Polyhydride Ceh9 with an Atomic-Like Hydrogen Clathrate Structure

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https://doi.org/10.1038/s41467-019-11330-6 OPEN

Polyhydride CeH9 with an atomic-like hydrogen clathrate structure

Xin Li 1, Xiaoli Huang1, Defang Duan1,2, Chris J. Pickard 2, Di Zhou1, Hui Xie1, Quan Zhuang1, Yanping Huang1, Qiang Zhou1, Bingbing Liu1 & Tian Cui1

Compression of hydrogen-rich hydrides has been proposed as an alternative way to attain the atomic metallic hydrogen state or high-temperature superconductors. However, it remains a

1234567890():,; challenge to get access to these states by synthesizing novel polyhydrides with unusually high hydrogen-to-metal ratios. Here we synthesize a series of cerium (Ce) polyhydrides by a

direct reaction of Ce and H2 at high pressures. We discover that cerium polyhydride CeH9, formed above 100 GPa, presents a three-dimensional hydrogen network composed of

clathrate H29 cages. The electron localization function together with band structure calculations elucidate the weak electron localization between H-H atoms and conﬁrm its metallic character. By means of Ce atom doping, metallic hydrogen structure can be realized via the

existence of CeH9. Particularly, Ce atoms play a positive role to stabilize the sublattice of hydrogen cages similar to the recently discovered near-room-temperature lanthanum hydride superconductors.

1 State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China. 2 Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Correspondence and requests for materials should be addressed to X.H. (email: [email protected]) or to T.C. (email: [email protected])

NATURE COMMUNICATIONS | (2019) 10:3461 | https://doi.org/10.1038/s41467-019-11330-6 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11330-6

he realization of metallic hydrogen is highly desirable Results and discussion Tbecause of its attractive properties, such as high energy Synchrotron X-ray diffraction experiments. The experiments density, high-temperature superconductivity, and super- were performed with a direct reaction of Ce and H2 samples up to fluidity. Pressure has been recognized as one of the most 159 GPa at room temperature. Compared with previously straightforward and robust way of getting metallic hydrogen, reported polyhydrides, the current reacted samples without laser where changes in bond strength and geometry are induced heating are reliable enough to detect the X-ray diffraction (XRD) without the complication of chemical adjustments. Tremendous signals. Moreover, excess hydrogen in the sample chamber efforts have been devoted, but unfortunately, solid hydrogen ensures the possible reaction of polyhydrides (Supplementary requires very high pressures to get into a metallic state1–6. Fig. 1). We have identified the formation of Ce polyhydrides Recently, metallization of solid hydrogen at 495 GPa has been under high pressures. The newly synthesized phases were char- reported, but such high pressure is a challenge for further acterized by XRD from 3 to 159 GPa as shown in Fig. 1a. The experimental exploration7. Systematic studies on polyhydrides evolution of d-spacing also proves the multiple phase transitions have indicated that nonhydrogen atoms maybe provide a che- with increasing pressure (Supplementary Fig. 2). Because of the mical precompression effect inside hydrogen-rich compounds, low-atomic scattering power of hydrogen, what we have directly which may potentially reduce the pressure required to metalli- determined are the positions of Ce atoms from XRD data. We zation8. Moreover, these polyhydrides also present intriguing have selected typical XRD patterns in Fig. 1b, and the results high-temperature superconductivity theoretically, such as H3S reveal dramatic changes in the formation of Ce polyhydrides. 9,10 11 with 203 K at 200 GPa , CaH6 with 235 K at 150 GPa , LaH10 At 3 GPa, just after the target sample loading into DAC, the 12 13 with 280 K at 210 GPa , and YH10 with 310 K at 250 GPa . diffraction pattern reveals that the sample crystallizes in a face- 14–16 Only the predicted high Tc exceeding 200 K of H3S and centered cubic (fcc) structure with the most possible space group 17,18 fi LaH10 have been undoubtedly con rmed by experiments Fm-3m (Supplementary Fig. 3). This is reminiscent of the cubic thus far. These discoveries have raised new prospects that poly- structure of the element Ce at similar pressures, but with hydrides could become a new family of high-temperature con- significantly large volume. With increasing pressure, the diffrac- ventional superconductors, but the principal experimental tion peaks broaden and shift abnormally, indicating a distortion challenge lies in the difficulty of synthesizing the novel hydrogen- in the fcc Ce sublattice (Supplementary Fig. 4). Above 33 GPa, rich compounds proposed by theory. distorted fcc phase gradually transfers to another cubic phase and Until recently, polyhydrides were usually synthesized under the new cubic phase is stable up to 72 GPa. We observe an high pressures combined with high-temperature condition. In emergence of tetragonal phase at 62 GPa with a small diffraction these polyhydrides, hydrogen atoms exist in different forms, peak. Note that the tetragonal phase can stabilize in a wide including unpaired H, H2,andH3 units, and extended H pressure range from 62 to 159 GPa. Above 80 GPa, a new phase networks. For instance, FeH5 prepared from a laser-heated with hexagonal symmetry appears and coexists with the diamond anvil cell (DAC) above 130 GPa have been proposed tetragonal phase. Above 103 GPa, the crystal symmetry and 19 as a layered structure built of atomic hydrogen only .NaH7 diffraction patterns remain hexagonal symmetry, but its unit cell was synthesized above 40 GPa and 2000 K, but it presents volume changes as discussed below. Currently, XRD patterns − nonmetallic properties with H3 and H2 units processing suggest that the original sample undergoes four phase transitions higher-frequency vibrons in Raman spectrum20.Subsequent under high pressures. Upon decompression to ambient condi- work turned to polyhydrides with three-dimensional hydrogen tions, the hexagonal phase was recovered to initial fcc phase. structure. These polyhydrides might be an analog of high-Tc Our P–V data points are plotted in Fig. 2 together with superconductor and the representative example was reported in literature data. The present data are less scattered thanks to the 12 a body-centered cubic phase of LaH10 with clathrate H32 cage , improved volume accuracy. The P–V data points are fitted with a which was synthesized above 170 GPa and 1000 K. The sur- third-order Birch–Murnaghan (BM) equation of state24, yielding prising high Tc near room temperature in lanthanum hydride is the zero pressure volume V0, the bulk modulus B0, and its mainly attributed to an intriguing H clathrate structure with pressure derivative B0´. At 3 GPa, the volume of fcc phase is large H-derived electron density of states at Fermi level and the 41.80 Å3/formula. Because of the very similar volumes between – 21 25 fi strong electron phonon coupling related to H cages .Then, CeH2 and CeH3 , we have identi ed fcc phase as CeH3 rather in-depth research in rare earth metal hydrides is urgently than CeH2, because H2 is excessive in the chamber and the needed, including successful synthesis of H clathrate structure enthalpy calculations indicate that CeH3 has a lower enthalpy and observation of superconductivity. value than CeH2 (Supplementary Fig. 5). Above 72 GPa, the + 26 For these synthesized polyhydrides, the laser-heating treatment mixing curve of Ce 4/2 H2 (here we use reported EOS of Ce 27 at sufficient pressure is the necessary condition of successful and H2 ) is in good agreement with our experimentally fitted synthesis, overcoming the possible energy barrier. In contrast, to compression curve of tetragonal phase, thus this phase is likely to obtain polyhydrides only through the high-pressure method is have an CeH4 stoichiometry. For the second cubic phase from 40 more feasible to explore the promising properties. Here, we report to 72 GPa, its volume is close to cubic CeH3 at 40 GPa, however, fi a study of the prototypical Ce hydrides to nd the one with the volume becomes close to tetragonal CeH4 at 72 GPa. In this atomic-like hydrogen sublattice and high-Tc superconductor pressure range, we can also intuitively see that the compression through cold-compression engineering. Among the Ce hydrides curve of second cubic phase is approaching the ideal mixing curve + reported in the past decade, the studies of Ce hydrides are limited of Ce 4/2 H2 and connects the compression curves of CeH3 and ≤ with a nonstoichiometric composition range (CeHx, x 3) by the CeH4 smoothly, indicating a process of hydrogen absorption 22,23 combination of Ce in excess hydrogen . In this work, the from CeH3 to CeH4 (Supplementary Figs. 6 and 7). In the experimental syntheses of Ce polyhydrides have been successfully pressure range of 80–103 GPa, anomalous compressibility fulfilled under high pressures up to 159 GPa. Five stoichiometries behavior of hexagonal phase is interpreted as an increase of the of Ce polyhydrides are identified from experimental and theo- hydrogen concentration, corresponding to the ideal mixing curve fi + fi retical results. Especially, we nd that synthesized CeH9 has of Ce 8/2 H2. Above 103 GPa, the tted compression curve of + unique clathrate structure with H29 cages and exhibits a structure hexagonal phase lies above the ideal mixing curve of Ce 8/2 H2 + with atomic hydrogen sublattice. and below the ideal mixing curve of Ce 9/2 H2. The calculation

2 NATURE COMMUNICATIONS | (2019) 10:3461 | https://doi.org/10.1038/s41467-019-11330-6 | www.nature.com/naturecommunications mmm B uvsrpeettevlm fmxue furatdC n H and Ce unreacted of mixtures of volume the represent curves aeeghwas wavelength ersn h Mequation BM the represent AUECOMMUNICATIONS NATURE 2 Fig. 1 Fig. https://doi.org/10.1038/s41467-019-11330-6 | COMMUNICATIONS NATURE odcmrsinu o19Gaa omtmeaue h lc rosso h bomlted fpaswt nraigpressure. increasing with peaks of trends abnormal the show arrows black The temperature. room at GPa 159 to up cold-compression re 0 fi = eet fCeH of nements 0. 1.)GPa, (13.4) 106.0 ; h vlto fXDpten fC oyyrdsa eetdpressures. selected at polyhydrides Ce of patterns XRD of evolution The h oueprfruaui safnto fpesr o eplhdie.Tesldpit ersn h experimental the represent points solid The polyhydrides. Ce for pressure of function a as unit formula per volume The B 0 = 3. 1.)GPa, (12.7) 135.9 λ = 3 - .19Å xessldhdoe eandtasaeta 5 P ssoni netdphotomicrographs inserted in shown as GPa 159 at transparent remained hydrogen solid Excess Å. 0.6199 Pm B -3 0 ´ n 21)1:41 tp:/o.r/013/447091306|www.nature.com/naturecommunications | https://doi.org/10.1038/s41467-019-11330-6 | 10:3461 (2019) | fi ab = CeH , so xeietlrsls The results. experimental of ts Intensity (a.u.) 4( 01 416 14 12 10 8 6 B 3 0 159 (GPa) Volume per f.u.(Å ) 3 14 33 44 57 62 72 76 84 93 103 118 140 ´ fi 30 35 40 15 20 25 4 = e) and xed), - I 4/ 4( 020 mmm fi e) and xed), V n CeH and 2theta (degree) 0 CeH CeH CeH EOS ofCe(Vohra etal. 1999) = 83(.)Å (0.9) 38.3 3 3 3 V - - - Fm Fm Fm 0 9 = - -3 -3 -3 P 74(.)Å (1.0) 47.4 6 06 80 60 40 m m m 3 calculation fittedexp. experiment 82 224 22 20 18 fi / tdrslsare results tted mmc CeH CeH 3 o CeH for 2 n eBi re Bail Le and 3 3+x - Pm - 3 Pm o CeH for -3 Pressure (GPa) 3 -3 + n calculation x n ; B experiment B 0 0 = 9 = a fi - 68 P 45(.)GPa, (3.6) 54.5 eet fCeH of nements yia R atrso eplhdie safnto fpesr upon pressure of function a as polyhydrides Ce of patterns XRD Typical 56(.)GPa, (2.5) 95.6 6 R R R R R 3 R R R wp p wp p / =15.14% wp wp p p =9.07% =8.41% =12.98% =17.10% =16.71% mmc =12.21% =7.28% CeH Ce +8/2H 0 2 4 160 140 120 100 01 416 14 12 10 h ahdcre ersn h acltdES h dotted The EOS. calculated the represent curves dashed The . 9- δ experiment 2theta (degree) B 9 2 0 B − ´ 0 δ ´ = epcieya eetdpesrs h incident The pressures. selected at respectively , Ce +4/2H CeH CeH CeH = 4( . ( 3.4 Ce +9/2H CeH CeH CeH 4 4 4 experiment fi CeH Difference Fitting at76GPaExperiment CeH CeH Difference Fitting at93GPaExperiment CeH CeH Difference Fitting at57GPaExperiment CeH Difference Fitting at159GPaExperiment calculation fitted exp. e) and xed), 9 9 9 82 224 22 20 18 fi 3 4 4 9- 4 9 calculation fitted exp. experiment - - - - - e) and xed), δ Pm l l l P 2 4/ 4/ 4/ 6 mmm mmm mmm 3 -3 / mmc 2 n V 0 = V 0 40(.)Å (0.5) 44.0 = 96(.)Å (0.3) 39.6 P – V aa h oi curves solid The data. 3 ARTICLE o CeH for b 3 h Rietveld The o CeH for 3 - Fm 4 -3 -I 4/ m 3 ; ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11330-6 of enthalpy in this pressure range shows that the hexagonal phase give the possible phase transitions and symmetry and match the is in favor of CeH9. structures of prediction; (ii) the volume changes obtained from XRD data indicate the stoichiometry of new phases; (iii) theoretical calculations help us to finally determine the crystal structure Theoretical calculation of Ce polyhydrides. The theoretical and stoichiometry through the calculated enthalpy difference, calculation is urgently required to determine the hydrogen posi- pressure–volume relation and phase stability. Particularly, Riet- tions and consequently the stoichiometry of Ce polyhydrides. fi fi veld re nement combines the calculated and experimental XRD Previous theoretical work has predicted ve stoichiometries of Ce results. The stoichiometries and crystal structures of Ce poly- polyhydrides: CeH3-R-3m, CeH4-I4/mmm, CeH8-P63mc, CeH9- 21 hydrides have been unraveled through the analysis of theoretical P63/mmc, and CeH10-Fm-3m . Inspired by the prediction on Ce simulation and XRD experiments. As shown in Fig. 1b, excellent polyhydrides, we did further detailed calculations on predicted Rietveld refinements of experimental pattern profiles enable us to structures of Ce polyhydrides to supplement our experimental unambiguously determine the crystal structures of these poly- results. Because Ce atoms with f electrons are an extremely hydrides. The experimental refined lattice parameters are sum- delicate system for density functional theory (DFT) calculations, marized in Supplementary Figs. 10 and 11. At 3 GPa, the patterns generalized gradient approximation (GGA) with Hubbard U of fcc phase match the known structure of CeH -Fm-3m. The correction was used for DFT calculations. We have done the 3 second cubic phase matches our predicted CeH3-Pm-3n structure structural searches at 0 K and 50, 100, and 150 GPa using and its crystal structure is presented in Supplementary Fig. 12. CALYPSO and AIRSS methods28,29. We reproduced the three – Nevertheless, the P V curves demonstrate that CeH3-Pm-3n phases CeH4-I4/mmm, CeH8-P63mc, and CeH9-P63/mmc, and ≤ ≤ would absorb hydrogen and become CeH3+x (0.7 x 1.1) as proposed a new stable phase CeH3-Pm-3n. As summarized in pressure increasing. The tetragonal phase matches the predicted Supplementary Fig. 8, we have investigated the chemical stability CeH4 with I4/mmm structure and its crystal structure is shown in of various Ce polyhydrides by calculating their formation Fig. 3a. In the pressure range of 80–103 GPa, although the pre- enthalpies at pressures of 0, 50, 100, and 150 GPa, relative to the dicted CeH9-P63/mmc has been observed. The abnormal increase products of their dissociation into constituent. At 0 GPa, the of d-spacing and lattice parameters indicates a significant formation enthalpies of all stoichiometries are positive, except expansion of volume (Fig. 2 and Supplementary Fig. 13), which is CeH3-Pm-3n. Both CeH4-I4/mmm and CeH8-P63mc phases lie attributed to absorption of hydrogen in the process of forming on the convex hull at 50 GPa. As pressure increases up to 100 CeH9-P63/mmc. Because of the continuous expansion of volume, GPa, CeH9-P63/mmc emerges, and CeH4-I4/mmm is the most δ ≤ we assign the phase in this pressure range noted as CeH9−δ ( stable stoichiometry. At 150 GPa, CeH3-Pm-3n, CeH4-I4/mmm, 0.85). Above 103 GPa, CeH9-P63/mmc is successfully synthesized, and CeH9-P63/mmc are energetically the most stable, falling on with Wyckoff positions Ce: 2d (2/3, 1/3, 1/4) and H:2b (0, 0, 1/4), the convex hull. Besides, we investigated the dynamical stability of 4f (1/3, 2/3, 0.149), and 12k (0.156, 0.312, 0.062) shown in Sup- proposed CeH3-Pm-3n, CeH4-I4/mmm, CeH8-P63mc, and CeH9- plementary Table 1. Most interestingly, CeH9-P63/mmc presents a P63/mmc by calculating their phonon dispersion curves, and three-dimensional network in H sublattice. Figure 3b–d shows found that all the phases are dynamically stable except CeH8- the crystal structure of CeH9-P63/mmc and the H structure within P63mc (Supplementary Fig. 9). CeH9-P63/mmc. Each Ce atom is surrounded by 29 H atoms forming H29 cage and there are 6 H4 rings, 6 H5 rings, and 6 H6 Crystal structures of Ce polyhydrides. By the joint of experi- rings in each H29 cage. Contiguous H29 cages share the rings and mental and theoretical results, the stoichiometries and crystal form an extended H network structure in three-dimensional structure of compounds were estimated as follows: (i) XRD data space. Note that the P–V relation of these polyhydrides measured

abe

1.6

H3S CeH 1.5 4

1.4

FeH5 cd1.3

Distance of nearest H (Å) LaH 1.2 10

CeH9 1.1 Atomic H

100 120 140 160 180 Pressure (GPa)

Fig. 3 Crystal structure determination of Ce polyhydrides. a The crystal structure of CeH4-I4/mmm. b The crystal structure of CeH9-P63/mmc. c The extended H network in CeH9-P63/mmc. d The H29 cage in CeH9-P63/mmc. Large brown and small green spheres represent Ce and H atoms, respectively. e The evolution of nearest-neighbor H–H distances in polyhydrides and atomic metallic H2 as a function of pressure. These data were gained in previous 9 19 12 19 work of H3S , FeH5 , LaH10 , CeH4-I4/mmm, and CeH9-P63/mmc (this work) and the calculation of atomic metallic hydrogen (solid curve for ref. and dash curve from ref. 12)

4 NATURE COMMUNICATIONS | (2019) 10:3461 | https://doi.org/10.1038/s41467-019-11330-6 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11330-6 ARTICLE by the XRD are in excellent agreement with the theoretical cal- nearest-neighbor H–H distances and volume collapse per H atom culation (Fig. 2), thus further confirming the stoichiometry. in Ce polyhydrides with other reported polyhydrides and metallic For the first time, the phase diagram of Ce–H system has hydrogen in Fig. 3e and Supplementary Fig. 16. Some poly- 30 been determined in experiment by the XRD patterns (Supple- hydrides consisted of H2 or H3 units, such as Xe(H2)8 and 20 mentary Fig. 14). The stable Ce polyhydrides in excess hydrogen NaH7 , present undissociated H–H distances of about 0.75 Å. At have increasing stoichiometry under high pressure. Further 132 GPa, the nearest-neighbor H-H distance of CeH4-I4/mmm is discussion about stoichiometries is given in Supplementary 1.54 Å, which is slightly smaller than 1.56 Å in high-temperature – Information. superconducting phase H3S. The nearest-neighbor H H distance Based on the above analysis, the polyhydrides especially CeH9- of CeH9-P63/mmc is closest to atomic metallic hydrogen (pre- 31 – P63/mmc have been discovered merely through the cold- dicted I41/amd structure ) with nearest-neighbor H H distance compression treatment. The present synthesized path facilitates of 1.09 Å at 132 GPa, which is also much lower than 1.2 Å in 12 19 further experimental characterization in contrast with other LaH10 , 1.32 Å in FeH5 at the same pressure, indicating that Ce recently synthesized polyhydrides such as LaH10 and NaH7, atoms play a more positive on chemical precompression. Thus, which require laser-heating to overcome the energy barrier. the evolution of nearest-neighbor H–H distance is almost close to Besides, we have also combined high-pressure and high- atomic metal hydrogen, which may be realized by Ce atom temperature conditions to trigger the formation of CeH9-P63/ doping. mmc and high-temperature is realized through laser-heating We have calculated the electron localization function to figure fi treatment. By means of Rietveld Re nement (Supplementary out the bonding nature in the CeH9-P63/mmc. Figure 4a shows fi Fig. 15), we have con rmed the formation of CeH9-P63/mmc in that the lack of electron localization between Ce and H atoms these two synthetic routes—cold compression and high- indicates the absence of covalent bond characteristics and there is temperature annealing. a weak electron localization (about 0.5–0.6) between H atoms forming a 3D network structure with H4,H5, and H6 rings. In Fig. 4b, an analysis of the electronic band structure illustrates that Atomic-like hydrogen sublattice realized via CeH9-P63/mmc. CeH9-P63/mmc presents metallic character owing to overlap of The discovered Ce polyhydrides invite us to reveal rich chemistry the conduction and valence bands at the Fermi level. The of hydrogen atom at elevated pressures. We compared the projected electronic density of states shows that both H and Ce

0 0.5 1

b 01 4

–2

–4

–6 Energy (eV)

–8

–10

Ce –12 H

–14

GA HK G ML HDOS (state/eV/f. u.)

Fig. 4 Calculated electronic properties of CeH9-P63/mmc at 100 GPa. a Plot of electron localization function reveals a weak electron localization (about 0.5–0.6) between H atoms and conﬁrms the formation of H4,H5, and H6 rings as the units of extended H network. b Electronic band structure and Partial density of electronic states of CeH9-P63/mmc

NATURE COMMUNICATIONS | (2019) 10:3461 | https://doi.org/10.1038/s41467-019-11330-6 | www.nature.com/naturecommunications 5 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-019-11330-6 make a sizable contribution to the density of states at the Fermi Received: 25 March 2019 Accepted: 9 July 2019 level (Fig. 4b and Supplementary Fig. 17). The density of electronic states of each phase illustrates that the contribution of hydrogen to the Fermi level would increase as stoichiometry and pressure increase (Supplementary Fig. 18), further confirming the indispensable role of hydrogen in these polyhydrides. References In conclusion, we have successfully synthesized a series of 1. Huang, X. et al. High-pressure Raman study of solid hydrogen up to 300 GPa. cerium polyhydrides by direct reaction of Ce and H2 upon cold- Chin. Phys. B 25, 037401 (2016). 2. Zha, C.-S., Liu, Z. & Hemley, R. J. Synchrotron infrared measurements of compression up to 159 GPa. 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Acknowledgements Open Access This article is licensed under a Creative Commons The authors would like to thank professor Artem R. Oganov for many useful discussions, the Attribution 4.0 International License, which permits use, sharing, Beijing Synchrotron Radiation Facility HP-Station 4W2, Shanghai Synchrotron Radiation adaptation, distribution and reproduction in any medium or format, as long as you give Facility Beamline BL15U1 and its staff for allowing us to perform the XRD analyze. This appropriate credit to the original author(s) and the source, provide a link to the Creative work was supported by National Natural Science Foundation of China (Nos. 51572108, Commons license, and indicate if changes were made. The images or other third party 51632002, 11504127, 11674122, 11574112, 11474127, and 11634004), the 111 Project (No. ’ B12011), Program for Changjiang Scholars and Innovative Research Team in University material in this article are included in the article s Creative Commons license, unless (No. IRT_15R23) and National Found for Fostering Talents of basic Science (No. J1103202). indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from Author contributions the copyright holder. To view a copy of this license, visit http://creativecommons.org/ X.H. and T.C. designed the experiments. X.L. and D.Z. prepared the cells and loaded the licenses/by/4.0/. hydrogen. X.L. and Y.H. performed the XRD experiments. X.L. and X.H. analyzed the experimental data. C.J.P., H.X., Qu Z., and D.D. performed and analyzed the calculations. X.L., X.H., and T.C. wrote and revised the paper. Qi Z. and B.L. discussed the results and © The Author(s) 2019 commented on the paper.

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