New iron hydrides synthesised under pressure

12-05-2015

Many calculations have predicted exceptional properties of hydrides at high pressure, namely a high hydrogen storage capacity and a possible conventional high Tc superconductivity. Yet, these properties remain to be tested. Scientists working at the ESRF have observed an increase of hydrogen content in iron under pressure, with the formation of two novel hydrides, FeH~2 and FeH3. In particular, the structure of FeH~2 is remarkable with alternate planes of Fe and of atomic H.

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Most transition metals form hydrides in contact with H2, usually with a H:metal ratio close to 1. However, this chemistry is predicted to change under pressure. Numerous first-principles calculations have predicted that novel forms of polyhydrides should exist at high pressure, such as LiH2, LiH6 [1], FeH3 and FeH4 [2]. A rough rule is emerging, i.e. the hydrogen content in hydrides should increase significantly with pressure. So far, this rule and the existence of such unusual stoichiometries have yet to be confirmed experimentally.

In this study, we have discovered that under pressure the hydrogen content in iron dramatically increases in discontinuous steps: heating FeH in the presence of hydrogen leads to the formation of FeH~2 at 67 GPa and of FeH3 at 86 GPa.

Iron samples (~2 µm thick) were loaded in diamond anvil cells with hydrogen as a pressure medium, and YAG-laser heated using the online setup at the ID27 beamline. dhcp-FeH forms under pressure at 3.5 GPa and no changes in the X-ray diffraction (XRD) pattern were observed when purely compressing the sample up to 136 GPa. Laser-heating, however, allowed us to induce a chemical reaction, identified by the disappearance of the dhcp-FeH XRD peaks and the appearance of new XRD peaks (Figure 1). Rietveld refinement of the integrated new diffraction patterns give, at 67 GPa, FeH~2 with a tetragonal unit cell (space group I4/mmm), and at 86 GPa, FeH3 with a simple cubic unit cell (space group Pm-3m) corresponding to the structure predicted in Ref. [2] at 300 GPa. The structural positional parameters of the H atoms have been determined by first-principles density functional calculations. The new phase diagram for the Fe-H system is shown in Figure 2.

X-ray diffraction pattern for FeH, FeH2 and FeH3.

Figure 1. X-ray diffraction pattern for FeH, FeH2 and FeH3 (right: image plates, left: integrated patterns).

This discovery is a textbook case for the expected increase of the hydrogen stoichiometry in hydrides with pressure. The investigation of the electronic properties of these novel iron compounds could reveal unusual magnetism, dimensionality, proton zero point energy, and correlations. Apart from these fundamental issues, the observed stability of novel iron hydrides under high pressure is of significant importance for planetary interior modelling since iron and hydrogen are two of their main constituents [3]. In particular, hydrogen is considered as a possible light element in the Earth’s core, which is mainly composed of iron.

Phase diagram of the Fe-H system showing the different structures.

Figure 2. Phase diagram of the Fe-H system showing the different structures.

 

Principal publication and authors
New Iron Hydrides under Pressure, C.M. Pépin (a), A. Dewaele (a), P. Loubeyre (a) and M. Mezouar (b), Phys. Rev. Letter 113, 265504 (2014).
(a) Commissariat à l’Énergie Atomique, Direction des Applications Militaires Île de France Arpajon (France)
(b) ESRF

 

References
[1] E. Zurek, R. Hoffmann, N. Ashcroft, A. Oganov, and A. Lyakhov, Proc. Natl. Acad. Sci. U.S.A. 106, 17640 (2009).
[2] Z. Bazhanova, A. Oganov, and O. Gianola, Phys. Usp. 55, 489 (2012).
[3] D. Stevenson, Nature (London) 268, 1300 (1977).

 

Top image: Sketch of a diamond anvil cell and microscope image of the sample studied.