An '''iron-hydrogen alloy''' is a [[metallic compound]] or [[alloy]] made by combining [[iron]] and [[hydrogen]].
The term may refer to iron with a very low percentage of hydrogen, absorbed in the molten state at ordinary pressures; or to a [[crystal]]line [[intermetallic compound]] with [[chemical formula]] FeH. The latter is stable only at very high pressures (over 30.000 times [[standard atmospheric pressure]]), and metastable at ambient pressure at very low temperatures.
This high-pressure metallic compound is often called '''iron hydride''', but this term also applies to a few other non-metallic, low-pressure [[iron hydride|iron-hydrogen compounds]].
== Hydrogen as impurity==
At atmospheric pressure and room temperature only 10−6 parts of hydrogen will enter solid iron. Molten iron can absorb up to 0.08% hydrogen by weight, even under high pressure.[ Even at pressures above 100 MPa the concentration does not exceed 0.05% (atomic).][
The inclusion of hydrogen causes iron to become brittle and break up due to expansion of cracks and defects in the solid.][ Hydrogen acts as a hardening agent, preventing [[dislocation]]s in the iron atom [[crystal lattice]] from sliding past one another. Varying the amount of hydrogen and the form of its presence in the iron hydride (precipitated phase) controls qualities such as the [[hardness (materials science)|hardness]], [[ductility]], and [[tensile strength]] of the resulting iron hydride. Iron hydride with increased hydrogen content can be made harder and stronger than iron, but such iron hydride is also less [[ductile]] than iron.
==High-pressure binary hydrides==
The common form of iron is the "α" form, with [[body centred cubic]] (bcc) crystalline structure;][ in the absence of reactive chemicals, at ambient temperature and 13 [[gigapascal|GPa]] of pressure it converts to the "ε" form, with [[hexagonal close packing]] (hcp) structure.][ In an atmosphere of hydrogen at ambient temperature, α-Fe retains its structure up to 3.5 GPa (35,000 [[standard atmospheric pressure|atmospheres]]), with only small amounts of hydrogen diffusing into it forming a solid interstitial solution.][
Starting at about 3.5 GPa of pressure, hydrogen {{chem|H|2}} rapidly [[diffusion|diffuses]] into metallic iron (with [[diffusion length]] of about 500 [[millimetre|mm]] per 10 s at 5 GPa][) to form a crystalline solid with formula close to FeH. This reaction, in which the iron expands significantly, was first inferred from the unexpected deformation of steel gaskets in [[diamond anvil cell]] experiments. In 1991 [[J. V. Badding]] and others identified the compound by [[X-ray diffraction]] as having the approximate composition FeH0.94 and [[double hexagonal close packed]] (dhcp) structure. ][
Since then the phase diagram of this high-pressure iron-hydrogen compound has been intensively investigated up to 70 GPa. Three stable crystalline forms have been observed, denoted "ε'" (the original dhcp form),][ "ε" ([[hexagonal close packed]], hpc), and "γ" ([[face centered cubic]], fcc).][ A fourth metastable "α" form ([[body-centered cubic]], bcc) has also been identified.][ In all these phases the packing of iron atoms is less dense than in pure iron. The hcp and fcc forms have the same iron lattice but have different number of hydrogen neighbors, and have different local magnetic moments.][ The hydrogen and iron atoms are electrically neutral in this compound.][.
At low temperatures the stable forms are bcc below 5 GPa and ε' (dhcp) above 5 GPa at least up to 80 GPa; at higher temperatures γ (fcc) exists at least up to 20 GPa.][ The triple point ε'-γ-melt is predicted to be at 60 GPa and 2000 K. ][ Theoretical calculations however predict that, at 300 K, the stable structures should be dhcp below 37 GPa, hcp between 37–83 GPa, and fcc above 83 GPa.][
Other hydrogenated forms FeH''x'' with ''x'' = 0.25 ({{chem|Fe|4|H}}), ''x'' = 0.50 ({{chem|Fe|2|H}}), and ''x'' = 0.75 ({{chem|Fe|4|H|3}}) have been the subject of theoretical studies.][
These compounds dissociate spontaneously at ordinary pressures, but at very low temperatures they will survive long enough in a [[metastable]] state to be studied.][ At ordinary temperatures, rapid depressurization of FeH from 7.5 GPa (at 1.5 GPa/s) results in metallic iron containing many small hydrogen bubbles; with slow depressurization the hydrogen diffuses out of the metal.][
Hydrogen is a solid at room temperature above 5 GPa. ][
=== The ε' (dhcp) form ===
[[File:Closest packing ABAC.png|thumb|right|The double hexagonal close packed (dhcp) structure with ABAC alignment of FeH. Each sphere is an iron atom. Hydrogen are located in the interstices.]]
The best-known form of FeH (characterized by [[V. E. Antonov]] and others, 1989) has a double hexagonal close packed structure (dhcp). It consists of layers of hexagonal packed iron atoms, offset in a pattern ABAC; which means that even-numbered layers are vertically aligned, while the odd-numbered ones alternate between the two possible relative alignments. The c axis of the [[unit cell]] is 0.87 [[nanometre|nm]]. Hydrogen atoms occupy [[octahedral cavity|octahedral cavities]] between the layers. The hydrogen layers come in vertically aligned pairs, bracketing the B and C layers and shifted like them.][ For each hydrogen added the unit cell expands by 1.8 [[angstrom|Å]]3 (0.0018 nm3). This phase was denoted ε', after the similar structure that iron assumes above 14 GPa.][
This form of FeH is rapidly created at room temperature and 3.8 GPa from hydrogen and α-iron.][ The transformation entails an expansion by 17% to 20%in volume.][ The reaction is complex and may involve a metastable hcp intermediate form; at 9 GPa and 350 there are still noticeable amounts of unreacted α-Fe in the solid.][ The same form is obtained from by reacting hydrogen with the higher-pressure hcp form of iron (ε-Fe) at 1073 K and 20 GPa for 20 min;][ and also from α-iron and {{chem|H|2|O}} at 84 GPa and 1300 K.][
This form of FeH is stable at room temperature at least up to 80 GPa,][ but turns into the γ form between 1073 and 1173 K and 20 GPa.][ There may be several dhcp forms with slightly different stoichometries,][ but the material is stoichometric above 10 GPa in the presence of excess hydrogen.][
This material has metallic appearance and is an [[electrical conductor]].][ Its [[resistivity]] is higher than that of iron, and decreases down to a minimum at 8 GPa. Above 13 GPa the resistivity increases with pressure. The material is [[ferromagnetic]] at the lowest pressure range, but the ferromagnetism begins to decrease at 20 GPa and disappears at 32 GPa t.][
The bulk [[elasticity modulus]] of this compound is 121±19GPa, substantially lower than iron's 160 GPa. This difference means that at 3.5 GPa FeH has 51% less volume than the mixture of hydrogen and iron that forms it.][
The speed of compressional sound waves in FeH rises as pressure rises, at 10 GPa it is at 6.3 [[kilometre|km]]/[[second|s]], at 40 GPa 8.3 km/s and 70 GPa 9 km/s.]
The dhcp form of FeH can be preserved in a metastable form at ambient pressures by first lowering the temperature below 100 K.
===The γ (fcc) form===
At 30 GPa and 1600 K a [[face centered cubic]] (fcc) crystal form of FeH is produced, first described by [[M. Yamakata]] and others in 1992.[ It can also be obtained at 54 GPa and 1650 K.][ This form can be prepared from the [[gamma (Greek letter)|γ ]] phase that iron assumes at high temperatures below 20 GPa, without structural change in the iron arrangement; and has therefore been called the γ phase of FeH.][ This form too is stoichometric or nearly so (''x'' ≈ 1)][ and has a [[specific volume]] slightly larger than ε'-FeH.][
The [[triple point]] where γ-FeH, ε'-FeH, and α-Fe with included hydrogen coexist is at 280°C and 5 GPa.][ As temperature increases a higher hydrogen pressure is needed to make the γ phase, and at 400°C it requires 7 GPa.][ The γ form is stable at room temperature from 19 to over 68 Gpa.][ This form reverts to the ε' (dhcp) form when the pressure islowered to 12 GPa.][
The γ structure can be created from ε' between 1073 and 1173 K and 20 GPa,][ but persists upon cooling to 673 K, presumably in a metastable state.][
This form has also been obtained by reaction of [[cementite|iron carbide]] {{chem|Fe|3|C}} and hydrogen at about 1650 K and 30 GPa:][
: 2{{chem|Fe|3|C}} + 3{{chem|H|2}} → 6FeH (fcc) + 2C (diamond)
Below that temperature the carbide is stable in the presence of hydrogen. Indeed, at about 1400 K metallic iron (which under those conditions has the hcp structure) will react with [[paraffin]] C''n''H2''n''+2 to form {{chem|Fe|3|C}} (and sometimes [[Hägg carbide|{{chem|Fe|5|C|2}}]]) with release of hydrogen.][
Its constitutive parameters are V0= 53.8 ±0.3 Å3, K0 = 99 ±5 GPa, K' = 11.7 ±0.5.][
===The ε (hcp) form ===
A [[hexagonal close packed]] (hcp) from of FeH also exists at lower pressure hydrogen, also described by M. Yamakata and others in 1992. This is called the ε phase (no prime)][. The hcp phase is not ferromagnetic,][ probably [[paramagnetic]].][ This appears to be the most stable form in a wide pressure range.][ It seems to have a composition between {{chem|FeH|0.42}}.][
The hcp form of FeH can be preserved in a metastable form at ambient pressures by first lowering the temperature below 100 K.][
===The α (bcc) form ===
A metastable body-centered cubic (bcc) form of FeH was described by [[Y. Fukai]] and others in 1982.][
==Melting point==
These iron-hydrogen alloys melt at a significantly lower temperature than pure iron:][
]
|
'''Pressure (Gpa)''' | |
7.5 | 10 | 11.5 | 15 | 18 | 20 |
'''Melting point (C)''' | |
1150 | 1473 | 1448 | 1538 | 1548 | 1585 |
The slope of the melting point curve with pressure (dT/dP) is 13 K/GPa.[
==Occurrence in the Earth's core==
Very little is known about the composition of Earth's [[inner core]]. The only parameters that are known with confidence are the speed of the [[pressure wave|pressure]] and [[shear wave|shear]] sound waves (the existence of the latter implying that it is a solid). The pressure at the boundary between the inner core and the liquid [[outer core]] is estimated at 330 GPa,][ still somewhat beyond the range of laboratory experiments. The density of the outer and inner cores can only be estimated by indirect means. The inner core was at first thought to be 10% less dense than pure iron at the predicted conditions,][ but this presumed "density deficit" has later been revised downwards: 2 to 5% by some estimates][ or 1 to 2% by others][.
The density deficit is thought to be due to mixture of lighter elements such as [[silicon]] or [[carbon]].][ Hydrogen has been thought unlikely because of its volatility, but recent studies have uncovered plausible mechanisms for its incorporation and permanence in the core. It is estimated that hcp FeH would be stable under those conditions.][ Iron-hydrogen alloys could have been formed in a reaction of iron with water in [[magma]] during the formation of the earth. Above 5 GPa, iron will split water yielding the hydride and [[ferrous]] ions:][
:3Fe + {{chem|H|2|O}} → 2FeH + FeO
Indeed, Okuchi obtained [[magnetite]] and iron hydride by reacting [[brucite|magnesium silicate]], [[magnesium oxide]], [[silica]] and water with metallic iron in a diamond cell at 2000 C.][ Okuchi argues that most of the hydrogen accreted to Earth should have dissolved into the primeval magma ocean; and if the pressure at the bootom of the magma was 7.5 GPa or more, then almost all of that hydrogen would have reacted with iron to form the hydride, which then would have sunk to the core where it would be stabilized by the increased pressure.][ Moreover, it appears that at those pressures iron binds hydrogen in preference to carbon.][
Based on density and sound velocity measurements at room temperature and up to 70 GPa, extrapolated to core conditions, Shibazaki and others claim that the presence of 0.23±0.06% hydrogen in weight (that is, a mean atomic composition of FeH0.13±0.03) would explain a 2 to 5% density deficit.][ and match the observed speed of [[pressure wave|pressure]] and [[shear wave|shear]] sound waves in the solid inner core.][ A different study predicts 0.08 to 0.16% (weight) hydrogen in the inner core,][ while others proposed from 50% to 95% FeH (by mole count) If the core has this much hydrogen it would amount to ten times as much as in the oceans.][
The liquid outer core also appears to have density 5 to 10% lower than iron.][ Shibazaki and others estimate that it should have a somewhat higher proportion of hydrogen than the inner core, but there is not enough data about molten FeH''x'' for accurate estimates.][ Narygina and others estimate 0.5 to 1.0% (weight) of hydrogen in the melt.][
== References ==
][
Surendra K. Saxena, Hanns-Peter Liermann, and Guoyin Shen (2004), "Formation of iron hydride and high-magnetite at high pressure and temperature". Physics of the Earth and Planetary Interiors, volume 146, pages 313-317. {{doi|10.1016/j.pepi.2003.07.030}}
]
[
J.V. Badding, R.J. Hemley, and H.K. Mao (1991), "High-pressure chemistry of hydrogen in metals: in situ study of iron hydride." ''Science' , American Association for the Advancement of Science, volume 253, issue 5018, pages 421-424 {{doi|10.1126/science.253.5018.421}}
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[
A. S. Mikhaylushkin, N. V. Skorodumova, R. Ahuja, B. Johansson (2006), [http://proceedings.aip.org/resource/2/apcpcs/837/1/161_1 "Structural and magnetic properties of FeHx (x=0.25; 0.50;0.75)"]. In: ''Hydrogen in Matter: A Collection from the Papers Presented at the Second International Symposium on Hydrogen in Matter (ISOHIM)'', AIP Conference Proceedings, volume 837, pages 161–167 {{doi|10.1063/1.2213072}}
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Takahiro Matsuoka, Naohisa Hirao,Yasuo Ohishi, Katsuya Shimizu, Akihiko Machida and Katsutoshi Aoki (), "Structural and electrical transport properties of FeHx under high pressures and low temperatures". High Pressure Research, volume 31, issue 1, pages 64–67 {{doi|10.1080/08957959.2010.522447}
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{{cite journal|last=Sakamaki|first=K|coauthors=Takahashi, E.; Nakajima, Y.; Nishihara, Y.; Funakoshi, K.; Suzuki, T.; Fukai, Y.|date=May 2009|title=Melting phase relation of FeHx up to 20GPa: Implication for the temperature of the Earth's core|journal=Physics of the Earth and Planetary Interiors|volume=174|pages=192–201|doi=10.1016/j.pepi.2008.05.017}}
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Takuo Okuchi (1997), "Hydrogen partitioning into molten iron at high pressure: implications for Earth's core." ''Science'' (American Association for the Advancement of Science), volume 278, pages 1781-1784. {{doi|10.1126/science.278.5344.1781}}
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[[Category:Metal hydrides]]
[[Category:Ferrous alloys]]
[[fr:Hydrure de fer]]