High-Pressure Formation of Cobalt Polyhydrides - American Chemical

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

High-Pressure Formation of Cobalt Polyhydrides: A First-Principle Study Liyuan Wang, Defang Duan,* Hongyu Yu, Hui Xie, Xiaoli Huang, Yanbin Ma, Fubo Tian, Da Li, Bingbing Liu, and Tian Cui* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun, 130012, P. R. China ABSTRACT: The high-pressure phase diagram, crystal structures, and electronic properties of cobalt hydrides are systematically investigated in the pressure range of 1 atm to 300 GPa by first-principle calculations. Except for the experimentally found CoH, two new cobalt polyhydrides CoH2 and CoH3 are discovered at 10 and 30 GPa, respectively. The crystal structure of CoH2 is determined to have cubic symmetry with the space group Fm3̅m and then transforms into the I4/mmm phase above 42 GPa. In addition, CoH3 with Pm3̅m is stable between 30 and 300 GPa, which can be used as a potential hydrogen storage material with a high volumetric hydrogen density of 425 g H2/L. All the cobalt polyhydrides exhibit metallic and ionic characteristics at high pressure. Furthermore, application of the Allen−Dynes-modified McMillan equation estimated no superconductivity for cobalt polyhydrides.

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INTRODUCTION Metallic hydrogen has been predicted theoretically to be a hightemperature superconductor at high pressure.1,2 However, the experimental evidence of metallic hydrogen up to 495 GPa remains insufficient.3 To reduce the external pressure for metallization, many scientists have tried to explore hydrogenrich compounds under high pressure.4 As Ashcroft5 suggested, the hydrogen-rich compounds could transform into metal under relatively lower pressure than pure hydrogen via chemical precompression. Encouragingly, a number of theoretical studies have showed that some hydrogen-rich compounds at high pressure are good superconductors with remarkably superconducting critical temperatures Tc that are higher than 200 K (e.g., H3S,6,7 CaH6,8 YH10,9 and LaH1010). In fact, hydrogen-rich compounds are not only high-temperature superconductors but can also be used as potential hydrogen storage materials.11 For example, CH4 can absorb several H2 to form the hydrogen-rich compound (H2)4CH4 at high pressure with high molecular hydrogen density (33.4 wt %).12 Under pressure, most transition metals can react with hydrogen to form metal hydrides, and the ratio of hydrogen and metal is usually close to 1. However, recent experiments identified polyhydrides of transition metals under high pressure. For example, a new rhodium dihydride RhH2 with high volumetric hydrogen density of 163.7 g H2/L was discovered at 8 GPa at room temperature.13 Iridium trihydride IrH3 was first theoretically predicted and was successfully synthesized above 65 GPa at room temperature by in situ X-ray diffraction measurement.14 In the case of FeH with Fm3m ̅ phase, it is the only stable stoichiometry up to 136 GPa and at room temperature.15 However, a dihydride FeH2 with the space group I4/mmm was © XXXX American Chemical Society

observed at 67 GPa by laser heating. With increasing pressure, another trihydride FeH3 with Pm3m ̅ was discovered under laser heating at ∼1400 K above 87 GPa. Recently, a new pentahydride FeH5 with I4/mmm was synthesized at 130 GPa in a laser-heated diamond anvil cell.16 At ambient conditions, cobalt adopts a face-centered cubic structure (fcc) with the space group Fm3m ̅ . In compressing the mixture of cobalt and hydrogen at 523 K−623 K, the hydrogento-metal atomic ratios x reached 0.6 at 7 GPa. At an increased pressure of 9 GPa, cobalt monohydride with fcc structure was formed, which can be retained in a metastable state at an atmosphere below 200 K.9 The neutron diffraction at 95 K determined the cobalt monohydride as a NaCl-type structure (space group Fm3̅m) with a = 3.7124(5) Å.17 In this structure, Co atoms occupied the fcc sublattice and H atoms filled the octahedral sites, and this structure was an isostructure with FeH.18 Hereafter, the X-ray diffraction showed that CoH was observed above 4.5 GPa and room temperature. Further pressure increase of up to 22 GPa showed no occurrence of hydrogen content increase over H/Co = 1.19 Cobalt is adjacent to Fe and Rh in group VIIIB. We argue that new cobalt polyhydrides may be synthesized at high pressure. In this work, we extensively explore the energetically stable stoichiometries and structures of cobalt hydrides CoHn (n = 1−5) in the pressure range of 1 atm to 300 GPa by first-principle calculation. Two new cobalt polyhydrides CoH2 and CoH3 are predicted to be stable above 10 and 30 GPa, respectively. At low pressure, CoH2 adopts the Fm3̅m phase and transforms to the Received: September 18, 2017

A

DOI: 10.1021/acs.inorgchem.7b02371 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry I4/mmm phase, which is an isostructure with FeH2.15 The CoH3 with Pm3̅m phase, which is similar to FeH3,15 is stable up to 300 GPa. Furthermore, we explore electronic properties, magnetic properties, and superconductivity of cobalt hydrides at high pressure. Unfortunately, no superconductivity in Co−H system is observed.

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consistent with experimental reports.19 When pressure increases to 10 GPa, a new stoichiometry CoH2 emerges, but CoH has the most negative enthalpy of formation. When the pressure reaches 50 GPa, another numerous H content stoichiometry CoH3 falls on the convex hull, which can coexist with CoH and CoH2. In addition, the stoichiometry CoH keeps the most stable phase at up to 300 GPa. Meanwhile, CoH4 and CoH5 consistently remain unstable. Furthermore, we construct the pressure−composition phase diagram of Co−H system presented in Figure 2. For CoH,

COMPUTATIONAL METHOD

We have employed the evolutionary algorithm Universal Structure Predictor: Evolutionary Xtallography (USPEX), which is implemented in the USPEX code,20−22 to predict the candidate stoichiometries and structures of CoHn (n = 1−5) at 0 K and in the pressure range of 1 atm to 300 GPa. Structural geometry optimizations, electronic structure, and magnetic properties were performed using Perdew−Burke−Ernzerhof parametrization of the generalized gradient approximation,23 as implemented in the Vienna ab initio simulation package code.24 The projector augmented-wave method25 was adopted for all electrons of Co and H. The valence electrons and cutoff radius for Co and H were 3d74s2, 1s1, and 2.3 au, 0.8 au, respectively. A plane-wave cutoff energy of 700 eV and k-point meshes of spacing 2π × 0.03 Å−1 was employed for all structures to ensure that the enthalpy calculations converged to higher than 1 meV per atom. The phonon calculations were carried out using a supercell approach26 in the PHONOPY code.27 The electron− phonon coupling (EPC) for superconducting properties were performed by using the density function perturbation theory by the Quantum ESPRESSO code.28 The norm-conserving pseudopotentials and a kinetic energy cutoff of 80 Ry were adopted. The q-point meshes of 6 × 6 × 6 for Fm3̅m-CoH, Fm3̅m-CoH2, and I4/mmm-CoH2 and 8 × 8 × 8 for Pm3̅m−CoH3 were selected.

Figure 2. Pressure−composition phase diagram of the Co−H system.

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it adopts the face-centered cubic Fm3̅m (4 fu/cell) structure, which is consistent with previous experiments, as shown in Figure 3a. At 10 GPa, the Fm3̅m phase is predicted for CoH2 and it transforms to the I4/mmm phase at 42 GPa. In the Fm3̅m phase, Co atoms form an fcc sublattice and the H atoms occupy all the tetrahedral interstitial sites in Figure 3b, similar to the structure of RhH2.13 For the I4/mmm phase, the H atoms occupy the 4e and 4c sites, which can be interpreted as stackings of

RESULTS AND DISCUSSIONS The variable-cell structural predictions for CoHn (n = 1−5) are implemented with considering the unit cell sizes ranging from 1 to 4 formula units (fu). Figure 1 shows the computed formation

Figure 1. Convex hull diagram for Co−H system with respect to Co and H2 at given pressures. Solid lines represent the thermodynamically stable compounds, and dashed lines represent the metastable ones.

enthalpies of the most stable ground state structures at each stoichiometry relative to the Co metal and solid H2 in their most stable forms at given pressures. The known structures of P63/ mmc, Fm3̅m for Co, and P63/m, C2/c, and Cmca for H229 are selected as the reference. The thermodynamically stable phase on the convex hull can be synthesized experimentally, whereas the phases above the convex hull are mestable. At 1 atm, no compound is stable with respect to the elemental decomposition, and this is consistent with the fact that no cobalt hydrides exist at ambient conditions. At a pressure of 5 GPa, CoH becomes the only stable stoichiometry against elemental dissociation, which is

Figure 3. Crystal structures of (a) Fm3m ̅ -CoH, (b) Fm3m ̅ -CoH2, (c) I4/mmm-CoH2, and (d) Pm3̅m-CoH3. Blue and pink spheres denote the Co and H atoms, respectively. B

DOI: 10.1021/acs.inorgchem.7b02371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry square lattices, isostructure with FeH2,15 as depicted in Figure 3c. For CoH3, it forms a Pm3̅m structure, wherein H atoms are located at the eight vertexes and the center of four faces, and Co atoms are in the center of other two faces, isostructure with FeH3,15 as shown in Figure 3d. From our first-principle calculation, we noted that Co can absorb H2 to form CoH2 with Fm3̅m phase at 10 GPa, which transformed to I4/mmm phase at 42 GPa and to form CoH3 with Pm3̅m phase at 30 GPa. However, X-ray diffraction experiment at room temperature asserted that except for CoH, no change of the H content up to 22 GPa was observed.19 The same process was observed for the Fe−H system. FeH was the only stable hydride up to 136 GPa at room temperature. However, I4/mmm-FeH2, Pm3̅m-FeH3, and even I4/mmm-FeH5 were synthesized by laser heating at 67, 86, and 130 GPa, respectively.15,16 Our predicted that the structures of I4/mmm-CoH2 and Pm3m ̅ -CoH3 are similar to FeH2 and FeH3, respectively. Moreover, Co is adjacent to Fe and they have similar electronegativity values (1.88 for Fe and 1.83 for Co). Therefore, we considered that I4/mmm-CoH2 and Pm3m ̅ -CoH3 can be synthesized by laser heating at high pressure. In addition, the solubility of hydrogen in cobalt increases with increasing pressure, indicating that the hydrogen storage capacity of cobalt hydrides increased. Moreover, CoH3 has a high volumetric hydrogen density of 425 g H2/L and a hydrogen gravimetric density of 4.9 wt %. The equation of state (EOS) of Co, CoH, CoH2, and CoH3 are determined by fitting the pressure as a function of volume to the third-order Birch−Murnahan EOS.30 P=

between experimental and theoretical data was mainly attributed to the temperature effect. The P(V) data points of Co, CoH, CoH2, and CoH3 were fitted with a Birch−Murnahan EOS, as presented in Figure 4 together with the experimental data.19

Figure 4. Calculation equation of states (EOS) of Co, CoH, CoH2, and CoH3 are presented by solid lines. (inset) Calculated EOS of CoH compared with experimental data (ref 19).

Note that the calculated volumes of CoH at different pressures are slightly smaller than the experimental data due to the temperature effect. In CoH 2 , the volume collapsed at approximately 3.14% as a result of the Fm3m ̅ to I4/mmm transition, suggesting a first-order phase transition character. Furthermore, we found that the volume increase per H atom, starting from CoH to CoH2 and from CoH2 to CoH3, was similar to that in ∼1.9 Å3. This finding will provide theoretical guidance for further experimental synthesis. The shortest distances of Co−Co, Co−H, and H−H in all the stable structures of cobalt polyhydrides at given pressure are listed in Table 2. The shortest distances of Co−Co is 2.620 Å in

7/3 ⎪ ⎛ V ⎞5/3⎤⎧ 3 ⎡⎛ V0 ⎞ 3 B0 ⎢⎜ ⎟ − ⎜ 0 ⎟ ⎥⎨ 1 + (4 − B0′) ⎪ ⎝ V ⎠ ⎥⎦⎩ 2 ⎢⎣⎝ V ⎠ 4

⎫ ⎡⎛ V ⎞2/3 ⎤⎪ ⎬ × ⎢⎜ 0 ⎟ − 1⎥⎪ ⎢⎣⎝ V ⎠ ⎥⎦⎭

The calculated lattice constant and the equilibrium volume at zero pressure, the bulk modulus, and its pressure derivative are listed in Table 1. Lattice constant a = 3.71 Å is in excellent

Table 2. Shortest Distances of Co−Co, Co−H, and H−H in All the Stable Structures of Co, CoH, CoH2, and CoH3 at Given Pressures

Table 1. Lattice Constants at 0 GPa and Parameters (V0 Zero Pressure Volume, B0 Bulk Modulus, B0′ its Pressure Derivative) of the Equation of State Obtained by the Birch− Murnahan Fit for Co, CoH, CoH2, and CoH3 structure P63/mmc-Co Fm3m ̅ -Co Fm3̅m-CoH Other theoretical studies31 experimental data Fm3̅m-CoH2 I4/mmm-CoH2 Pm3̅m-CoH3

a (Å) 2.46 3.13 3.71 3.70

c (Å)

V0 (Å3/fu)

3.96

10.87 10.23 12.72 12.72

17

3.7124 4.00 2.60 2.61

19

9.08

13.2 16.03 15.35 17.84

B0 (GPa)

B0′

224.6 278.2 211.3 230.45

4.08 4.41 4.48 4.88

pressure (GPa)

Co−Co (Å)

Co−H (Å)

H−H (Å)

0 100 200 300 0 100 200 300

2.620 2.456 2.257 2.211 2.478 2.271 2.177 2.118

1.853 1.504 1.601 1.563

2.620 1.737 1.609 1.563

Fm3̅m-Co

19

190 194.4 212.3 214.2

structure Fm3m ̅ -CoH Fm3̅m-CoH2 I4/mmm-CoH2 Pm3̅m-CoH3 P63/mmc-Co

4.03 4.17 4.06

Fm3̅m-CoH at 0 GPa, 2.456 Å in Fm3̅m-CoH2 at 100 GPa, 2.257 Å in I4/mmm-CoH2 at 200 GPa, and 2.211 Å in Pm3̅m-CoH3 at 300 GPa, which are longer than those in bulk Co at corresponding pressures, namely, 2.478, 2.271, 2.177, and 2.118 Å, respectively. The Co lattice is expanded by the incorporation of an increased concentration of H atoms, consistent with the transition metal hydride nature. Furthermore, the shortest distances of H−H in Fm3̅m-CoH at 0 GPa, Fm3̅mCoH2 at 100 GPa, I4/mmm-CoH2 at 200 GPa, and Pm3̅m-CoH3 at 300 GPa are 2.620, 1.737, 1.609, and 1.563 Å, respectively. The shortest bonding H−H distance in solid H2 is 0.73−0.76 Å at

agreement with other theoretical data of a = 3.70 Å31 and experimental data with a = 3.71 Å which was measured by neutron diffraction at 95 K.17 Moreover, the equilibrium volume V0 = 12.72 Å was underestimated by 3.6% compared with that of experimental data with V0 = 13.2 Å,19 which was measured by Xray diffraction at room temperature. All the calculations in this work were performed at 0 K, whereas the experimental data19 were obtained at room temperature. Thus, the difference C

DOI: 10.1021/acs.inorgchem.7b02371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

overlap population (BOP). BOP is widely used to obtain the bonding nature of bulk crystals. A high value of the BOP indicates a high degree of covalency, whereas a low value indicates ionic interaction. Table 3 shows that the BOP value of Fm3m ̅ -CoH, Fm3̅m-CoH2, I4/mmm-CoH2, and Pm3̅m-CoH3 are equal with 0.20 |e|, 0.30 |e|, 0.25 |e|, and 0.10 |e| at 5, 10, 200, and 200 GPa, respectively. These values are close to 0.22 |e| of ionic NaCl, indicating an ionic bonding feature. The dynamical stabilities of Fm3̅m-CoH, Fm3̅m-CoH2, I4/ mmm-CoH2, and Pm3̅m-CoH3 at accessible pressure are performed by calculating the phonon dispersion curves, as depicted in Figure 6. No imaginary phonon mode is observed in

these pressures. This distance is shorter than that of cobalt polyhydrides. Therefore, we conclude that no paired hydrogen occurs in the ground-state cobalt hydrides. The electronic localization function (ELF) is calculated for Fm3̅m-CoH, Fm3̅m-CoH2, I4/mmm-CoH2, and Pm3̅m-CoH3 at 5, 10, 200, and 200 GPa, respectively. The bonding type can be revealed from the ELF. The ELF value of 1 corresponds to strongly localized electrons. The ELF value of 0.5 suggests the electron gas, and the value is closest to 0, thus reflecting nonelectron localization. The 2D ELF maps of Co−H system are shown in Figure 5. The ELF value between Co and the nearest H

Figure 5. Calculated ELF: (a) Fm3̅m-CoH at 5 GPa for the (01̅0) plane, (b) Fm3̅m-CoH2 at 10 GPa for the (01̅1̅) plane, (c) I4/mmm-CoH2 at 200 GPa for the (010) plane, and (d) Pm3̅m-CoH3 at 200 GPa for the (001) plane.

Figure 6. (a−d) Phonon dispersion curves for Fm3̅m-CoH, Fm3̅mCoH2, I4/mmm-CoH2, and Pm3̅m-CoH3 at 5, 10, 200, and 200 GPa, respectively.

atom is less than 0.5, implying that no covalent interaction exists between Co and H atoms. Moreover, we implement the Bader charge analysis to understand the bonding features between Co and H atoms, as shown in Table 3. The charge transfer from Co to H atoms indicate the ionic nature of Co−H bonding. To further demonstrate the bonding behavior of Co−H compounds, we applied Mulliken population analysis to explore the bond

the entire Brillouin zone, demonstrating the dynamical stabilities of the above structures. The spin-polarized electronic band structure and projected density of states (DOS) for Fm3̅m-CoH are shown in Figure 7. The calculations are carried out by considering two spin channels, majority spins, and minority spins. The majority spin Co_d orbits are completely occupied, leading to a strong ferromagnetic behavior for the monohydride CoH. The magnetic moment of CoH amounts to 1.167 μB, which is in good agreement with other theoretical data with 1.18 μB.31 The electronic band passes through the Fermi level, clearly exhibiting a metal aspect. We examined the possibility of a highspin state of structures Fm3m ̅ -CoH2, I4/mmm-CoH2, and Pm3m ̅ CoH3, and the nonmagnetic solution is the most stable. Thus, their band structures and DOS are calculated without considering spin polarization, as shown in Figure 8. They are metallic with bands crossing the Fermi level, and Co_d orbits mainly dominate the DOS at the Fermi level. To explore the possibility of superconductivity for Co−H system, Tc was estimated by using the Allen−Dynes-modified McMillan equation:32

Table 3. Calculated BOP Value of Co−H and Bader Charge for Fm3̅m-CoH, Fm3̅m-CoH2, I4/mmm-CoH2, and Pm3̅mCoH3 at 5, 10, 200, and 200 GPa, Respectively Co−H (|e|)

atom

charge

σ(e)

Fm3m ̅ -CoH

0.20

Fm3̅m-CoH2

0.30

I4/mmm-CoH2

0.25

H1 Co1 H1 H2 Co1 H1 H2 H3 H4 Co1 Co2 H1 H2 H3 Co1

1.2662 8.7338 1.2484 1.2485 8.5031 1.1502 1.1502 1.0716 1.0716 8.7775 8.7790 1.0595 1.0595 1.0595 8.8216

−0.2662 0.2662 −0.2484 −0.2485 0.4969 −0.1502 −0.1502 −0.0716 −0.0716 0.2225 0.2210 −0.0595 −0.0595 −0.0595 0.1784

structure

Pm3m ̅ -CoH3

0.10

Tc =

⎤ ⎡ 1.04(1 + λ) exp⎢ − ⎥ 1.2 ⎣ λ − μ*(1 + 0.62λ) ⎦

ω log

where μ* is the Coulomb pseudopotential, ωlog is the logarithmic average of phonon frequencies, and λ is the EPC parameter. This equation is accurate for materials with λ < 1.5, and the empirical value of μ* often selects 0.1−0.13. The EPC parameter λ, ωlog, D

DOI: 10.1021/acs.inorgchem.7b02371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. Spin-polarized electronic band structure and projected density of states (DOS) for Fm3̅m-CoH at 5 GPa.

and Tc and the electronic DOS at the Fermi level N(εf) are calculated at different pressures, as listed in Table 4. For Fm3̅mCoH at 5 GPa, we found that its electron−phonon interaction is relatively weak due to the small EPC parameter λ of 0.28. On the other hand, the value of ωlog is relatively low with 361.8 K because cobalt is heavy. With a typical μ* of 0.1, the superconducting critical temperature Tc is estimated to be 0.11 K at 5 GPa. For most of the hydrides, the electron−phonon is strong, but not for CoH. This encourages us to study electronic DOS at Fermi level N(εf) that is commonly accepted to relate to electron−phonon coupling strength. We thus compare N(εf) of CoH with that of YH10,10 which is predicted to have a high λ with 2.56 at 250 GPa, and find that the d electrons with 24% of Y and s electrons with 36% of H are the main contributors to N(εf). For CoH, although the value of N(εf) with 16.8 states/spin/Ry/fu is high, contributions from the Co_d orbital dominate the DOS with 99.9%, and H_s contributions are negligible with 0.05%, as shown in Figure 7b. This result suggested that H atom contributions to N(εf) are important on determining the EPC. For CoH2 and CoH3, the H atom contributions to N(εf) are still small as well, shown in Figure 8. Therefore, there is not unreasonable to show the electron−phonon coupling parameters of them are low and the Tcs are estimated to become close to 0 K.

Figure 8. Electronic band structure and projected DOS for (a) Fm3m ̅ CoH2 at 10 GPa, (b) I4/mmm-CoH2 at 200 GPa, and (c) Pm3̅m-CoH3 at 200 GPa along the selected high symmetry lines, where the dotted lines at zero indicate the Fermi level.

Table 4. Calculated Electron−Phonon Coupling Parameters (λ), the Logarithmic Average Phonon Frequency ωlog, Electronic Density of States at the Fermi level N(εf) (states/ spin/Ry/fu), and Superconducting Critical Temperatures Tc of Cobalt Polyhydrides at Different Pressures structure

P (GPa)

λ

ωlog (K)

N(εf)

Tc (K) μ* = 0.1

Fm3m ̅ -CoH Fm3̅m-CoH2 I4/mmm-CoH2 Pm3̅m-CoH3

5 10 200 200

0.28 0.23 0.23 0.19

361.8 351.2 744.2 861.2

16.8 11.7 6.45 2.87

0.11 0 0 0

evolutionary algorithm USPEX and first-principle calculations. We found novel structures of Fm3m ̅ -CoH2, I4/mmm-CoH2, and Pm3̅m-CoH3 at high pressure, which were ionic metallic crystals without superconductivity. At 10 GPa, Co reacted with H2 to form CoH2 with Fm3̅m. As pressure increased to 30 GPa, the CoH3 with Pm3m ̅ was formed. We found that the content of

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CONCLUSION In summary, the high-pressure crystal structures and properties of the Co−H system were investigated systematically by the E

DOI: 10.1021/acs.inorgchem.7b02371 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(12) Mao, W. L.; Mao, H. K. Hydrogen storage in molecular compounds. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 708−710. (13) Li, B.; Ding, Y.; Kim, D. Y.; Ahuja, R.; Zou, G. T.; Mao, H. K. Rhodium dihydride (RhH2) with high volumetric hydrogen density. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 18618−18621. (14) Scheler, T.; Marqués, M.; Konôpková, Z.; Guillaume, C. L.; Howie, R. T.; Gregoryanz, E. High-pressure synthesis and characterization of iridium trihydride. Phys. Rev. Lett. 2013, 111, 215503. (15) Pépin, C. M.; Dewaele, A.; Geneste, G.; Loubeyre, P.; Mezouar, M. New Iron Hydrides under High Pressure. Phys. Rev. Lett. 2014, 113, 265504. (16) Pepin, C. M.; Geneste, G.; Dewaele, A.; Mezouar, M.; Loubeyre, P. Synthesis of FeH5: A layered structure with atomic hydrogen slabs. Science 2017, 357, 382−385. (17) Antonov, V. E.; Antonova, T. E.; Fedotov, V. K.; Hansen, T.; Kolesnikov, A. I.; Ivanov, A. S. Neutron scattering studies of gammaCoH. J. Alloys Compd. 2005, 404, 73−76. (18) Bazhanova, Z. G.; Oganov, A. R.; Gianola, O. Fe−C and Fe−H systems at pressures of the Earth’s inner core. Phys.-Usp. 2012, 55, 489− 497. (19) Kuzovnikov, M. A.; Tkacz, M. High pressure studies of cobalt− hydrogen system by X-ray diffraction. J. Alloys Compd. 2015, 650, 884− 886. (20) Oganov, A. R.; Glass, C. W. Crystal structure prediction using ab initio evolutionary techniques: Principles and applications. J. Chem. Phys. 2006, 124, 244704. (21) Oganov, A. R.; Lyakhov, A. O.; Valle, M. How Evolutionary Crystal Structure Prediction Works and Why. Acc. Chem. Res. 2011, 44, 227−237. (22) Lyakhov, A. O.; Oganov, A. R.; Stokes, H. T.; Zhu, Q. New developments in evolutionary structure prediction algorithm USPEX. Comput. Phys. Commun. 2013, 184, 1172−1182. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Kresse, G.; Furthmüller, J. Efficiency of ab initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (25) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (26) Parlinski, K.; Li, Z. Q.; Kawazoe, Y. First-Principles Determination of the Soft Mode in Cubic ZrO2. Phys. Rev. Lett. 1997, 78, 4063− 4066. (27) Togo, A.; Oba, F.; Tanaka, I. First-principles calculations of the ferroelastic transition between rutile-type and CaCl2-type SiO2 at high pressures. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 1374106. (28) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502. (29) Pickard, C. J.; Needs, R. J. Structure of phase III of solid hydrogen. Nat. Phys. 2007, 3, 473−476. (30) Birch, F. Finite strain isotherm and velocities for single-crystal and polycrystalline NaCl at high pressures and 300 K. J. Geophys. Res. 1978, 83, 1257−1268. (31) Riane, R.; Abdiche, A.; Hamerelaine, L.; Guemmou, M.; Ouaini, N.; Matar, S. F. Ab initio investigations of the electronic and magnetic structures of CoH and CoH2. Solid State Sci. 2013, 22, 77−81. (32) Allen, P. B.; Dynes, R. C. Transition temperature of strongcoupled superconductors reanalyzed. Phys. Rev. B 1975, 12, 905−922.

hydrogen in the Co−H system increased with as pressure increased. The formation of cobalt polyhydrides and crystal structure was similar to those of iron polyhydrides, which were discovered in a laser-heated diamond anvil cell. Therefore, CoH2 and CoH3 could be synthesized at high pressures. This work will provide theoretical guidance for further experimental synthesis and for exploring other transition metal hydrides.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected] (D.D.). *E-mail: [email protected] (T.C.). ORCID

Tian Cui: 0000-0002-9664-848X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51632002, 51572108, 11674122, 11634004, 11504127, 11574109, 11404134), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_15R23), Jilin Provincial Science and Technology Development Project of China (20170520116JH), the 111 Project (No. B12011), National Key Research and Development Program of China (under Grant No. 2016YFB0201204), and National Fund for Fostering Talents of basic Science (No. J1103202). Parts of calculations were performed in the High Performance Computing Center (HPCC) of Jilin University and TianHe-1(A) at the National Supercomputer Center in Tianjin.

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DOI: 10.1021/acs.inorgchem.7b02371 Inorg. Chem. XXXX, XXX, XXX−XXX