might tend to arise as structural defects, either related to the kinetics of crystal growth and/or created to accommodate local magnetic stresses with iron atoms nearby on the condition that the local environment contains enough structural space to allow such a relaxation.” The essential finding is that the local magnetic stress could be a more important factor to consider in the occurrence of tetrahedral iron in a structure than the OH/Fe ratio. For reasons clearly stated within the study of Sassi and Rosso2 and again above, the Drits model by design was not the simulation target. It was not found in the structural search performed because, given its design, there of course was no way for it to be found as a result of the constrains imposed on the stoichiometry of the formula unit and the number of formula units in the unit cell. For example, goethite and lepidocrocite, two well-characterized minerals with the FeOOH stoichiometry, also could not have been found because “the number of formula units in the unit cell is not
n the comments made by Manceau1 on the study by Sassi and Rosso,2 some concerns were raised regarding the potential for the structure theoretically found and identified as ferrihydrite (Michel model3) to be hydromaghemite. Here, we address these concerns by showing that the structure theoretically generated cannot be unambiguously compared to hydromaghemite. Many of the concerns raised are, in fact, oblique to the given study. Notwithstanding, we reiterate clearly stated methodological details and caveats regarding the theoretical structural search performed, which disallow for the hydromaghemite prospect to manifest. We furthermore highlight the scholarly importance of examining experimental X-ray diffraction (XRD) and pair distribution function (PDF) of synthesized ferrihydrite samples from a variety of independent sources, as embodied in a broader set of relevant literature. First, it is important to restate the context and goals of the study performed by Sassi and Rosso.2 Given the current state of the relevant experimental literature, which spans at least 3 decades, there is no doubt that the atomic structure of what is generally referred to as nanomineral ferrihydrite remains a matter of unresolved debate. With the hypothesis that this has arisen in part as a result of natural structural and compositional variability in this material that depends upon factors such as exact synthesis conditions, aging time, etc., Sassi and Rosso2 designed and executed a first computational exploration intended not on identifying a singularly exclusive structure but instead to “develop a fundamental understanding of the prospective relationship between various proposed ferrihydrite structures and water content.” This structure search was limited in extent and comprehensiveness, as clearly stated, within a composition space primarily relevant to the model developed by Michel et al. in 2007,3 simply because, given our stated purpose, it can easily be represented as a virtual bulk lattice. In synergy with the statement made by Manceau1 indicating that PDF-based structural refinements allow for multiple solutions, this structural search explored a limited range of conformation space for thermodynamically stable structure possibilities other than the Michel model but also conceptually tested the controversial presence of tetrahedral iron in the structure, from an electronic and magnetic structure perspective. Sassi and Rosso2 acknowledged the variable amounts of OH and H2O relative to Fe in ferrihydrite as well as the experimental difficulty of separating the amount of structural and adsorbed water playing a role in determining a specific composition for ferrihydrite. While the discussions about both the Fe5O8H stoichiometry of the Michel model being depleted in OH and H2O4 and the Fe5O8H·4H2O stoichiometry being © XXXX American Chemical Society
Received: June 5, 2019 Published: June 28, 2019 A
DOI: 10.1021/acsearthspacechem.9b00160 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Comment
ACS Earth and Space Chemistry the same. Indeed, to find goethite or lepidocrocite, an integer multiple of four FeOOH formula units per unit cell is required, but in this work we have 10 FeOOH per unit cell.”, as stated in the computational details of Sassi and Rosso.2 Aware of the limitations induced by such constraints, Sassi and Rosso2 further noted in the conclusion that the structural search performed, “although exhaustive in one sense, also has not yet gone so far as to consider structure types that are based on a different number of formula units per unit cell. We have limited this search to two Fe5O8H formula unit per unit cell. A great deal of additional work would be required to explicitly and fully encompass the optimal amount of H, OH, or H2O relative to Fe, and this does not exclude the possibility of finding additional stable structure types within a different compositional framework.” For similar reasons, hydromaghemite with a composition of Fe5O8H·0.25H2O,9 cannot be found by Sassi and Rosso2 because of the composition space limitations stated above. Five compositions of Fe5O8H·nH2O stoichiometry were investigated, with n = 0, 0.5, 1, 1.5, and 2, but n = 0.25 was not considered. While it would be extremely interesting to perform complementary structural searches that investigate the composition space for which OH/Fe ≥ 1, specifically tailoring a search to find the Drits model would require a more complicated approach to handle, for example, the associated partial occupancies of Fe and O atoms in the unit cell. In the study of Sassi and Rosso,2 the designation of “ferrimagnetic ferrihydrite” never occurs. Instead, “ferrimagnetic Michel model” is used throughout the manuscript as a reference to the Michel et al. structure found in 2007.3 The fact that the Michel model turns out to possess a ferrimagnetic ground state, according to the density functional theory (DFT), is a coincidence that could lead to misinterpretation of the terminology used in the manuscript. We note that this result is in agreement with previous DFT calculations performed by Pinney et al.10 on the Michel model in 2009 (i.e., prior to the publication of the 2010 study on ferrifh11). It should also be noted that the enthalpy difference between the antiferromagnetic and ferrimagnetic state of the Michel model has been calculated to be only 1 kJ/mol.10 The discussion about XRD fits of ferrihydrite is particularly interesting. As stated by Manceau,1 ferrihydrite contains seven lines in its XRD when it is well-crystallized. However, in our case, we question the relevance of using the XRD of seven-line ferrihydrite as a reference to assess whether or not a phase can be identified as ferrihydrite, because seven-line ferrihydrite is obtained under highly specific conditions. In the list of references to seven-line ferrihydrite provided by Manceau,1 it is indicated that seven-line ferrihydrite is obtained with the presence of silicon,12 which is known to inhibit the transformation of ferrihydrite to other more thermodynamically stable iron oxide phases. Hence, more crystalline and larger ferrihydrite particles (∼20 nm) can be obtained. The other references13−15 used by Manceau1 primarily focus on the synthesis of a Sr-based iron oxide phase (hexaferrite, SrFe12O19), in which seven-line ferrihydrite was identified as an intermediate phase. In these studies, the influence of Sr on ferrihydrite formation and stabilization is not mentioned. While there is no doubt that seven-line ferrihydrite exists naturally in silicon-rich environments, the specific conditions required to obtain seven-line ferrihydrite were not used in the experiment performed by Michel et al.3 Therefore, the XRD of six-line ferrihydrite, which is a common observed phase under
the conditions of the Michel et al.3 experiment, was used as a reference. In the comments made by Manceau,1 the sentence “The Xray diffraction (XRD) patterns of 6Fh and 7Fh both fit well by the Drits model,6,15 ...” is inaccurate and has motivated the following discussion, in which the goal is to objectively compare the Drits and Michel models regarding their ability to fit XRD patterns. While the Drits model on its whole (i.e., defect-free and defective phases taken together) provides a good fit to the XRD of six-line ferrihydrite,6 the fit of the XRD for seven-line ferrihydrite performed by Granados-Miralles et al.13 (ref 6 in comments by Manceau1) found that “... the zcoordinate and the site occupation for the iron atom were refined to significantly different values.”, as indicated by the authors in the Supporting Information. While the authors only used the defect-free phase (f phase) from Jansen et al.7 in their fit, this raises questions about the ability for the defect-free phase alone to properly describe the most crystalline seven-line phase of ferrihydrite obtained in these experiments. The idea that defect-free and defective phases should not be dissociated to successfully fit the XRD pattern of ferrihydrite is further strengthened by the comments of Manceau saying “According to the Drits model,15 ferrihydrite is a mixture of a defect-free phase (f phase) and a defective phase (d phase). The volume percentages of the two components were estimated to be 67:33 for the Drits sample (6Fh) studied by XRD,15 and 50:50 for the Jansen sample studied by neutron diffraction.27”, which indicates that different ferrihydrite samples would yield a different ratio of defect-free and defective phases. Without the flexibility brought by the multiphase combination and the partial site occupation, a fit to XRD would probably be less successful. In comparison, the Michel model has less degrees of freedom because it has a full occupation for the atom sites and does not have an analogue of the defective phase (d phase) of the Drits model. This lack of flexibility makes a fit to the XRD pattern more challenging but possible. While Manceau bases his interpretations on studies using the Drits model, he overlooked other studies that used the Michel model to fit the XRD pattern of two- and six-line ferrihydrite samples.16,17 In particular, Smith et al.17 proposed a new synthetic route to two-line ferrihydrite and suggested that “ferrihydrite has a consistent, repeatable structure independent of variation in the synthetic method, water content of the sample, or particle size of the crystallites, and that its structure can be adequately described by the proposed hexagonal (Michel) model.” The idea that the core of ferrihydrite remains similar from two to six lines and that all disorder is projected to occur at the particle surface has been explored by Hiemstra,18 who has developed a surface depletion model for the Michel model, in which ferrihydrite consists of a defect-free mineral core and a defect- and water-rich surface layer that is depleted in the Fe2 and Fe3 polyhedral sites (label names referring to two iron sites in the Michel model). From such an array of investigations, it is interesting to extrapolate to the notional prospect that both the Michel and Drits models could have two complementary components. The Michel model of 2007, which has been put forward by others as a representation of the core of ferrihydrite, could correspond by analogy to the defect-free (f phase) component of the Drits model, and the surface depletion model suggested by Hiemstra18 could correspond by analogy to the defective (d phase) component of the Drits model. While this analysis opens the possibility that, depending on the conditions, B
DOI: 10.1021/acsearthspacechem.9b00160 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
Comment
ACS Earth and Space Chemistry
permanent magnets: HAADF-STEM study and magnetic properties. J. Solid State Chem. 2018, 264, 124−133. (16) Wang, X.; Zhu, M.; Koopal, L. K.; Li, W.; Xu, W.; Liu, F.; Zhang, J.; Liu, Q.; Feng, X.; Sparks, D. L. Effects of crystallite size on the structure and magnetism of ferrihydrite. Environ. Sci.: Nano. 2016, 3, 190−202. (17) Smith, S. J.; Page, K.; Kim, H.; Campbell, B. J.; Boerio-Goates, J.; Woodfield, B. F. Novel Synthesis and Structural Analysis of Ferrihydrite. Inorg. Chem. 2012, 51, 6421−6424. (18) Hiemstra, T. Surface and mineral structure of ferrihydrite. Geochim. Cosmochim. Acta 2013, 105, 316−325.
different structures for the core of ferrihydrite could exist, it also raises the question about the potential for different phases of ferrihydrite to simultaneously coexist. Especially, while the Michel model3 could be representative of a dry phase of ferrihydrite, the Drits6 model could well represent a wet phase counterpart. Hence, the premise of the initial computational study performed by Sassi and Rosso2 is fully validated.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Michel Sassi: 0000-0003-2582-3735 Kevin M. Rosso: 0000-0002-8474-7720 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Manceau, A. Comment on “Roles of Hydration and Magnetism on the Structure of Ferrihydrite from First Principles”. ACS Earth Space Chem. 2019, DOI: 10.1021/acsearthspacechem.9b00018. (2) Sassi, M.; Rosso, K. M. Roles of Hydration and Magnetism on the Structure of Ferrihydrite from First Principles. ACS Earth Space Chem. 2019, 3, 70−78. (3) Michel, F. M.; Ehm, L.; Antao, S. M.; Lee, P. L.; Chupas, P. J.; Liu, G.; Strongin, D. R.; Schoonen, M. A. A.; Phillips, B. L.; Parise, J. B. The structure of ferrihydrite, a nanocrystalline material. Science 2007, 316, 1726−1729. (4) Rancourt, D. G.; Meunier, J. F. Constraints on structural models of ferrihydrite as a nanocrystalline material. Am. Mineral. 2008, 93, 1412−1417. (5) Jambor, J. L.; Dutrizac, J. E. Occurrence and constitution of natural and synthetic ferrihydrite, a widespread iron oxyhydroxide. Chem. Rev. 1998, 98, 2549−2585. (6) Drits, V. A.; Sakharov, B. A.; Salyn, A. L.; Manceau, A. structural model for ferrihydrite. Clay Miner. 1993, 28, 185−207. (7) Jansen, E.; Kyek, A.; Schäfer, W.; Schwertmann, U. The structure of six-line ferrihydrite. Appl. Phys. A: Mater. Sci. Process. 2002, 74, S1004−S1006. (8) Manceau, A.; Skanthakumar, S.; Soderholm, L. PDF analysis of ferrihydrite: Critical assessment of the under-constrained akdalaite model. Am. Mineral. 2014, 99, 102−108. (9) Barron, V.; Torrent, J.; de Grave, E. Hydromaghemite, an intermediate in the hydrothermal transformation of 2-line ferrihydrite into hematite. Am. Mineral. 2003, 88, 1679−1688. (10) Pinney, N.; Kubicki, J. D.; Middlemiss, D. S.; Grey, C. P.; Morgan, D. Density Functional Theory Study of Ferrihydrite and Related Fe-Oxyhydroxides. Chem. Mater. 2009, 21, 5727−5742. (11) Michel, F. M.; Barron, V.; Torrent, J.; Morales, M. P.; Serna, C. J.; Boily, J. F.; Liu, Q. S.; Ambrosini, A.; Cismasu, A. C.; Brown, G. E. Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 2787−2792. (12) Berquo, T. S.; Banerjee, S. K.; Ford, R. G.; Penn, R. L.; Pichler, T. High crystallinity Si-ferrihydrite: An insight into its Néel temperature and size dependence of magnetic properties. J. Geophys. Res.: Solid Earth 2007, 112, B02102. (13) Granados-Miralles, C.; Saura-Muzquiz, M.; Bojesen, E. D.; Jensen, K. M. O.; Andersen, H. L.; Christensen, M. Unraveling structural and magnetic information during growth of nanocrystalline SrFe12O19. J. Mater. Chem. C 2016, 4, 10903−10913. (14) Grindi, B.; Beji, Z.; Viau, G.; BenAli, A. Microwave-assisted synthesis and magnetic properties of M-SrFe12O19 nanoparticles. J. Magn. Magn. Mater. 2018, 449, 119−126. (15) Grindi, B.; BenAli, A.; Magen, C.; Viau, G. M-SrFe12O19 and ferrihydrite-like ultrathin nanoplatelets as building blocks for C
DOI: 10.1021/acsearthspacechem.9b00160 ACS Earth Space Chem. XXXX, XXX, XXX−XXX
