Nonmetallic FeH6 under High Pressure

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Nonmetallic FeH under High Pressure Shoutao Zhang, Jianyan Lin, Yanchao Wang, Guochun Yang, Aitor Bergara, and Yanming Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04125 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 11, 2018

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The Journal of Physical Chemistry

Nonmetallic FeH6 under High Pressure Shoutao Zhang,1‡ Jianyan Lin,1‡ Yanchao Wang,2 Guochun Yang,*,1 Aitor Bergara*,3,4,5 and Yanming Ma*,2 1

Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China 2 State Key Lab of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China 3

Departmento de Física de la Materia Condensada, Universidad del País Vasco, UPV/EHU, 48080 Bilbao, Spain Donostia International Physics Center (DIPC), 20018 Donostia, Spain 5 Centro de Física de Materiales CFM, Centro Mixto CSIC-UPV/EHU, 20018 Donostia, Spain 4

ABSTRACT: High pressure induces unexpected chemical and physical properties in materials. For example, hydrogen-

rich compounds under pressure have recently gained much attention as potential room temperature superconductors and iron hydrides have also raised a significant interest as candidates for the main constituents of the Earth’s core. It is well known that pressure induces insulator-to-metal transitions, whereas pressure-induced metal-to-insulator transitions is rare, especially for transition metal hydrides. In this article we have extensively explored the structural phase diagram of iron hydrides by using ab initio particle-swarm optimization. We have found a new stable stoichiometry, FeH6, above 213.7 GPa with C2/c symmetry. Interestingly, C2/c FeH6 presents an unexpected non-metallicity, and its band gap becomes larger with increasing of pressure. This is in sharp contrast with P21/m FeH4. Non-metallicity of C2/c FeH6 mainly originates from pressure-induced hybridization between Fe and H orbitals. This new compound shows a unique structure with a mixture of non-bonded hydrogen atoms in a helical iron framework. The strong Fe-Fe interaction and ionic Fe-H bonds are responsible for its structural stability. In addition, we have also found a more stable tetragonal FeH2 structure with the same I4/mmm symmetry as the previously proposed one, whose x-ray diffraction pattern perfectly agrees with the experiment.

1. INTRODUCTION Pressure reduces interatomic distances, modifies electronic orbitals, and changes bonding patterns. As a consequence, it becomes a versatile tool to produce new materials with unexpected physical and chemical properties.1–4 For example, it is well known pressure induces insulator-to-metal transitions but, even more interestingly, counterintuitive and rare pressure driven metal-toinsulator transitions have been predicted and experimentally observed in very few systems, such as alkali and alkaline earth metals (e.g. Li5, Na6 and Ca7), and their compounds (e.g. Li4C8 and Ca2N9). Pressure induced confinement of electrons at the interstitial regions is mainly responsible for their nonmetallic transition.10,11 In order to better understand pressure-induced nonmetallic transitions, it is really interesting to explore this unusual phenomenon in other compounds as well.

Hydrogen-rich (H-rich) compounds2,12–19 have become a recent interesting alternative to obtain metallic hydrogen at experimentally accessible pressures. Interestingly, some of them show a strong electron-phonon interaction becoming potential high-temperature superconductors.12–14 Recently, theoretical calculations predicted that compressed sulfur hydrides could be hightemperature superconductors.17,20 Interestingly, this prediction has been confirmed when compressed hydrogen sulfide was observed to show a superconducting transition temperature (Tc) of 203 K,2 which boosted the research on H-rich compounds.21,22 Among the hydrides, transition metal hydrides have drawn much attention due to their low pressure synthesis, diverse stoichiometries, catalytic transformations, and interest as hydrogen storage materials.23–32 More importantly, hydrogen content in these hydrides remarkably increases with pressure,33 which becomes an effective way to obtain a great variety of H-rich compounds.

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To the best of our knowledge, besides FeH4,34 all the reported transition metal hydrides are metallic at high pressure,35 and part of them are also excellent candidates as high-temperature superconductors.32,36–38 On the other hand, iron hydrides have recently gained a significant attention for geophysics as potential candidates for the main constituents of the Earth’s core.39–42 Although Fe and H are not reactive at ambient pressure, several iron hydrides can be stabilized under pressure. For example, it is believed they can be formed at the Earth’s core by the reaction of iron and water under high pressures.39,40,43 Therefore, analyzing their relative stabilities is important in determining the chemical composition of the Earth’s core and understanding its dynamics. Additionally, characterizing the structures and properties of iron hydrides is also crucial to solve some technological problems related with hydrogen degradation of ferrous metals.44,45 Diverse ratios of Fe and H in iron hydrides lead to the appearance of different physical and chemical properties.33 Up to now, structural and electronic properties of FeH, FeH2, FeH3, and FeH4 have been reported both theoretically and experimentally.33,46– 48,34 Recently, FeH5, consisting of atomic hydrogen layers, has been synthesized by compressing a mixture of iron and hydrogen above 130 GPa.49 Interestingly, the presence of non-bonded hydrogen atoms in FeH5 provides an approach to metallic atomic hydrogen. Moreover, FeH5 shows a calculated Tc value of 51 K.50 Very recently, Kvashnin et al have carried out a variablecomposition structure prediction with the USPEX algorithm to search for new Fe-H binary compounds below 150 GPa. 51 According to their predictions, Fe-H system shows an interesting and complex structural phase diagram. Actually, considering that pressure produces unusual H-rich compounds, such as BaH6,52 LaH8,21 YH10,22 and LaH10,53 we could also expect iron hydrides with a higher hydrogen content might also be stable under high pressure. In this work we extensively explored the structural phase diagram of iron hydrides with a variety of FeHx (x = 1 - 12) stoichiometries with the aid of the advanced swarm structural search method.54,55 A new more stable tetragonal structure for FeH2 with the same I4/mmm symmetry as the previously proposed one33 was identified, which perfectly agrees with the experimental data. Additionally, we have also found that iron hexahydride (FeH6), with the highest hydrogen content among the stable iron hydrides, becomes stable above 213.7 GPa in a monoclinic C2/c structure. Its unique atomic arrangement of non-bonded hydrogen atoms embedded in a helical Fe framework induces an unexpected nonmetallic character. With increasing pressure, its band gap becomes larger and larger. Its non-metallicity can be attributed to pressure-induced hybridization between Fe and H orbitals. Our study provides an opportunity for

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understanding pressure-induced non-metallic transitions in transition metal hydrides. 2. COMPUTATIONAL DETAILS Nowadays, structural prediction is an effective way to find new materials with intriguing properties.56–58 Here, we carried out our structure search for FeHx (x = 1-12), at the selected pressures of 100, 200, and 300 GPa by using a particle-swarm optimization algorithm as implemented in the CALYPSO code.54,55 This methodology is effectively capable of finding stable or metastable structures only depending on the given chemical composition. It has been successfully applied to a number of systems, from element solids to binary and ternary compounds.4,5,20,59–62 Details on the structural search method are described in the Supporting Information. Geometrical optimization and electronic property calculations were performed with density functional theory (DFT)63,64 as implemented in the Vienna Ab initio Simulation Package (VASP) code.65 Here, the Perdew– Burke–Ernzerhof (PBE)66 functional was selected as a compromise between accuracy and computational efficiency. The all-electron projector augmented-wave (PAW)67 pseudopotentials of Fe and H we have considered treat 3s23p63d74s1 and 1s1 electrons as valence electrons, respectively. A kinetic-energy cutoff of 800 eV and a Monkhorst-Pack scheme68 with a k-point grid of 2π × 0.03 Å–1 were found to give the required energy convergence. Phonon calculations were performed by using a supercell approach with the finite displacement method69 as implemented in the Phonopy code.70 Finally, Bader’s Quantum Theory of Atoms in Molecules (QTAIM) analysis was adopted for the charge transfer analysis.71 3. RESULTS AND DISCUSSION To find stable Fe-H compounds with higher hydrogen content we performed a structural search on FeHx (x = 1 - 12) at 0 K and selected pressures of 100, 200, and 300 GPa. After each initial structural search, some of the structures with lower enthalpies were selected to do a refined structural optimization and stability analysis. Based on the results of our structure search, all the already known iron hydrides were well reproduced, confirming our methodology is applicable to Fe-H system. For example, among others, the experimental cubic Pm3m FeH333 and the recently discovered tetragonal I4/mmm FeH549 were readily found. However, it is also interesting to note we have found another tetragonal structure for FeH2, which is more stable than the previously proposed one (Figure S1),33 as will be discussed later. The same structure has been also identified in a recent and independent work.51

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The Journal of Physical Chemistry enthalpies over the explored pressures. (b) Schematic illustration of the pressure stability region of Fe-H compounds.

The determination of the relative stabilities of any compound is a necessary prerequisite before studying their applications. The relative stability of each considered iron hydride stoichiometry can be obtained by calculating the formation enthalpy, ∆H = [H(FeHx) – H(Fe) – xH(H2)/2]/(1 + x). Here, H(FeHx) is the enthalpy of the studied stoichiometry, H(Fe) and H(H2) are the enthalpies of elemental Fe and H2 solids, respectively. A negative ∆H indicates that a compound is energetically stable relative to elemental Fe and H2 solids, and the compounds located on the convex hull are stable against decomposition into other compositions (Figure 1a). At 100 GPa, FeH, FeH2, FeH3, FeH5, and FeH6 are found to be stable. With further compression, FeH2 becomes unstable with respect to FeH and FeH3 and FeH6 decomposes into FeH5 and H2 at 200 GPa. However, FeH4 becomes stable and FeH6 again sits on the convex hull at 300 GPa. Figure 1b shows their stability pressure ranges. Although ion dynamics in compounds with light elements might substantially change their total energy due to the large zero-point energy (ZPE) contribution and, therefore, modify their structural stability,51,72 in the iron hydrides we have studied the inclusion of the ZPE does not change their relative stabilities (Figure S7).

As mentioned above, although the tetragonal I4/mmm FeH2 structure we are predicting here has the same space group as the one proposed in Ref.33, it is different. The difference is in the occupation of H2 atoms (Figure S2). Specifically, although in the FeH2 structure we are predicting H2 atoms bond with iron atoms located in the same plane, in the previously proposed one H and Fe atoms lay on different planes. In comparison with the structure proposed in Ref.33, the unique atomic arrangement of the new structure we are predicting here shows a smaller ∆U and ∆(PV), leading to a lower enthalpy. Additionally, the simulated x-ray diffraction pattern of our structure is in very good agreement with the experimental one (Figure S3), and it is ferromagnetic in its stable pressure range (Figure S4), which also agrees with a theoretical study of Pépin et al.33 More interestingly, we are also predicting a new stable stoichiometry under pressure, FeH6, becoming another example of how pressure promotes the formation of polyhydrides with unusually high hydrogen-to-metal ratios. FeH6 stabilizes into an orthorhombic structure (space group Cmcm, 8 formula units per cell, Figure S5) at 100 GPa. The Cmcm structure transforms into an orthorhombic structure with Cmmm symmetry at 106.8 GPa (Figure S6). Then, Cmmm FeH6 becomes unstable above 115 GPa, in good agreement with a recent work of Kvash51 nin et al. Under further compression, a monoclinic FeH6

with C2/c symmetry (4 formula units per cell) becomes stable above 213.7 GPa. A pressure induced potential synthesis of FeH6 starting from FeH5 and H2 is also shown in Figure S8. Interestingly, contrary to the trend of the internal energy (∆U), ∆(PV) plays a crucial role in stabilizing C2/c FeH6 (Figure S8). C2/c FeH6 contains one equivalent Fe occupying 4e (0.0000, 0.3653, 1.2500) position, and four inequivalent H’s sitting at 8f (0.1889, 0.9929, 1.3450), 8f (0.0441, 0.1566, 0.5144), 4e (0.0000, 0.3855, 0.7500), and 4c (0.2500, 0.7500, 0.5000) sites. In more detail, each Fe atom shows a thirteen-fold coordination with H atoms (Figure S9), and Fe polyhedrons form a three-dimensional Fe framework (Figure 2a). On the other hand, the structure shows helical four-membered Fe chains along the a-axis (Figure 2b). The distance between the nearest-neighbor Fe atoms is 2.32 Å at 220 GPa, which is slightly shorter than 2.48 Å in Im-3m Fe phase at ambient conditions75 and comparable to 2.31 Å in I4/mmm FeH5 at 220 GPa. There is no bonding between hydrogen atoms, as the nearestneighbor H-H distance is 1.19 Å, which is much longer than 0.74~1.0 Å of quasimolecular H276,36,37 and linear H3 units,77,78 but much closer to the H-H distance in metallic atomic hydrogen.79 Thus, monoclinic C2/c FeH6 can be viewed as non-bonded hydrogen atoms embedded in the iron frame.

Figure 1. (a) Phase stabilities of various Fe-H compounds at 100, 200, and 300 GPa. The elemental Fe solid with P63/mmc symmetry,73 the P63/m, C2/c, and Cmca-12 phases74 of elemental H2 solids were used to calculate formation



The calculated phonon spectra of the predicted C2/c FeH6 structure do not present any imaginary frequency mode, indicating it is dynamically stable (Figure 3a and Figure S10). Further analysis of the projected phonon density of states (PHDOS) shows that H states are larger than the Fe states above 17.2 THz (Figure 3b). It is interesting to note that the highest vibrational frequency of 70.8 THz, is much lower than in quasimolecular H2 or linear H3.77,78 This also reflects the weak interaction between hydrogen atoms. While Cmcm FeH6 is also metallic (Figure S11), C2/c FeH6 shows an unexpected non-metallic behavior. To accurately describe electronic properties, band structure and projected density of states of C2/c FeH6 were calculated by using Heyd-Scuseria-Ernzerhof80,81 (HSE06) screened hybrid density functional. C2/c FeH6 exhibits an indirect band gap of 0.93 eV at 220 GPa (Figure 3c). Its non-metallicity can be attributed to pressure-induced hybridization of Fe and H orbitals. The relaxed C2/c FeH6 structure at 0 GPa is a narrow-band semiconductor of band gap 0.17 eV, with bands associated to Fe 3d orbitals close to the Fermi level (Figure S12). Compression leads to an effective hybridization between Fe 3d orbital and Fe 3p and H 1s states, as their contribution just below the Fermi level at Γ enhances with pressure (see Figures S13 and S14), favoring the pressure induced increasing of the gap (Figures 3f and S14). This observation is similar to ionic compounds such as MgO282 and Li2O2.83 Although P21/m FeH4 has a band gap value of 0.40 eV at 241.7 GPa,34 its band gap eventually closes at 1000 GPa. In other words, it becomes metallic at high pressure (Figure S15). This is due to pressure-induced effective Fe 3d orbital extension (Figures S15 and S16).

Figure 3. (a) Phonon dispersion curves and (b) PHDOS projected on Fe and H atoms for C2/c FeH6 at 220 GPa, where the PHDOS of Fe and H are depicted with green and blue lines, respectively. (c) Electronic band structure calculated using the HSE06 hybrid functional for C2/c FeH6 at 220 GPa. The contribution of different atomic orbitals (Fe 4s, Fe 3p, Fe 3d, and H 1s) is denoted by the diameter of the circles, where, for clarity, the sizes of the circles associated to Fe 3d orbital are made ten times smaller. The horizontal dashed lines represent the Fermi energy. (d) COHP of adjacent Fe-Fe and Fe-H pairs in C2/c FeH6 at 220 GPa. (e) Difference charge density (crystal density minus superposition of isolated atomic densities) of C2/c FeH6 at 220 GPa. (f) Pressure dependence of the band gap for C2/c FeH6 calculated using the HSE06 hybrid functional. Additionally, we have analyzed the interatomic interaction and chemical bonding in C2/c FeH6. Crystal orbital Hamilton population (COHP) analysis gives information on the interatomic interaction. Below the Fermi level, negative COHP means bonding states, whereas positive COHP represents antibonding ones. The more negative is the COHP, the stronger is the interatomic interaction. Here, the decomposed COHPs of adjacent Fe-Fe and Fe-H pairs in FeH6 were calculated as implemented in the LOBSTER package.84,85 As shown in Figure 3d, the contribution of bonding states is much larger than the antibonding ones below the Fermi level, favouring the stabilization of C2/c FeH6. Moreover, COHP of Fe-Fe is more negative than that of Fe-H. Fur-

Figure 2. Crystal structure of FeH6 with C2/c symmetry. (a) View of ten-membered Fe units. (b) View of helical four-membered Fe chains along the a-axis direction.


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ther analysis indicates that Fe-Fe interaction mainly arises from the overlap of 4s-3d, 3p-3d, and 3d-3d orbitals. The integrated COHPs (ICOHPs) can be used to measure the bond strength. Here we have calculated the ICOHPs of Fe-Fe and Fe-H in Fm-3m FeH, Pm-3m FeH3, I4/mmm FeH5, and C2/c FeH6 at 220 GPa (Table S2) to compare their bond strengths. The resulting ICOHPs of Fe-Fe and Fe-H in C2/c FeH6 are smaller than those in I4/mmm FeH5 and slightly larger than those in Fm-3m FeH and Pm-3m FeH3, indicating the Fe-Fe and Fe-H bonds in our predicted C2/c FeH6 play a key role in stabilizing this structure. ICOHPs of C2/c FeH6 at 220 GPa (Table S2) are more negative than those at 0 GPa, which also supports that pressure helps stabilizing C2/c FeH6. The Fe and H interaction originates from the hybridization of Fe 4s, Fe 3d, and H 1s states (Figure S13). To further verify the Fe-H bonding type, charge density differences are displayed in Figure 3e, which shows an electronic depletion around Fe atoms and accumulation nearby H atoms, revealing a charge transfer from Fe to H, associated to the Fe-H ionic bond. This is further supported by the Bader charge analysis. In order to check if the oxidation state of Fe changes, as it happens in other elements under pressure,56,57,86,87 we have analyzed the relation between the charge transfer in the Fe-H bond and the hydrogen content in all the stable iron hydrides. According to our calculations, the charge transferred from Fe to H gradually reduces with increasing the hydrogen content in the iron hydrides (Figure S18), indicating it does not change the oxidation state of Fe. Very recently, Kvashnin et al made an extensive study of the structural phase diagram of Fe-H system below 150 GPa.51 To further strengthen our results, we have performed an extensive structural search including another fifteen fraction stoichiometries in the H-rich region (i.e. FeyHx; y = 2, x= 3, 5, 7, 9, 11; y = 3, x = 4, 5, 7, 8, 10, 11, 13, 14, 16, 17) at 200 and 300 GPa. As expected, our predicted insulating C2/c FeH6 structure clearly remains stable after including all these fractional stoichiometries (Figure S19). A more detailed information can be found in the Supporting Information.

an unexpected non-metallicity, becoming another nonmetal among transition metal hydrides at high pressure. More intriguingly, FeH6 is still non-metallic at 1000 GPa, whereas FeH4 becomes metallization at this pressure. This new compound shows a unique structure with a mixture of non-bonded hydrogen atoms in a helical iron framework. Its non-metallicity mainly originates from pressure induced hybridization between Fe and H orbitals. ASSOCIATED CONTENT Supporting Information

Computational details, calculated Birch-Murnaghan equation of states for FeH6 compound, structural parameters of Fe-H compounds, Bader atomic charge of Fe-H compounds.

AUTHOR INFORMATION Corresponding Authors

*[email protected] *[email protected] *[email protected] ORCID Shoutao Zhang: 0000-0002-0971-8831 Guochun Yang: 0000-0003-3083-472X Author Contributions ‡These authors contributed equally.

Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by Natural Science Foundation of China under Nos. 21573037 and 11704062, the Postdoctoral Science Foundation of China under grant 2013M541283, the Natural Science Foundation of Jilin Province (20150101042JC), and the Fundamental Research Funds for the Central Universities (2412017QD006). A.B. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (IT756-13).

4. CONCLUSIONS In summary, we have extensively explored the structural phase diagram of iron hydrides with a variety of FeHx (x = 1 - 12) stoichiometries, by using ab initio particleswarm optimization. Besides finding a new tetragonal structure for FeH2 with I4/mmm symmetry, which agrees with experimental observations, we have predicted a new stable stoichiometry, FeH6, above 213.7 GPa with C2/c symmetry. This is another example of how pressure promotes the formation of polyhydrides with unusually high hydrogen-to-metal ratios. Although pressure usually induces insulator-metal transitions, C2/c FeH6 shows

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Nonmetallic FeH6 under High Pressure

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