Iron–hydrogen alloy
Varying the amount of hydrogen, as well as controlling its chemical and physical makeup in the final iron hydride (either as a solute element, or as a precipitated phase), hastens the movement of those dislocations that make pure iron ductile, and thus controls and undermines its qualities.
One such process, known as hydrogen roasting, is more commonly applied to metals such as tungsten and molybdenum, but can be used to produce iron-hydrogen alloys.
At room temperature, the most stable form of pure iron is the body-centred cubic (BCC) structure called alpha-iron or α-iron.
At 910 °C (1,670 °F) pure iron transforms into a face-centred cubic (FCC) structure, called gamma-iron or γ-iron.
At the very high cooling rates produced by quenching, the hydrogen has no time to migrate but is locked within the crystalline structure and forms martensic iron hydride.
Annealing is the process of heating the iron-hydrogen alloy to a sufficiently high temperature to relieve local internal stresses.
It does not create a general softening of the product but only locally relieves strains and stresses locked up within the material.
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 analysed a sample using X-ray diffraction, as having an approximate composition FeH0.94 and double hexagonal close packed (DHCP) structure.
[8] These compounds dissociate spontaneously at ordinary pressures, but at very low temperatures they will survive long enough in a metastable state to be studied.
[5] High pressure stability of different iron hydrides was systematically studied using density-functional calculations and evolutionary crystal structure prediction by Bazhanova et al.,[7] who found that at pressures and temperatures of the Earth's inner core only FeH, FeH3 and an unexpected compound FeH4 are thermodynamically stable, whereas FeH2 is not.
The best-known high-pressure phase in the iron-hydrogen system (characterized by V. E. Antonov and others, 1989) has a double hexagonal close packed (DHCP) structure.
[8][10] The reaction is complex and may involve a metastable HCP intermediate form; at 9 GPa and 350 °C there are still noticeable amounts of unreacted α-Fe in the solid.
This difference means that at 3.5 GPa FeH has 51% less volume than the mixture of hydrogen and iron that forms it.
The only parameters that are known with confidence are the speed of the pressure and shear sound waves (the existence of the latter implying that it is a solid).
The inner core was at first thought to be 10% less dense than pure iron at the predicted conditions,[1][5] but this presumed “density deficit” has later been revised downwards: 2 to 5% by some estimates[9] or 1 to 2% by others.
[6] The density deficit is thought to be due to mixture of lighter elements such as silicon or carbon.
[1] Hydrogen has been thought unlikely because of its volatility, but recent studies have uncovered plausible mechanisms for its incorporation and permanence in the core.
Above 5 GPa, iron will split water yielding the hydride and ferrous ions:[6] Indeed, Okuchi obtained magnetite and iron hydride by reacting magnesium silicate, magnesium oxide, silica and water with metallic iron in a diamond cell at 2000 C.[5][11] Okuchi argues that most of the hydrogen accreted to Earth should have dissolved into the primeval magma ocean; and if the pressure at the bottom 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.
[9] and match the observed speed of pressure and shear sound waves in the solid inner core.
[6] Similar, but without extrapolations in pressure, theoretical estimates give a narrower range of concentrations 0.4-0.5% (weight),[7] however, this results to too low mean atomic mass of the inner core (43.8-46.5) and hydrogen seems to be less likely than other elements (S, Si, C, O) to be the main light alloying element in the core.
