Wüstite (Fe1−xO) – Thermodynamics and crystal growth

Maki Hamada; Steffen Ganschow; Detlef Klimm; George Serghiou; Hans-Josef Reichmann; Matthias Bickermann

doi:10.1515/znb-2022-0071

Open Access Published by De Gruyter May 25, 2022

Wüstite (Fe_1−xO) – Thermodynamics and crystal growth

Maki Hamada , Steffen Ganschow , Detlef Klimm , George Serghiou , Hans-Josef Reichmann and Matthias Bickermann

From the journal Zeitschrift für Naturforschung B

https://doi.org/10.1515/znb-2022-0071

Abstract

Iron(II) oxide, wüstite, is the iron oxide with the lowest oxygen content. Under ambient conditions it is metastable for two reasons: (1) it undergoes eutectoid decomposition to Fe and Fe₃O₄ below ≈570° C, and (2) depending on temperature, it is thermodynamically stable only for very low oxygen partial pressures, down to below 10⁻²⁰ bar. Hence, for the growth of single crystals from the melt, the growth atmosphere must contain reducing components to keep the oxygen partial pressure on the required low level. With Ar/CO₂/CO mixtures this aim can be reached. It is shown experimentally and by thermodynamic calculation, that the grown crystals contain carbon inclusions. Theoretically it is shown that wüstite crystals without carbon inclusions could be grown in humid N₂/H₂ mixtures. First experiments are presented in this article, but a further adjustment of experimental parameters is required.

Keywords: crystal growth; inclusions; iron oxide; thermodynamic equilibrium

1 Introduction

1.1 Geological aspects

Iron(II) oxide, also known as wüstite, is one end-member of magnesiowüstite (Mg,Fe)O solid solutions, one of the main constituents of the lower mantle of our planet. The core–mantle boundary, the so-called D ″ layer, is regarded as the chemically most active region of the Earth [1]. There, liquid iron from the core reacts with crystalline (Mg,Fe)SiO₃ and magnesiowüstite to form MgSiO₃, stishovite, FeSi, and FeO. Via this route iron permeates upward in the mantle and oxygen enters the core. This model provides an explanation for the density of the outer core which is about 10% lower than that of pure iron under the given (p, T)-conditions [2, 3 2,3]. The heterogeneous D ″ layer has a thickness of only 200…300 km but affects global phenomena such as wobbling of the Earth’s rotation axis, changes of its geomagnetic field, and drift of large tectonic plates [3, 4 3,4]. Since the Earth’s deep interior cannot be observed directly, laboratory experiments on synthetic samples in diamond anvil cells subjected to conditions corresponding to depths of thousands of kilometres are of great value for the geophysical sciences. Data gathered from these experiments greatly help to validate and improve models of the Earth’s interior.

1.2 Crystal growth

Over the past decades, single crystals of wüstite have been grown using various techniques. Burmeister used a Czochralski technique to pull crystals from a cold crucible. This technique could be used because the high intrinsic electrical conductivity of the material allowed melting of it without external heat sources [5]. The crystals were pulled at nearly 1 cm/h into a cavity which was held at a temperature well above the eutectoid decomposition of wüstite which occurs below ≈570° C. The grown crystals were then cooled to room temperature within minutes. Following this procedure Burmeister obtained wüstite single crystals of a few millimetres in diameter and up to 5 cm long. The author argues that for relatively short growth duration the amount of oxygen purging the cavity results in only a marginal shift of composition of the grown crystals. In etching experiments he observed, however, onset of magnetite formation at grain boundaries. A tri-arc furnace was used by Hayakawa et al. [6] to melt wüstite in an Ar atmosphere, and to pull crystals whose lattice parameters where measured at high temperatures. Wet chemical analysis indicated that the composition of the grown crystals was Fe 0.905 … 0.908 O. Bowen & Kingery [7] grew crystalline wüstite films by chemical vapor deposition on MgO substrates. The transport agent was HCl at low pressure (50 Torr), the deposition temperature was between 640 and 900° C, and growth rates reached 20 μm/h. Differences in thermal expansion between crystal and substrate were considered responsible for the relatively high dislocation density of those films reaching 10⁶/cm². A Verneuil process was used by Chen & Peterson [8] to grow crystals for use in diffusion experiments. Berthon et al. [9] developed an optical float zone growth of wüstite to avoid possible contamination originating from a container, and grew crystals with different Fe:O ratios with rates reaching 1 cm/h. The large temperature gradient associated with optical heating proved beneficial for suppression of eutectoid decomposition.

1.3 Thermodynamic aspects

Chemical equilibria between iron, iron oxides, and the reducing agent carbon have been exploited by humans for centuries. Already ca. 3000 years ago, iron metal was produced first, and gave its name to this period “iron age” of human history. Carbon is the most important reducing agent for oxidic iron ores in blast furnaces for the production of iron metal. The harmful impact of carbon dioxide on our climate, however, brings hydrogen as an alternative reducing agent more into focus now.

For the Fe–O–C system, Baur and Glaessner introduced in 1903 phase diagrams where the ratio of CO (reducing) to CO₂ (oxidizing) is plotted versus the temperature T [10]. Such diagrams for CO–CO₂ and H₂–H₂O atmospheres demonstrate that hydrogen is, especially at high T, very strongly reducing [11].

This article, however, is devoted to the growth of wüstite ( Fe 1 − x O ) crystals. This phase has the lowest oxidation state of all iron oxide phases and has in the Baur/Glaessner diagrams with a CO–CO₂ atmosphere a wider phase field than with an H₂–H₂O atmosphere.

2 Experimental

Growth of FeO single crystals was attempted using the micro-pulling-down technique. Steep temperature gradients associated with this technique [12], [13], [14] are advantageous for fast cooling of the grown crystal in order to suppress or completely avoid the eutectoid reaction around 570° C [6]. The starting materials, iron(II) oxalate dihydrate (Alfa Aesar, Puratronic^® 99.999%) or Fe₂O₃ (Alfa Aesar, Puratronic^® 99.998%), were melted in an inductively heated platinum crucible with an inner diameter of 13 mm and a total height of 35 mm. The tip of the conical crucible bottom had an orifice of 0.8 mm diameter. The crucible was covered on top with a platinum lid and placed in porous alumina tubes for thermal insulation. The entire growth setup was placed inside a water-cooled vacuum chamber evacuated to <10⁻² mbar and refilled to normal pressure with a gas mixture of 5 vol% CO + 10 vol% CO₂ in Ar, or with a humid mixture of 2.5 vol% H₂ in N₂ (cf. next paragraph). Growth was initiated on randomly oriented FeO fragments obtained in previous experiments, or simply on the tapered tip of a platinum wire. Further details of the growth procedure are described in Ref. [15].

Thermodynamic equilibrium calculations were performed with the FactSage software and database system [16, 17 16,17]. Due to the technical relevance of the Fe–O system in metallurgy, it has been investigated extensively. Due to this, reliable thermodynamic data can be used for the calculations [18].

3 Results

Depending on the actual growth conditions, in particular the growth rate, the crystals where either around 1 mm in diameter and a few centimeters long (low growth rate) or rather compact bodies of several millimeters diameter and length (high growth rate, cf. Fig. 18.3 in Ref. [19]). The diameter of the former was easily controlled by adjustments of the heating power, whereas the latter grew in a less controlled fashion depending on the (pre)set conditions. However, with respect to phase constitution, the growth rate turned out to be crucial: Pulling with very high rate (e.g., 50 mm/min) yielded phase pure wüstite crystals, as detected by X-ray powder diffraction analysis (Figure 1), whereas crystals grown at lower rates (tested ≤20 mm/min) always contained significant amounts of magnetite.

$Figure 1: X-ray diffraction powder pattern (CuKα radiation) of a Fe 1 − x O ${\text{Fe}}_{1-x}\text{O}$ crystal, showing only the rocksalt F m 3 ‾ m $Fm\overline{3}m$ phase. From the fitted lattice parameter a = 4.308 $a=4.308$ Å, x = 0.054 $x=0.054$ is calculated for the composition [20].$

Figure 1:

X-ray diffraction powder pattern (CuKα radiation) of a Fe 1 − x O crystal, showing only the rocksalt F m 3 ‾ m phase. From the fitted lattice parameter a = 4.308 Å, x = 0.054 is calculated for the composition [20].

Field Emission Gun – Scanning Electron Microscopy (FEGSEM) analysis was conducted on a shiny flat interior section of a grown Fe 1 − x O crystal using a Carl Zeiss SIGMA HD VP Field Emission SEM. No carbon coating was used. To avoid charging, conductive tape was employed. Backscatter electron imaging was performed with a four quadrant solid state angle selective backscatter (AsB) detector for imaging compositional variation. Chemical analysis was performed using an Oxford AZtecEnergy Energy dispersive X-ray analysis system by means of an 80 mm² silicon-drift energy dispersive X-ray detector. Carbon is present throughout the sample, and it is possibly interstitially located in the crystal (Figure 2). Confirmation and exclusion of other possibilities would however require TEM investigation.

Figure 2:

FEGSEM image (BSE contrast) of a wüstite crystal that was grown in an Ar/CO₂/CO atmosphere. Carbon is present throughout the sample both in the darker (points 11 & 12) and lighter (points 13 & 14) regions. There is more carbon in the darker regions. The investigated sample is shown in the insert.

First experiments were conducted to grow crystals in a mixture of H₂ and H₂O as an oxygen delivering medium, to avoid contamination with carbon. For this purpose the growth chamber was rinsed with N₂ containing 2.5 vol% H₂ (forming gas) and saturated in a bubbler with water at ambient laboratory conditions (20° C, resulting saturated vapor pressure p H 2 O = 23.5 mbar). Stable growth was possible at low pulling rate, but at high rates (20 mm/min) the diameter increased in an uncontrollable manner (Figure 3). According to powder XRD analysis, the grown crystal was composed of magnetite and hematite indicating that oxygen activity during growth was significantly higher than intended. We value this experiment as a proof of concept that should be further improved in order to ensure stable growth conditions.

Figure 3:

Wüstite crystal grown in an atmosphere containing H₂/H₂O. The arrow indicates pulling direction.

4 Discussion

For the recent crystal growth experiments, the focus was put on the stabilization of Fe²⁺ in the solid during crystallization from a Fe_1-xO melt ( x = 0 . 81 … 0 . 96 ) [15, 21]. For this purpose, an Ellingham-type predominance diagram of the system Fe–O₂ was used where the logarithm of the oxygen partial pressure log [ p O 2 ] is plotted as a function of the temperature T. Depending on these parameters, different iron oxides, or metallic iron, are stable. The regions where one phase prevails are phase fields, and it turns out that the phase field of wüstite ( Fe 1 - x O ) is narrow. In addition, it undergoes below T e ≈ 570 ∘ C eutectoid decomposition to α-Fe (bcc structure) and magnetite (Fe₃O₄) [15].

The p O 2 ( T ) pressures for which wüstite is stable are very low, down to <10⁻²⁰ bar, and cannot be reached by any “pure” gas that is commercially available. A mixture of 85% Ar + 10% CO₂ + 5% CO, however, was found to supply such p O 2 ( T ) pressures that stabilize wüstite. This is demonstrated in Figure 4 where the green dashed line crosses the gray predominance field almost centrally.

$Figure 4: Predominance diagram of the system Fe–C–O2 with a fixed ratio Fe:C = 9:1. The green dashed line shows the p O 2 ( T ) ${p}_{{\text{O}}_{2}}(T)$ that is created by the Ar:CO2:CO = 85:10:5 mixture used by Ganschow et al. [15].$

Figure 4:

Predominance diagram of the system Fe–C–O₂ with a fixed ratio Fe:C = 9:1. The green dashed line shows the p O 2 ( T ) that is created by the Ar:CO₂:CO = 85:10:5 mixture used by Ganschow et al. [15].

The predominance diagram in Figure 4 is more detailed than the diagrams used in recent publications [15, 21], because carbon is included in the calculation of the stability fields. This third component carbon adds one degree of freedom to the system, and it turns out that some details depend on the Fe:C ratio. Figure 4 shows the case of excess iron, Fe:C = 9:1. For this, Fe in its different modifications, body-centered cubic (bcc) and face-centered cubic (fcc), coexists with iron carbide Fe₃C or with C, in the bottom right part. With an excess of carbon, metallic iron is not stable and a solid mixture of C and Fe₃C coexists; this case, however, is not shown here. All other phase fields are identical for both cases.

Under ambient conditions Fe 1 − x O is not an equilibrium phase for two reasons: (1) it undergoes eutectoid decomposition, as mentioned above, and (2) it is thermodynamically stable only under extremely reducing atmospheres. Nevertheless, wüstite crystals can be held metastably under ambient conditions for years. It should be noted that conversions between the iron oxides Fe₃O₄ and Fe₂O₃ were recently observed to occur at variance with thermodynamic expectations, due to kinetic effects [22].

In practice this means that the gray existence field of Fe 1 − x O in Figure 4 is extended to lower T. There, in the vicinity of T e ≈ 570 ∘ C, a tiny phase field “C + Fe 1 − x O ” appears (red), which means that carbon (as graphite) and wüstite can coexist. Even more surprising is the neighboring blue phase field “C + Fe₃O₄” where graphite coexists with magnetite. Taking this into account, the observation of small graphite inclusions in wüstite crystals which are grown in an Ar/CO₂/CO atmosphere is not very surprising.

In the introduction, hydrogen was mentioned as an alternative reducing agent that would avoid the incorporation of carbon into wüstite crystals. The equilibrium reaction H 2 O ⇄ H 2 + 0.5 O 2 can deliver a temperature-dependent oxygen partial pressure p O 2 ( T ) that rises with T — in a similar way as the equilibrium CO 2 ⇄ CO + 0.5 O 2 [23], but with the exclusion of carbon. Addition of water vapour to a gas flow has to be performed by a bubbler. The final water content can be adjusted by heating the bubbler to an appropriate temperature.

Figure 5 describes this situation in terms of a predominance (stability) diagram where a mixture of 97.5% N₂ + 2.5% H₂ was used as a basis. If this gas mixture is saturated with water in a bubbler, the following saturation vapor pressures of water in the gas flow can be achieved, depending on the bubbler temperature: 20° C: 23.5 mbar; 40° C: 73.9 mbar; 80° C: 471 mbar. The dashed lines in Figure 5 represent the p O 2 ( T ) values that are created by these water concentrations in the gas.

$Figure 5: Ellingham diagram of the system Fe–N2–H2–H2O, with 2.5% H2 in N2. The wüstite predominance field is gray. The dashed color lines represent the p O 2 ( T ) ${p}_{{\text{O}}_{2}}(T)$ functions that are supplied if the gas is saturated with water at the given temperature.$

Figure 5:

Ellingham diagram of the system Fe–N₂–H₂–H₂O, with 2.5% H₂ in N₂. The wüstite predominance field is gray. The dashed color lines represent the p O 2 ( T ) functions that are supplied if the gas is saturated with water at the given temperature.

According to the calculations, fairly low bubbler temperatures of the order of 20…40° C were expected to supply useful oxygen concentrations in the atmosphere. Using higher bubbler temperatures would result in technical problems, because condensation of water at cooled parts of the growth chamber prohibited the required H₂O content in the atmosphere. It should be noted that too low p O 2 would be fatal, because Fe metal is formed which reacts with the Pt crucible. The fcc phases of the metals form solid solutions, with a liquidus minimum of 1519° C — leading to failure of the inductively heated crucible [24].

In this context, the presence of Fe₃O₄ in the grown crystal can be understood, at least below the eutectoid decomposition of wüstite. However, the observation of Fe₂O₃ is very surprising because this phase field is not touched. A possible explanation might be leaks of the vacuum chamber (which is rather unlikely) or the gas routing through the bubbler.

5 Summary

The growth of Fe 1 − x O crystals is challenging because the stability field of this phase in the iron–oxygen system is a narrow corridor: For too high oxygen partial pressures p O 2 ( T ) , Fe₃O₄, and Fe₂O₃ form. For too low p O 2 ( T ) pressures, on the other hand, metallic iron is formed which reacts with the metallic crucibles used in the experiments. Maintaining a conducive oxygen partial pressure is especially difficult, because p O 2 ( T ) changes with T over several orders of magnitude, but can be achieved with an appropriate Ar/CO₂/CO mixture, as shown in Figure 4. Then, the equilibrium CO 2 ⇄ CO + 0.5 O 2 delivers a self-adjusting p O 2 ( T ) . It turns out, however, that for low T ≈ 600 … 700 ∘ C carbon (as graphite) coexists under such conditions in equilibrium with Fe 1 − x O .

Contamination with carbon could be avoided using H 2 O ⇄ H 2 + 0.5 O 2 , hence, a gas mixture containing hydrogen and water in similar concentrations, as a self-adjusting oxygen source. Figure 5 shows that this seems feasible; but first growth experiments did not yet lead to satisfactory results.

Dedicated to Professor Martin Lerch on the occasion of his 60th birthday.

Corresponding author: Steffen Ganschow, Leibniz-Institut für Kristallzüchtung, Max-Born-Str. 2, 12489 Berlin, Germany, E-mail:steffen.ganschow@ikz-berlin.de

Acknowledgments

We thank Nicola Cayzer for her help with the Carl Zeiss SIGMA HD VP Field Emission SEM measurements. We also acknowledge the use of a Zeiss Crossbeam Cryo FIB/SEM bought with EPSRC grant EP/P030564/1 and thank Fraser Laidlaw for help with these electron microscopy measurements. We also thank Louise Adams for materials processing of the sample and Hans-Peter Nabein for X-ray diffraction measurements.

Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: None declared.
Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Received: 2022-04-28

Accepted: 2022-05-05

Published Online: 2022-05-25

Published in Print: 2022-06-27

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