Effect of nickel on the high-pressure phases in FeH

Helene Piet, Andrew V. G. Chizmeshya, Bin Chen, Stella Chariton, Eran Greenberg, Vitali B. Prakapenka, and Sang-Heon Shim

Phys. Rev. B 104, 224106 – Published 21 December 2021

Abstract

Hydrogen-rich metal alloys and compounds have drawn interest from planetary geophysics and condensed matter physics communities because of their potential for deep hydrogen storage in planets and high-temperature superconductivity. We find that a small amount of Ni can alter the phase behavior in the FeH alloy system. Ni can stabilize the double hexagonal close-packed (dhcp) structure in FeH up to the liquidus at 33 GPa, which is in contrast with the stability of the face-centered cubic (fcc) structure in Ni-free FeH at the same conditions. Above 60 GPa, Ni suppresses the stability of the tetragonal ${FeH}_{2}$ phase but stabilizes fcc FeH at higher temperatures. At the same pressure range, we find tetragonal ${FeH}_{2}$ and cubic ${FeH}_{3}$ to be stable at temperatures above 2500 K without Ni. Therefore, in planetary interiors, Ni will expand the stability field of dense close-packed structures in the FeH system. If the Ni content is low, then ${FeH}_{2}$ and ${FeH}_{3}$ can play an important role in the cores of hydrogen-rich planets. Also, our study demonstrates that a secondary alloying component can severely impact the high-pressure stability of polyhydrides.

Received 6 July 2021
Revised 25 October 2021
Accepted 14 December 2021

DOI:https://doi.org/10.1103/PhysRevB.104.224106

Physics Subject Headings (PhySH)

Equations of state Pressure effects

Hydrogen storage materials

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Helene Piet^1,*, Andrew V. G. Chizmeshya², Bin Chen³, Stella Chariton⁴, Eran Greenberg⁴, Vitali B. Prakapenka⁴, and Sang-Heon Shim ^1,†

¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
²School of Molecular Sciences, Arizona State University, Tempe, Arizona, USA
³Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, Hawaii, USA
⁴Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois, USA

^*helene.m.piet@gmail.com
^†shdshim@asu.edu

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Vol. 104, Iss. 22 — 1 December 2021

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Images

Figure 1
X-ray diffraction patterns obtained at $\sim 30 GPa$ for FeH (a) (x-ray energy was 30 keV; run 231) and FeNiH (b) (x-ray energy was 37 keV; run 131b). The red arrow highlights the 200 peak of fcc FeH. Each heating sequence reads from bottom to top. The green and red ticks show the expected peak positions of the fcc and dhcp phases from their equations of state [7] and crystal symmetry, respectively. dhcp: double hexagonal close-packed structure; fcc: face-centered cubic structure.
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Figure 2
X-ray diffraction patterns obtained at $\sim 70 GPa$ for FeH (a) (run 103) and for FeNiH (b) (run 130). The notations are the same as Fig. 1. Because of the small sample chamber size in LHDAC, some diffraction patterns show peaks from the rhenium gasket. There was no direct contact between the sample and the gasket. dhcp: double hexagonal close-packed structure; fcc: face-centered cubic structure; tet- ${FeH}_{2}$ : tetragonal ${FeH}_{2}$ structure [7]. The colored ticks show expected peak positions of different phases from their equations of state [7] and crystal symmetry. These diffraction patterns were measured with a 37 keV x-ray beam.
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Figure 3
X-ray diffraction patterns of metallic Fe in $H_{2}$ at 89 GPa showing (a) dhcp FeH (green ticks) at 300 K before heating, (b) ${FeH}_{2}$ (orange ticks), and ${FeH}_{3}$ (blue ticks) at 1888 K. (c) As we keep heating at 2539 K, the ${FeH}_{3}$ peaks become weaker. (d) After quench, fcc FeH (red ticks) is present in the pattern. X-ray energy was 37 keV. The diffraction patterns were collected during run 203. Because of the small sample chamber size in LHDAC, some diffraction patterns show peaks from the rhenium gasket. There was no direct contact between the sample and the gasket. dhcp: double hexagonal close-packed structure; fcc: face-centered cubic structure; tet ${FeH}_{2}$ : tetragonal ${FeH}_{2}$ structure [7]; cub ${FeH}_{3}$ : cubic ${FeH}_{3}$ structure [7]. The colored ticks show expected peak positions of different phases from their equations of state [7, 10] and crystal symmetry. Information on the crystal structures of some of these phases are available in [10].
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Figure 4
Pressure-volume relationship of phases in FeNiH (circles) and FeH (squares) at 300 K. The dashed curves represent equations of state from the literature for hcp Fe (black) [27], dhcp FeH (green) [7], fcc FeH (red) [6], tetragonal ${FeH}_{2}$ (yellow) [7], and cubic ${FeH}_{3}$ (blue) [7]. The solid curves represent equations of state from this study for dhcp (Fe,Ni)H (green), fcc (Fe,Ni)H (red), and (Fe, $Ni) H_{2}$ (yellow). The values used in this table are provided in the Supplemental Material as a table [23].
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Figure 5
Pressure-temperature conditions of data obtained for (a) FeH and (b) FeNiH. The colors denote the observed structures: dhcp FeH (green) fcc FeH (red), tetragonal ${FeH}_{2}$ (yellow), and cubic ${FeH}_{3}$ (blue). The phase boundaries and melting curve for FeH below 20 GPa are from a multianvi press study [32]. The melting curve of (Fe,Ni)H below 20 GPa is from a multianvil press study in [34]. Data points from these studies are also presented. Effect of H on the melting temperature is not known above 20 GPa. We presented instead the melting of Fe and FeNi alloy in the figure [35, 36]. Diamond-anvil cell data for FeH are included in (a) for comparison [6, 7, 26, 33]. The estimation method for temperature uncertainty can be found in the caption of Table 1. bcc: body-centered cubic structure; dhcp: double hexagonal close-packed structure; fcc: face-centered cubic structure; tet ${FeH}_{2}$ : tetragonal ${FeH}_{2}$ structure [7]; cub ${FeH}_{3}$ : cubic ${FeH}_{3}$ structure [7].
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Figure 6
An optical image showing eye-shaped structures (highlighted by the circles with run numbers) found after laser heating in runs 131b–431b. We interpret that such a structure forms by heating above the liquidus temperature of FeH alloys. The diffraction patterns from the transparent areas at the center of the heating spots show much weaker diffraction peak intensities form metal alloy samples, suggesting that the metal melt may have migrated to colder adjacent areas along the thermal gradients and the transparent areas are filled with hydrogen after melting. The green squared area is an unheated area which can be compared with the heated areas indicated by the colored circles. See Table 1 for more information on each run.
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