Investigation of the IronâSulfide Phase Transformation in Nanoscale

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Investigation of the Iron Sulfides phase transformation in nanoscale Pengpeng Bai, Shuqi Zheng, Changfeng Chen, and Hui Zhao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg500333p • Publication Date (Web): 28 Jul 2014 Downloaded from http://pubs.acs.org on August 11, 2014

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Crystal Growth & Design

Investigation of the Iron Sulfide phase transformation in nanoscale Pengpeng Bai, Shuqi Zheng *, Changfeng Chen, Hui Zhao State Key Laboratory of Heavy Oil Processing and Department of Materials Science and Engineering, China

University of Petroleum, Beijing 102249, P R China

E-mail:[email protected]

Abstract Iron sulfides, polymorphous substance, play a key role in geology, marine systems, material science and the life sciences. The crystal structures and phase transitions of iron-sulfur compounds are complex and elusive. In this study, we fabricated most known iron sulfides, mackinawite was the first reaction product and declined gradually until disappeared; the following Cubic FeS and troilite presented a characteristic of layer growth. Using high-resolution transmission electron microscopy, we directly observed for the first time the transition of cubic FeS into greigite and that of mackinawite into greigite. Greigite, which is extremely unstable in an oxygen-free H2S environment, was found to play an intermediate role in the transition into a more stable phase.

Keywords: iron sulfides; crystal structure; crystal growth; phase transition; H2S.

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1. Introduction Iron sulfides occur in various crystallographic phases and many of them play a key role in the world. At least seven different solids consisting only of iron and sulfur through the formation of different stoichiometric ratios are currently known: mackinawite (FeS), cubic FeS, troilite (FeS), pyrrhotite (Fe1-xS), smythite (Fe9S11), greigite (Fe3S4), pyrite (FeS2), and marcasite(FeS2).1 Studies on the phases of iron sulfides can be traced back to the last century or earlier. Geologists, physicists, chemists, biologists, and materials scientists have expressed keen interest in iron-sulfur compounds.2-4 Physicists reported the partial phonon densities of states (DOS) of iron sulfide,5 and the density functional theory were used in the modeling of a number of iron sulfide like mackinawite and cubic FeS.6-8 More new phases composed with iron and sulfides were found by geologists or prepared in the laboratory with the development of science and technology during the recent fifty years,9-11 and then nanoscale iron sulfides were synthesized and characterized by various methods in last several years.12-18 The phases of iron-sulfur have distinct characteristics relevant to the origin of life on earth and interesting properties related to photovoltaic and the steady state flux of hydrogen.19-22 Intriguingly, iron-sulfur compounds have the capacity to transform into different phases under certain conditions, such as high temperature or pressure;23-25 the laws and mechanisms of the phase transitions are significant to synthesis new iron-sulfur phases and even study the origin of life deeply. Previous studies have yielded various results on the phase transitions of iron-sulfur compounds in geological systems, microbiological systems, and materials science. During a heating progress in inert atmosphere, mackinawite has been found to transform to greigite above 100 °C, and then to a mixture of pyrrhotite and magnetite above 245 °C;26 Pyrite has been found to transform to pyrrhotite between 2


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500-600 °C, and pyrrhotite remains stable even at 900 °C;27 Troilite, which has a hexagonal structure at low temperature, has been found to transform to a MnP-type structure at 140 °C (α-transition), and then to a 1C superstructure of NiAs-type structure at 315 °C (β-transition).28-29 In addition, troilite can convert to other crystal structures under high pressure; the transition sequence at 0 °C is as follows: troilite → antiferromagnetic MnP-type structure (3.4 GPa) → monoclinic structure (6.7 GPa) → nonmagnetic MnP-type structure (40-135 GPa) → distorted NaCl-type structure (135-400 GPa).28, 30 Iron-sulfur compounds can even evolve only with H2S at relatively low temperature and pressure. Ian b. Butler and David Richard found mackinawite can convert to greigite, and then to pyrite at 60 °C in oxygen-free aqueous solution of H2S,31 and Y. Li et al. found mackinawite can transform to pyrrhotite (Fe9S10) at 150 °C, and then to pyrrhotite (Fe7S8) above 300 °C in H2S atmosphere.32 Some materials are sensitive to the high energy induced by TEM beam or focused ion beam (FIB), such as aluminium magnesium alloy, and the beam energy can induce damage or reconfiguration of the specimen structure.33 Among the iron-sulfide compounds, mackinawite has been found could convert to other stable phases by high energy electron-beam irradiation, since 1970 when Horiuchi et al. reported greigite could form from mackinawite reacted with amorphous sulfur in a TEM with heating, there has been a problem of where the electrons involved in the oxidation reaction came from.34 Lennie et al. showed that the total composition after the transition remained FeS.26 Though above studies showed mackinawite could transition to other phases under the electron-beam irradiation, the phase change is not absolute, Mihá ly Pó sfai described the transition of mackinawite to greigite in magnetotactic bacteria, on p880 he writes “We did not observe any changes in the crystal (either as mackinawite or as greigite) while it was exposed to the electron beam.” 3 3



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Previous studies are significative and helpful to our work, however, they are not detailed enough and direct observations are much more scarce. Various types of phase transitions found in minerals, and they include displacive phase transitions, orientational order-disorder phase transitions, and cation ordering transitions.35 Among the numerous phases in Fe-S system, there are complex mechanisms of phase transformation. As a solution to this conundrum, we fabricated most of the known iron sulfides (i.e., mackinawite, cubic FeS, greigite, troilite, and the coexisting domains of the two phases in transition) through the reaction between steel and H2S under a pressure of 1 MPa in an oxygen-free environment at different time, and the stage of transformation was determined between 18h to 21h. We also observed the transitions of cubic FeS, greigite, and mackinawite in nanoscale via high-resolution transmission electron microscopy (HRTEM); the transition of cubic FeS and greigite is the first time.

2. Experimental procedure The experimental material was a commercial X52 pipeline steel, which had a microstructure of typical ferrite and pearlite. Its chemical composition (wt %) is as follows: C, 0.13; Si, 0.4; Mn, 1.5; P, 0.02; S, 0.003; Cr, 0.3; and Fe balance. The specimen was machined in a disc 3mm in thickness and 30 mm in outer diameter. Prior to each experiment, the specimen was ground with a silicon carbide paper of 400 grit to 1500 grit, degreased with acetone, thoroughly rinsed with distilled water, and dried quickly in cold air to avoid oxidation. The test was conducted in an autoclave (70 MPa) made by Corrtest Company. The test solution was oxygen-free distilling water with H2S at 1 MPa pressure and the text solution temperature was 50 °C. Before the test, the autoclave was desecrated using high purity N2 for 12 h. The experiment was begun when the required test conditions had been reached and the text solution pH was 4.21 by actual measurement. The specimens were taken out from the autoclave, rinsed 4


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with distilled water and absolute alcohol, dried in air at the end of the experiment, and then stored in an oxygen-free box. The reaction products were characterized by Scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). SEM images were taken with an FEI Quanta 200F scanning electron microscope. The crystal structures of the reaction products were characterized by a Bruker AXS XRD-D8 Focus x-ray diffractometer with CuKα radiation, measurement was in the 2θ range of 10-90° with a scanning step of 2 (deg/min). Selected area electron diffraction (SAED) patterns, and high-resolution transmission electron microscopy (HRTEM) images were recorded on an FEI F20 field-emission transmission electron microscope with an acceleration voltage of 200 kV. About 0.2mg mash sample dispersed in absolute alcohol onto a carbon-coated copper grid for TEM observed. In order to avoid the influence by high energy of electron beam, a low light was chosen to observe the iron-sulfide compounds, and took picture quickly. The crystal structures models of iron sulfides were made by CrystalMaker.

3. Results and discussion The microscopic characteristic and crystal growth of iron-sulfur compounds. In our prior work, we showed that the types of iron sulfides change with the reaction time of iron and H2S, and the changes affect the steady state flux of hydrogen in steel,20 and then the growth and conversion of iron sulfides engaged our attention and interest. The formation of iron-sulfur compounds on steel surface in solutions saturated with H2S has been studied using electrochemistry techniques by Shoesmith et al.; they found mackinawite was the first reaction product which formed by both solid state and precipitation processes, and cubic FeS and 5



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troilite were the subsequent phase due to the rapture of mackinawite base layer, and the sequences of reaction products was mackinawite → cubic FeS → troilite → pyrrhotite → pyrite.36-38 However, the conversions of iron sulfides have not been studied very well. Under our experimental conditions, iron sulfides are formed not only in the progress of the crystal growth but also in the progress of the phase transition. Different phases are highly distinct from one another. We found mackinawite was the initial reaction product; it is consistent with previous studies. Mackinawite, an important mineral that interests many researchers in various fields, possesses a tetragonal layer structure, in which the Fe atoms are linked in a tetrahedral coordination to four equidistant sulfur atoms.39 In present work, mackinawite was not stable in aqueous solution with high concentration of H2S; through changing reaction time, we determined the period of transition was between 18h to 21h (Fig. 1). Mackinawite had the tendency of declining and disappearing, meanwhile troilite began to form and became the main phase gradually at this stage; in addition, the XRD pattern shows some peaks of greigite appeared at 19h, one hour later, pyrite is found also. Always, greigite is considered as a kinetic product, and very rare in aqueous solution with high concentration of H2S. It's worth mentioning that the amount of greigite, pyrite, cubic FeS and mackinawite are few after 21h, meanwhile, troilite is the main phase, and it is probably because the test condition is more conducive to the growth of troilite. This period of transition is very critical because hexagonal FeS (pyrrhotite and troilite) was deemed to inhibit corrosion of steel in wet H2S environment, and the protection performance was remarkable.40

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Figure 1. XRD patterns of products at 18, 19, 20 and 21 h. The main type of products at 18h is mackinawite, and the percentage of cubic FeS and troilite is relatively few. One hour later, greigite can be found from the XRD pattern; Two hours later, pyrite appeared; Three hours later, the main products were troilite, meanwhile, a small amount of greigite, pyrite, cubic FeS and mackinawite can be found.

Figure 2 shows SEM of iron-sulfur compounds formed on the steel substrates in test situation at 18h, 19, 20 and 21 h; various types of iron sulfides simultaneously appears during this period, and even the coexisting domains of the two phases in transition could be found. Scattered beam-shaped products can be seen on the surface of fest-shaped products in Fig. 2A, which has been identified as troilite by some researchers.41 Troilite has a NiAs-type superstructure with a unit cell of a = √3 A and c = 2C (where A and C refer to a NiAs-type structure), and some studies showed the microtopography 7



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of troilite generated at the steel surface in wet H2S environment is needle-like.42 Combining the SEM and XRD pattern, we think most of the beam- or needle-shaped troilite crystals generate on the surface of mackinawite, due to the mackinawite film hindered the outward motion of iron ions, result in a new iron-lack phase growth.43 Fig. 2C and 2D show the low-magnification and high-magnification images of the iron-sulfur compounds formed at 19h, different from the product at 18h, more flower-like compound can be seen instead, it can be considered as a fast growing period for troilite. One hour later, large amount of flower-like troilite appeared in a vision, and the macrocosm appearance of sample became shiny, in addition, few framboid-shaped pyrite (FeS2) grains have been observed (Fig. 2F). The compounds generated at 21h are similar to the products at 20h; furthermore, more framboid-shaped pyrite crystals are dispersive arrangement around troilite (Fig. 2H). A schematic diagram of the growth of iron-sulfur compounds on the steel has been given as show in Fig.3. First, the lack-S phases such as mackinawite are formed on the surface of steel due to the excessive iron ions from the dissolution of steel (Fig. 3 A). Secondly, after the surface of steel covered with the lack-S phases extensively, the lack-Fe phases such as troilite are formed due to the decrease of iron ion on the solid-liquid interface (Fig. 3 B,C), meanwhile, some phases (mackinawite and cubic FeS) convert to other lack-Fe phases via solid phase transition. At last, the lack-Fe phases become the main phase and growth rapidly (Fig. 3 D). Greigite, which can be considered as a minor phase from XRD pattern, unfortunately, cannot be identified from the SEM, more likely, it has no certain shapes and just a transition phase. The other main phase is the cubic FeS with an octahedron-shaped, and it is relatively studied less because of its unstable character relevant to the other phases. The octahedron-shaped crystals are widespread on the surface of steel (Fig 2E, Fig 4A). Furthermore, the high-magnification image showing the cubic FeS 8


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has a character of layer growth (Fig. 4 B), and the growing beam-shaped troilite is observed and its layer-by-layer growth threads are clearly visible also (Fig. 4 C,D).

Figure 2. SEM of diverse iron-sulfur compounds formed at18h, 19, 20 and 21 h. The labels M, C, and T stand for Mackinawite, Cubic FeS, and Troilite, respectively. (A) Low-magnification image and (B) High-magnification image of the iron-sulfur compounds formed at 18h. Various shaped compounds coexisted and the perfect beam-shaped iron sulfide with lengths between 40 µm to 60 µm. (C) Low-magnification image and (D) High-magnification image of the iron–sulfur compounds formed at 19h. Some single crystals fused with one another, an approximate flower-like iron-sulfur compound appeared. (E) Low–magnification image and (F) High–magnification image of the iron–sulfur compounds formed at 20h. Large amount of flower-like troilite crystals growth quickly. (G) Low–magnification image and (H) High–magnification image of the iron–sulfur compounds formed at 21h. Flower-like troilite became the main phase during 3 hours, and mackinawite almost disappeared.

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Figure 3. Schematic diagrams of the growth of iron-sulfur compounds on the steel.

The crystal growth mechanism has been studied for more than 100 years, there are two serious theories among the numerous theories. First one is the two-dimensional nucleation mechanism, which was proposed by Kossel in 1927 and modified by other researchers in the subsequent studies.44 The other one is the spiral growth mechanism, which was first proposed by F. C. Frank in 1949, suggesting that crystal growth could cause screw dislocations-linear defects oriented to the growing surface normally, and forming the core of a lattice structure partial akin to a spiral staircase.45 The height of the growing layer was measured as a single unit cell.46 The growth steps in numerous crystals have been 10


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observed directly by advanced technologies during the past several years,47 in especial, many studies show the progress of crystal growth in situ by advanced atomic force microscopy (AFM). Microscopic spirals always appear on the surfaces of crystal grown from solution, and at least three kinds of spirals layers (single, double and triple) could be found by single dislocation outcrops, depending on the growth condition.48 Gen Sazaki et al. found that two adjacent single steps coincide to form a double-bilayer, and the steps of two single bilayers detach with increasing radii of their spiral steps gradually at the center of a spiral growth hillock.49 Unfortunately, it is impossible to observe the growth of iron sulfides by the in situ AFM, because the growing solution contained H2S, which is hypertoxic and corrosive. In present work, the layer growth steps of cubic FeS and troilite are found by SEM, and the concentric closed-loops induced by a pair of dislocations with opposite sign are clearly visible (Fig 4B, Fig 4E), meanwhile, no double or triple spirals layers are found. Fig 4C and 4F show the schematic diagrams of the concentric closed-loops spiral steps of cubic FeS and troilite, respectively, A and B marks indicated the pair of screw dislocations. The crystal structures and transformations of iron-sulfur compounds Using HRTEM and SAED, we studied the crystal structures and transformations of iron sulfides. Iron-sulfur compounds are a kind of polymorphous substance, occurrence in almost all base types of crystal structures. It is different to identify mix phases and find the part transition grains just with XRD, because XRD only yield information on the average structure of the crystals. So we choose TEM as our technique to study the crystal structures and phase transitions of iron sulfides; most specimens contain diverse crystal structures that can be best studied by HRTEM and SAED techniques due to the high resolution and accuracy.

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Figure 4. SEM and 3D schematic diagrams of a single crystal of FeS showing the growth front arising from a pair of dislocations of opposite sense. The labels C and T stand for Cubic FeS, and Troilite, respectively. (A) Low-magnification image and (B) High-magnification image showing perfect cubic FeS has a character of layer growth. (C) 3D schematic diagrams model of the cubic FeS. (D) Low-magnification image showing that beam-shaped troilite crystals distribute on the surface of bulk FeS scattered. (E)The growing beam-shaped troilite has a growth pattern along one direction. (F) 3D schematic diagrams model of the beam-shaped troilite.

The crystal structures of iron-sulfur compounds Mackinawite (Tetragonal FeS) has been studied very well, so we did not discuss its crystal structure in present work. Instead, we investigated cubic FeS and greigite which play key roles in 12


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phase transition perhaps. Cubic FeS, which has a sphalerite structure, is much rarer geological mineral. Mainly, it is the product of the reaction of iron with H2S and has been found in magnetotactic bacteria in recent years. 50-51

Although it is not been found in nature, it is worth an in-depth study because it may be a precursor

to mackinawite or others.52 The crystal structure was identified through selected area electron diffraction (SAED) patterns (Fig. 5). The SAED patterns may serve as the [100], [211], and [111] of cubic FeS. Obvious SAED patterns of the twins were also observed.

Figure 5. SAED patterns in three different zone axes of the cubic FeS (A) Low-magnification image showing the submicrometer grain. (B to D) The SAED patterns along three different zone axes may be the [100], [211], and [111] of cubic FeS, respectively. The 13



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SAED patterns of the twins can be identified in the zone axes of [211] and [111].

Greigite, which has an inverse spinel structure (space group Fd3m and Z = 8), was discovered in the sediments of Miocene Lake in California by Skinner et al..53 It is a widely occurring mineral and has been found to cause the magnetotactic bacteria to be oriented in magnetic fields.54 In our study, we found a few gains with the structure of greigite through TEM (Fig. 6), and the SAED pattern indicates that the structure may be that of greigite [211]. The lattice spacings of 5.8, 3.5 Å in the corresponding HRTEM are in good agreement with the value of the lattice spacings of the (111) and (022) planes of the greigite. The HRTEM image shows few defects randomly distributed throughout the crystal. Two stacking faults parallel to the (111) plane were observed as indicated by the black arrows (the white arrows indicate the crystal boundary).

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Figure 6. HRTEM image and SAED pattern of the greigite (A) Low-magnification image showing the truncated octahedron-shaped grain. The images studied are located inside the black box. (B) HRTEM image obtained from the edge of the truncated octahedron-shaped grain. The lattice spacings of 5.8, 3.5 Å in HRTEM are in good agreement with the value of the lattice spacings of the (111) and (022) planes of the greigite. The HRTEM image shows few defects randomly distributed throughout the crystal. Two stacking faults parallel to the (111) plane were observed as indicated by the black arrows. The white arrows indicate the crystal boundary. (C) The corresponding SAED pattern of HRTEM may be the [211] of greigite. (D) The atoms image from black box of (B).

Figure 7. SAED pattern of pyrite. (A) Low-magnification image showing the framboidal-shaped crystal; (B) SAED pattern corresponding to [211] of the pyrite.

Pyrite, which has a cubic structure, is the most common sulfide mineral in Earth surface 15



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environments. Many researchers studied the pyrite and it appears as framboidal-shaped without shape-controlled synthesis. David Richard and his collaborators have contributed many significant works in the research of pyrite, and they have synthesized framboidal-shaped pyrite in H2S environment and studied this type of pyrite in detail.1 The structure of pyrite through TEM (Fig. 7) and the SAED pattern indicates that the structure may be that of pyrite [211]. The phase transformations of iron-sulfur compounds Many studies have shown that the properties of iron-sulfur compounds change with the phase transition, like magnetism, and cause new phenomena. However, the transitional phase is seldom observable because it is an ephemeral progress. The true transformed phase may be ignored because few phases are unstable in certain environments and act as transition states, which rapidly change to a stable phase. The phase transitions of numerous phases of iron-sulfur compounds have complex mechanisms, the various types of which are found in minerals.35 In this study, we observed the progress of the transformation of mackinawite, cubic FeS, and greigite through SAED pattern and HRTEM. The study about phase transitions of mackinawite is the most reported and mackinawite was regarded as a precursor which can convert to other phases (i.e., greigite, pyrrhotite and pyrite). The reports about phase transition of Cubic FeS is poor except it can convent to mackinawite gradually, concerned with the instability in nature.1 In a previous study, the disappearance of the initial metastable phases (mackinawite and cubic FeS) was considered to be a transformation into troilite and pyrrhotite intermediates and finally to the stable pyrite phase through redissolution and recrystallization.41 We basically agree with the explanation of previous studies for this phenomenon, however, how mackinawite and cubic FeS transform into other phases has not yet been investigated in 16


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detail. We observed the partial transition of mackinawite into greigite (Fig. 8) and the partial transition of cubic FeS into greigite (Fig. 9), an apparently random distribution of these phases.

Figure 8. Transformation of mackinawite into greigite (A) Low-magnification image showing the plate-shaped grain. The studied images are located inside the black boxes. (B) SAED pattern in the zone axes of [101] of the mackinawite obtained from a flake-shaped grain at 6 h period. (C) SAED pattern and (D) corresponding HRTEM image of a mixed mackinawite and greigite from the plate-shaped grain. Additional reflections appear (arrows) relative to (B), the SAED patterns showed two sets of diffraction can that may be the [101] of the mackinawite and [211] of greigite. (D) Crystal types of iron sulfides identified through the 17



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distinctions of the d spacings. The crystal boundary (black arrows) is obvious. Abundant crystal defects (white arrows) were observed and it was most likely the cause of the phase transition.

The transformation of mackinawite to greigite has been studied for a long time through various methods and has been found as a common phenomenon in different conditions, especially in magnetic bacteria, most likely through the movement of Fe atoms between neighboring S layers.1 In our study, the appearance of new reflections of reciprocal lattice rows and changed lattice spacings shows that mackinawite was changing into greigite. The SAED pattern (Fig. 8C) may be interpreted as a composite of mackinawite [101 ] and greigite [211] projections. The HRTEM shows a complex structure in its changing phase, with mass defects, like small translations and dislocations.

Figure 9. Transformation of cubic FeS into greigite

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(A) Low-magnification image showing the cubic FeS in transition obtained at the 21 h period. The studied images relocated inside the black boxes. (B) HRTEM image, (C) corresponding Fast Fourier transform (FFT) pattern, and (E) SAED pattern of the cubic FeS in transition. Additional reflections appear (arrows) relative to the zone axes of [001] of the cubic FeS. (D) The SAED patterns showed two sets of diffraction may be the [001] of the cubic FeS and [001] of greigite. (B) The d spacings are distinct, the phase boundary (black arrows) is obvious, and the stacking faults (white arrows) and twinned crystals (white arrows) are evident. All these findings verify that cubic FeS is capable of transforming into greigite in oxygen-free environments.

Few studies have shown that cubic FeS may gradually be transformed into mackinawite at room temperature.55 In our work, the transformation of cubic FeS into greigite is first observed, following by the appearance of new reflections of reciprocal lattice rows, which shows that cubic FeS was indeed changing to greigite. The SAED pattern (Fig. 9E) may be interpreted as a composite of cubic FeS [001] and greigite [001] projections, and the HRTEM shows the transition phase of cubic FeS into greigite. The obvious phase boundary, which was with several atomic layers thick, may be seen as the transition region along the dark arrows. The stacking fault and twinned crystal along the white arrows were also seen. Cubic FeS has a zincblende structure (F 43m; space group 216; face centered cubic unit cell), and the lattice parameters a = b = c = 5.423 ± 0.001 Å, with a Fe-S bond length of 2.348 Å. The S atoms are arranged on the nodes of a face-centered cubic lattice, while one half of the tetrahedral holes are occupied by the Fe atoms.50 Figure 10 shows the cubic FeS structure. Greigite has a spinel structure (space group Fd3m), and the unit cell of greigite contains 56 atoms 19



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of which 24 are Fe and 32 are S. The Fe atoms are divided into two sub-lattices, 8 Fe atoms occupying tetrahedral coordinated sites and 16 on octahedral sites and the sulfur atoms are bonded in a close-packed cubic lattice.53 Figure 11 shows the greigite structure.

Figure 10. Cubic FeS structure viewed along the off-axis (A) and c axis (B). Large and small spheres represent Fe and S atoms, respectively.

Figure 11. Greigite structure viewed along the off-axis (A) and c axis (B). Large and small spheres represent Fe and S atoms, respectively.

Previous studies proposed the cubic FeS convert to mackinawite in the H2S environment,55 whereas a different result from the present study shows cubic FeS convert to greigite, not 20


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mackinawite. The XRD pattern shows mackinawite declines and disappears from 18-21h; meanwhile, cubic FeS still exists as a main phase. So it is obvious that most of the cubic FeS is not converting to macknawite in our condition, instead to another phase. Furthermore, the coexistence region of cubic FeS and greigite is found under TEM, combining the XRD date, we think cubic FeS transform to greigite in aqueous H2S solution. The reasons why cubic FeS converts to greigite at low temperature (50°C) in H2S solution are complex. First, the unbroken iron sulfide films which formed at 18h can prevent the diffusion of iron ion toward the solid-liquid interface, result in the lack of iron ion in the H2S solution during the growth of cubic FeS, and then cubic FeS has to develop to another phase to continue grow. Secondly, H2S is an important factor in the transition process due to its oxidization, a part of Fe2+ can be oxidized to Fe3+ in the H2S solution.32 Above factors cause the phase transition of cubic FeS. Fig.9 shows the [001] of cubic FeS is parallel to [001] of greigite, and (220) of cubic FeS is parallel to (220) of greigite, therefore, the cubic close-packed S arrangement of cubic FeS is retained in greigite. Meanwhile, the excess iron atoms, which are removed during the oxidation of Fe2+ to Fe3+, transfer in the crystals and revolve around S atom to seek an equilibrium state. Ultimately, the arrangements of atoms result in approach to the structure of greigite. In our work, mackinawite and cubic FeS were also observed as transforming into greigite, a precursor in the formation of pyrite in anoxic sulfate-reducing sedimentary environments.56 Considering the phenomenon we observed and those in previous studies, we deduce that greigite is a transitional phase that is more unstable than cubic FeS in oxygen-free H2S environments. Correspondingly, greigite is stable in magnetotactic bacteria. Thus, we think that greigite is most likely a bridge from one phase to another, the length of which depends on environmental conditions (pressure, 21



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temperature, and oxide). Though the whole transition mechanism have not been studied exhaustive extremely thorough, we found greigite play a key role in the entire reaction as a transitional phase. Published papers have demonstrated greigite is the precursor to pyrite,56 we think the discovered pyrite in our study came from greigite. However, the origin of troilite is still indistinguishable.

4. Conclusions In summary, through immersion of steel in oxygen-free solutions saturated with H2S at 50 °C and 1MPa pressure for 18h-21h, we found mackinawite is the first reaction product and convent to other iron-sulfur compounds rapidly. Cubic FeS, the following product with an octahedral shape, has a character of layer growth, and may be come from solid phase transition, also convent to other iron-sulfur compounds. Troilite, the overriding phase in the last with a beam shape, presents the characteristics of layer growth and has a growth pattern along one direction. Especially, we found greigite play a transitional phase role in the transition of mackinawite and cubic FeS to others, and it is extremely unstable in oxygen-free hydrogen sulfide environments. For the first time, through the use of high-resolution transmission electron microscopy, we demonstrated that cubic FeS can be transformed into greigite just like mackinawite. We also observed there are abundant defects (stacking fault, twin crystal, dislocation) in the cubic FeS, which maybe concerned with its instability.

AUTHOR INFORMATION Corresponding Author

*Email: [email protected] 22


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT: This work was financially supported by the Natural Science Foundation of China (No.51171208 and 51271201) and the Science Foundation of China University of Petroleum, Beijing (No. LLYJ-2011-41). REFERENCES (1) Rickard, D.; Luther, G.W. Chem. Rev. 2007, 107, 514. (2) Johnson, D.C.; Dean, D.R.; Smith, A.D.; Johnson, M.K.D.C. Annu. Rev. Biochem. 2005, 74, 247–281. (3) Pó sfai, M.; Buseck, P.R.; Bazylinski, D.A.; Frankel, R.B. Science. 1998, 280(5365), 880–883. (4) Toulmin, P.; Barton, P.B. Geochim. Cosmochim. Acta. 1964, 28(5), 641–671. (5) Kobayashi, H.; Kamimura, T.; Alfe, D.; Sturhahn, W.; Zhao, J.; Alp, E.E. Phys.Rev.Lett. 2004, 93(19), 195503. (6) Devey, A.J.; Grau-Crespo, R.; de Leeuw, N.H. J. Phys. Chem. C. 2008, 112(29), 10960– 10967. (7) Devey, A.J.; Grau-Crespo, R.; de Leeuw, N.H. Phys Rev B. 2009, 79(19), 195126. (8) Devey, A.; de Leeuw, N.H. Phys Rev B. 2010, 82(23), 235112. (9) Taylor, L.A.; Mao, H.K. Science. 1970, 170 (3960), 850–851. (10)Nakazawa, H.; Osaka, T.; Sakaguchi, K. Nature. 1973, 242(114), 13–14. (11) Osaka, T.; Nakazawa, H. Nature. 1976, 259, 109–110. (12) Douglas, T.; Dickson, D. P. E.; Betteridge, S.; Charnock, J.; Garner, C. D.; Mann, S. Science. 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1995, 269(5220), 54–57. (13) Vanitha, P.V.; O’Brien, P. J. Am. Chem. Soc. 2008, 130(51), 17256–17257. (14) Jeong, H.Y.; Lee, J.H.; Hayes, K.F. Geochim. Cosmochim. Acta. 2008, 72(2), 493–505. (15) Yu, X.L.; Wang, Y.; Zheng, R.K.; Qu, J.F.; Chan, H.L. W.; Cao, C. B. Cryst. Growth Des. 2008, 9(3), 1293–1296. (16) Beal, J.H.L.; Prabakar, S.; Gaston, N.; Teh, G.B.; Etchegoin, P.G.; Williams, G.; Tilley, R.D. Chem. Mater. 2011, 23(10), 2514–2517. (17) Sines, I. T.; Vaughn II, D.D.; Misra, R.; Popczun, E.J.; Schaak, R.E. J. Solid. State Chem. 2012, 196, 17–20. (18) Han, W.; Gao, M. Cryst. Growth Des. 2008, 8(3), 1023–1030. (19) Cummins, D.R.; Russell, H.B.; Jasinski, J.B.; Menon, M.; Sunkara, M.K. Nano Lett. 2013, 13 (6), 2423–2430. (20) Zhou, C.; Zheng, S.; Chen, C.; Lu, G. Corros. Sci. 2013, 67, 184–192. (21) Cabrera-Sierra, R.; Sosa, E.; Pech-Canul, M.A.; Gonzá lez, I. Electrochim. Acta. 2006, 51 (8), 1534–1540. (22) Bai, Y.; Yeom, J.; Yang, M.; Cha, S.H.; Sun, K.; Kotov, N.A. J. Phys. Chem. C.

2013, 117(6), 2567–2573. (23) Li, F.; Franzen, H.F. J. Alloy. Compd. 1996, 238(1), 73–80. (24) Fei,Y.; Bertka, C.M.; Finger, L.W. Science. 1997, 275(5306), 1621–1623. (25) Berner, R.A. Science. 1962, 137(3531), 669–669. (26) Lennie, A.R.; Redfern, S.A.; Champness, P.E.; Stoddart, C.P.; Schofield, P.F.; Vaughan, D.J. Am. Mineral. 1997, 82(3), 302–309. 24


Page 24 of 27

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Bhargava, S.K.; Garg, A.; Subasinghe, N.D. Fuel. 2009, 88(6), 988–993. (28) King, H.E.; Prewitt, C.T. Acta. Crystallogr. B. 1982, 38(7), 1877–1887. (29) Wang, H.; Salveson, I. Phase Transit. 2005, 78(7-8), 547–567. (30) Ono, S.; Oganov, A.R.; Brodholt, J.P.; Vočadlo, L.; Wood, I.G.; Lyakhov, A.; Glass, C.W.; Côté, A.S.; Price, G.D. Earth Planet. Sci. Lett. 2008, 272(1), 481–487. (31) Butler, I.B; Rickard, D. Geochim.Cosmochim.Acta. 2000, 64(15), 2665–2672. (32) Li, Y.; Van Santen, R.A.; Weber, T. J. Solid. State Chem. 2008, 181(11), 3151–3162. (33) Mikmekova, S.; Matsuda, K.; Watanabe, K.; Ikeno, S.; Mu¨llerova´, I.; Frank, L. Mater. Trans. 2011, 3 (52), 292–296 (34) Horiuchi, S. Z. Anorg. AlIg. Chem. 1971, 386, 208. (35) Dove, M.T. Am. Mineral. 1997, 82(3), 213–244. (36)Shoesmith, D.W.; Taylor, P.; Bailey, M.G.; Owen, D.G. J. Electrochem. Soc. 1980, 127(5), 1007–1015. (37) Shoesmith, D.W.; Taylor, P.; Bailey, M.G.; Ikeda, B. Electrochim. Acta. 1978, 23(9), 903– 916. (38) Shoesmith, D.W.; Bailey, M.G.; Ikeda, B. Electrochim. Acta. 1978, 23(12): 1329–1339. (39) Wolthers, M.; Gaast, S.; Rickard, D. Am. Mineral. 2003, 88(11-12), 2007–2015. (40) Hao, W.; Zhang, L.; Yang, J.; Li, H.; Ding, J.; Lu, M. CORROS .2011. Paper No. 11293. (41) Wikjord, A.G.; Rummery, T.E.; Doern, F.E.; Owen, D.G. Corros.Sci. 1980, 20(5), 651–671. (42) Smith, S.N.; Brown, B.; Sun, W. CORROS. 2011, NACE International, Paper No.11081. (43) Ramanarayanan, T.A.; Smith, S.N. Corrosion. 1990, 46, 66–74. (44) Kossel, W. Nach. Ges. Wiss. Göttingen, 1927, 135–143. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(45) Frank, F. C., van der Merwe, J.H. Proc. R. Soc. Lond. A 1949, 198, 216–225. (46) Griffin, L.J. Phil. Mag. 1950, 41, 196. (47) Vekilov, P.G.; Alexander, J. I. Chem. Rev., 2000, 100 (6), 2061–2090. (48) Maiwa, K.; Plomp, M.; van Enckevort, W.J.P.; Bennema, P. J. Cryst. Growth. 1998, 186, 214–223. (49) Sazaki, G.; Asakawa, H.; Nagashima, K.; Nakatsubo, S.; Furukawa, Y. Cryst. Growth Des. 2014, 14 (5), 2133–2137. (50) Rickard, D. A.J. Elsevier Science.2012. (51) Pósfai, M.; Peter, R.; Dennis, A.; Richard, B. Am. Mineral. 1998, 83, 1469–1481. (52) Takeno, S; Zoka, H; Niihara, T. Am. Mineral. 1970, 55, 1639–1649. (53) Skinner, B.J.; Erd, R.C.; Grimaldi, F.S. Am. Mineral. 1964, 49(5–6), 543–555. (54) Lefèvre, C.T.; Menguy, N.; Abreu, F.; Lins, U.; Pósfai, M.; Prozorov, T.; Pignol, D.; Frankel, R.B.; Bazylinski, D.A. Science. 2011, 334(6063), 1720–1723. (55) Murowchick, J.B.; Barnes, H.L. Am. Mineral. 1986, 71(9–10), 1243–1246. (56) Hunger, S.; Benning, L.G. Geochem. Trans. 2007, 8(1), 1.

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"For Table of Contents Use Only" Investigation of the Iron Sulfide phase transformation in nanoscale. Pengpeng Bai, Shuqi Zheng *, Changfeng Chen and Hui Zhao

Brief Synopsis Through immersion of steel in aqueous H2S solution, we fabricated most known iron-sulfur compounds. The transition of cubic FeS into greigite and that of mackinawite into greigite were observed by TEM. Moreover, cubic FeS and troilite present a characteristic of layer growth; the concentric closed-loops induced by a pair of dislocations with opposite sign are clearly visible.

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Investigation of the IronâSulfide Phase Transformation in Nanoscale

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