ABSTRACT Hexagonal close-packed iron hydride, hcp FeH_x_, is absent from the conventional phase diagram of the Fe–H system, although hcp metallic Fe exists stably over extensive temperature

(_T_) and pressure (_P_) conditions, including those corresponding to the Earth’s inner core. _In situ_ X-ray and neutron diffraction measurements at temperatures ranging from 298 to 1073 K

and H pressures ranging from 4 to 7 GPa revealed that the hcp hydride was formed for FeH_x_ compositions when _x_ < 0.6. Hydrogen atoms occupied the octahedral interstitial sites of the

host metal lattice both partially and randomly. The hcp hydride exhibited a H-induced volume expansion of 2.48(5) Å3/H-atom, which was larger than that of the face-centered cubic (fcc)

CRYSTALLIZATION Article Open access 15 June 2022 INVERSION OF THE TEMPERATURE DEPENDENCE OF THERMAL CONDUCTIVITY OF HCP IRON UNDER HIGH PRESSURE Article Open access 09 October 2024

REDOX-STRUCTURE DEPENDENCE OF MOLTEN IRON OXIDES Article Open access 05 November 2020 INTRODUCTION Transition metals react with hydrogen to form hydrides, MH_x_, at hydrogen pressures of

several gigapascals (GPa; hereafter, the hydrogen pressure is referred to simply as “pressure”)1. Hydrogen molecules dissociate to hydrogen (H) atoms on the metal surface, where the H atoms

dissolve into the bulk to partially or fully occupy the interstitial sites of the metal lattice. The interstitial H atoms expand the volume of the metal lattice by 10–20% for a MH_x_

composition of _x_ = 11. The H composition _x_ varies as a function of the temperature (_T_) and pressure (_P_). Accordingly, the volume of hydride (_V_) varies via a hydrogen-induced volume

expansion. In addition to _V_, _x_ is an essential variable for describing the bulk state of a hydride. Hydrogen compositions for recovered specimens have been measured by neutron

diffraction or hot extraction of H2 gas at ambient pressure2. _In situ_ neutron diffraction was recently used to investigate the structure of iron deuteride under high _T–P_ conditions, and

the D composition was successfully determined3. Iron hydride (FeH_x_) has been intensively studied for half a century as a prototype of transition-metal hydrides2,3,4,5,6,7,8,9,10,11,12,13

and an endmember of the constituents of the Earth’s core5,14,15,16,17,18,19,20,21,22,23,24. Figure 1, plotted from previously reported data2,3,4,5,6,7,8,9,10, shows a schematic of the

_x–T–P_ diagram of the Fe–H system at temperatures from 300 to 1400 K and pressures ranging from 0 to 10 GPa 2,3,4,5,6,7,8,9,10. Three phases exist: a low-pressure α phase with a

body-centered cubic (bcc) structure, wherein Fe atoms occupy the vertexes of the bcc lattice; a high-temperature γ phase with a face-centered cubic (fcc) structure and a high-pressure εʹ

phase with a double hexagonal close-packed (dhcp) structure. The bcc and fcc phases are a solid solution of hydrogen for _x_ <1.0, whereas the dhcp phase is a monohydride for _x_ = 1 over

almost the entire stable _T–P_ region. The triple point is located at approximately 520 K and 5 GPa2,4,9,10, where the bcc, dhcp, and fcc phases have approximate H compositions of 0.1, 1.0,

and 0.5, respectively5,7,8,9,10. The phase stability for iron hydride in equilibrium with fluid hydrogen has been investigated, where the composition _x_ was uniquely fixed to the highest

value at a given _T–P_ condition for each hydride phase. A different hydride should form for _x_ values below the equilibrium _x_ surface. The hcp hydride is absent in the conventional phase

diagram although the hcp phase of metallic iron is stable at extensive _T–P_ conditions up to those corresponding to the Earth’s inner core25. We performed structural investigations on the

Fe–H system to explore the formation of the hcp hydride at _x–T–P_ conditions below the equilibrium _x_ surface. _In situ_ X-ray diffraction measurements revealed that the hcp hydride

appeared at ~800 K while the fcc hydride was cooled from ~1000 K at ~7 GPa under conditions where there was no coexisting fluid H2 (red arrow in the inset of Fig. 1). The hcp hydride

subsequently decomposed into dhcp FeH and bcc Fe at ~430 K upon further cooling. The crystal structure including the site occupancy of deuterium (D) atoms was investigated via _in situ_

neutron diffraction measurements of the hcp deuteride, which was prepared by cooling the fcc deuteride (blue arrow in the inset). In this communication, we present _x_–_T–P_ conditions for

the formation of the hcp hydride and the variation of _x_ with the fcc–hcp–dhcp structural transitions. Hydrogen-induced volume expansion and the _x–T_ relation, which are essential

properties for characterizing metal hydrides, are derived from the structural data and compared with those of the fcc hydride. RESULTS X-RAY DIFFRACTION MEASUREMENTS The X-ray diffraction

profiles were collected while cooling fcc FeH_x_ with _x_ ≈ 0.6 from 1073 K to 298 K at an initial pressure of ~7 GPa. In this experiment, the amount of aluminum trihydride (AlH3) pellets

that was used as an internal H source was reduced to a H/Fe molar ratio of ~0.6 to prevent transformation from fcc FeH_x_ to the monohydride dhcp FeH with additional H absorption. The

temperature was continuously decreased at 10 K/min, and the pressure was reduced from 7.1 to 5.0 GPa because of the thermal contraction of the reaction cell. Time-resolved X-ray diffraction

profiles were collected with an exposure time of 20 s/profile using the energy dispersion method26. Figure 2(a) Shows the evolution of the diffraction profile with decreasing temperature.

The observed profiles are divided into approximately three regions for ease of explanation: fcc-dominant (Fig. 2(b)), hcp-dominant (Fig. 2(c)), and dhcp-dominant regions (Fig. 2(d)). When

the temperature decreased to ~800 K, only one peak appeared (~60 keV, _d_ ≈ 2.0 Å) in addition to those of the fcc hydride, as shown in Fig. 2(b). The peak intensity increased with further

decrease in the temperature to ~750 K, whereas the peak intensity of the fcc peaks remained unchanged. Other new peaks appeared at ~650 K. These peaks were assigned as hcp lattice

reflections, indicating a transformation from the fcc to the hcp structure. The most intense 101 peak (denoted by the arrow in Fig. 2(c)), which appeared on the low-energy side of the

preceding 101 peak, was shifted to a higher energy with decreasing temperature, thereby merging with the energy level of the 101 peak in Fig. 2(b). We assigned the preceding peak at

temperatures of 800–750 K as a hcp 101 peak from “precipitated hcp hydride” and the peaks appearing at ~650 K were assigned to “transformed hcp hydride.” The fcc–hcp structural

transformation proceeded until ~480 K (Fig. 2(c)). The fcc 111 peak was shifted to a lower energy because of volume expansion with H absorption; by contrast, the hcp 101 peak was shifted to

a higher energy because of volume contraction with H desorption. The hcp hydride eventually decomposed into dhcp FeH_x_ (_x_ ≈ 1.0) and bcc Fe (Fig. 2(d)) at ~430 K. The diffraction profiles

showed peaks from the fcc and hcp hydrides at temperatures below ~650 and ~430 K, respectively. A small amount of the fcc and hcp hydrides remained in a metastable state, probably because

of slow transformation kinetics at lower temperatures. The diffraction peaks continued to shift to lower or higher energies in the metastable temperature ranges, and the corresponding peak

positions were used to calculate the lattice constants or atomic volumes of Fe, as described in the following paragraphs. NEUTRON DIFFRACTION MEASUREMENTS We performed neutron diffraction

measurements with the same reaction cell as that used in the X-ray diffraction experiments, except that the AlH3 pellet was replaced with an AlD3 pellet3,27,28. An excess amount of AlD3 with

a Fe/D molar ratio of ~1.5 was charged into the cell to completely deuterize a bulk Fe specimen, 3.0 mm in diameter and 2.5 mm in thickness, within a short time. However, deuterization of

the Fe disc took 90 min even at temperatures as high as 1073 K. By contrast, only a few minutes were required for the hydrogenation of Fe flakes, which were mixed with the BN powder that was

used in the X-ray diffraction experiments. This result was expected because the surface areas for the Fe disc and flakes differed by orders of magnitude. After the formation of the fcc

deuteride at 1073 K and 6.0 GPa was confirmed by neutron diffraction, the temperature was rapidly decreased to 673 K to prevent the fcc deuteride from achieving an equilibrium D composition;

the hcp deuteride was thus prepared. The temperature was further decreased to 573 K and finally to 300 K. A neutron diffraction profile was collected at each temperature with a few hours of

integration time. The pressure decreased from 6.0 to 4.2 GPa upon cooling to 300 K. Figure 3 shows the neutron diffraction profiles that were recorded at 1073 K and 6.0 GPa (a), 673 K and

5.1 GPa (b), and 300 K and 4.2 GPa (c). The corresponding simulated and experimental profiles, as fitted by Rietveld refinement29, are shown. It should be noted that the Fe and D

compositions of the fcc deuteride are denoted by _x_ʹ in the panels (a) and (b). The site occupancy of the Fe atoms in the fcc lattice deviated slightly from unity to _x_ʹ < 1.0 because

of the formation of vacancies at the Fe sites9,10,30. The Rietveld refinement only provides the ratio of the site occupancies between Fe and D atoms; hence, the Fe composition is described

by _x_ʹ. The diffraction peaks at 673 K showed that the hcp structure was the dominant component. Rietveld refinement using a hcp model structure with D atoms randomly occupying the

interstitial sites yielded site occupations of 0.48(1) (hereafter, the numbers in parentheses denote the experimental error) and 0.0 for the octahedral and tetrahedral sites, respectively,

and a deuterium composition of _x_ = 0.48(1). A very similar diffraction profile was observed for the hcp deuteride at 573 K and 4.8 GPa, yielding _x_ = 0.48(1). In the 300-K profile, the

dominant diffraction peaks originated from dhcp FeD and bcc Fe; the hcp deuteride decomposed, as observed in the X-ray diffraction experiments. For dhcp FeD, we used a stacking fault model

that was presented in the early neutron diffraction study8. Because bcc Fe and dhcp Fe(H/D) are ferromagnetic31,32, each diffraction peak contains a magnetic scattering component in addition

to a nuclear one. The magnetic moment was optimized to 2.1 (1.1) in Bohr magnetons (μB) for bcc Fe. No magnetic contribution considered for dhcp FeD because the peak intensity was too low

for the magnetic structure to be refined. The structural parameters that were optimized by Rietveld refinement are summarized in Table 1. _V–T_ RELATION The atomic volume of Fe, _v_Fe, which

is calculated by dividing the unit cell volume of iron hydride by the number of Fe atoms contained in the cell, was obtained for each hydride from its X-ray and neutron diffraction data.

The atomic volume was plotted as a function of temperature in the 298–1073 K range in Fig. 4. The atomic volumes of the fcc and hcp Fe metals, which were calculated using their equations of

state33,34, are also plotted as references. For the precipitated hcp hydride, only the 101 peak was observed at temperatures in the 730–800 K range (Fig. 2(b)). The atomic volume for the

precipitated hcp hydride was estimated from the measured _d_ values of the 101 peak and the axial ratio of _c/a_ = 1.600 that was obtained for the transformed hcp hydride. The atomic volumes

were also calculated for the fcc and hcp hydrides that remained as metastable states below the transformation and decomposition temperatures, respectively. The _v_Fe–_T_ relations in Fig. 

4a were used to derive the deuterium-induced volume _v_D = Δ_v_Fe/_x_1. Here, the excess amount of _v_D. that arises from the volume expansion of the metal lattice owing to the dissolution

of H/D atoms can be calculated using Δ_v_Fe = _v_Fe (hydride) − _v_Fe (reference metal). The value of _v_D for the hcp deuteride at 673 K and 5.1 GPa was found to be 2.51 (5) Å3/D atom using

Δ_v_Fe = 1.191 Å3, which was calculated from the _v_Fe (hcp FeD_x_) and the calculated _v_Fe (hcp Fe, which is plotted in Fig. 4b), and _x_ = 0.48(1). Using alternative data for Δ_v_Fe = 

1.166 Å3 and _x_ = 0.48(1) at 573 K and 4.8 GPa, we obtained _v_D = 2.45(4) Å3/D atom. We took an averaged value of 2.48(5) Å3/D atom for the _v_D of hcp deuteride. For dhcp FeD, a 2.42 (4)

Å3/D atom was obtained using the structural data that are listed in Table 1, where the hcp Fe volume was used as the reference volume. The volume of ferromagnetic dhcp deuteride contains an

unknown contribution from magnetic volume expansion; hence, the calculated value is an upper limit on _v_D. The volume data for fcc FeD_x_ that were obtained by the neutron diffraction

should be regarded with caution because the lattice volume for this deuteride is substantially reduced owing to vacancy formation in the metal lattice9,10,30. Hence, we used the _v_D value

of 2.21(4) Å3/D atom, as has been previously reported for vacancy-free fcc FeD_x_3. The _v_D of the hcp hydride is the largest volume among those for iron hydrides. _X_–_T_ RELATION The

expanded volume, Δ_v_Fe, for the fcc and hcp hydrides that was measured over a temperature range of 298–1073 K by X-ray diffraction was converted to H compositions using a proportionality

relation, _x_ = Δ_v_Fe/_v_H, in which _v_H = _v_D is assumed. Figure 5 shows the _x_–_T_ relations of the fcc and hcp hydrides that were calculated using _v_H = 2.21 and 2.48 Å3/H atom,

respectively. The _x_ value of the fcc hydride decreased slightly from 0.68 at 1073 K to 0.66 at 600 K, before increasing towards a saturated value of 1.0; the fcc monohydride and the dhcp

monohydride formed at temperatures below ~430 K. The value of _x_ ≈ 0.5 was obtained for the precipitated hcp hydride in the 700 to 800 K range, whereas the transformed hcp hydride showed a

monotonic decrease in _x_ with decreasing temperature below 650 K. Despite the very similar values of _x_ ≈ 0.6 for the hcp and fcc hydrides at ~600 K, opposing trends were observed for the

variation in the H compositions with the temperature below 600 K. DISCUSSION Iron hydride/deuteride with an hcp metal lattice was formed by the transformation from the fcc hydride/deuteride.

In the early studies6,8, the hcp hydride/deuteride formed as an intermediate metastable state during the hydrogenation/deuterization of bcc Fe to dhcp Fe(H/D). The “transformed” hcp

deuteride has the same crystal structure as that of the “intermediate” hcp deuteride as shown by the structural parameters presented in Table I and Table 3 of ref.8. The D atoms occupy the

octahedral interstitial sites of the host metal lattice both partially and randomly; The present study provided a D composition of 0.48 for hcp deuteride at 673 K and 5.1 GPa, and at 573 K

and 4.8 GPa. This value was slightly higher than 0.42 reported for hcp deuteride prepared at 623 K and 9.2 GPa8. The tetrahedral site occupation, reported for fcc FeD_x_3, or the formation

of layered octahedral superstructures, reported for hcp TcH_x_ and MnD_x_ at _x_ = ~1/235 was not observed. The hcp iron hydride, in both its stable and metastable states, formed for _x_ 

< 0.6 at pressures from 4 to 6 GPa (Table 1 and Fig. 5). This hcp hydride lies under the equilibrium _x_-surface in the phase diagram that is shown in Fig. 1. Controlling of the H

composition plays a key role in the formation of hcp hydride. In the neutron diffraction measurements, the bulk fcc deuteride transformed to the hcp deuteride through the nonequilibrium

state formed due to the relatively low diffusion rate of D atoms at the measured temperatures. In the X-ray diffraction measurements, the powder fcc hydride transformed to the hcp hydride

under the condition of insufficient hydrogen supply. Both of the transformations occurred near the stable _T–P_ region of dhcp phase; the hcp hydride, instead of the dhcp monohydride, was

preferentially precipitated. These results suggest the formation of hcp hydride over a wide _T–P_ region under controlled H composition. The most recent theoretical calculations have shown

that the hcp hydride becomes more stable than the dhcp hydride when _x_ <~0.5 at extensively high _T–P_ conditions36. Further phase studies of the Fe–H system in extended _x–T–P_

conditions are required to clarify the structural stability of the hcp hydride in terms of the H composition. The crystal structures of iron hydride that appeared sequentially in the cooling

experiments are drawn in Fig. 6. The fcc, hcp, and dhcp structures can transform into each other by sliding metal planes and tuning the H composition. At the fcc–hcp transformation

temperature of ~650 K, the H composition of both hydrides is _x_ = 0.6; hence, the fcc structure can transform to the hcp structure by simply altering the stacking sequence of the metal

planes from ABCABC∙∙∙ to ABAB∙∙∙ along the body diagonal axis of the cubic lattice. For decomposition at ~430 K, the dhcp structure can form by altering the sequence from ABAB∙∙∙ to

ABABACAC∙∙∙ along the _c_ axis of the hcp lattice and filling all of the octahedral sites with H atoms. Although each of the metal lattices has one octahedral site per Fe atom available for

H-atom accommodation, the spatial arrangements of these lattices are quite different. The octahedra consisting of Fe atoms at the corners are connected by corner sharing in the fcc lattice

but by face-sharing in the hcp lattice (Fig. 6). The dhcp lattice consists of a mixture of two configurations, as seen in its sequence ABABACAC∙∙∙. The face-sharing configuration

substantially shortens the first-neighbor distance between the H atoms. For the coexisting state at 673 K and 5.06 GPa, the fcc lattice constant of _a_ = 3.6901(3) Å and the hcp lattice

constants of _a_ = 2.60047(10) Å and _c_ = 4.2280(4) Å were obtained (Table 1). These values provide first-neighbor distances of 2.609 Å and 2.114 Å for the fcc and hcp structures,

respectively. The latter distance of 2.114 Å is very close to the critical distance of 2.1 Å, below which dissolved H atoms in metals cannot approach each other owing to interatomic

repulsion forces37. Dissolved H atoms can preferentially occupy second-neighbor octahedral sites to avoid violating the 2.1-Å rule in the half-filled hcp lattice but not in the hcp

monohydride. The 2.1-Å rule is a possible factor in the stabilization of the hcp structure for _x_ < 0.6. The hcp and fcc solid solutions exhibited opposing variations in _x_ with the

temperature. The two-step variation of the fcc hydride was interpreted in terms of a miscibility gap; solid solutions with high and low H compositions can coexist below a critical _T–P_

point, at which the H solubility gap vanishes. For the fcc hydride, the miscibility gap was confirmed experimentally10,13 and the critical pressure was located at 4.0_–_4.5 GPa at a critical

composition of _x_ ≈ 0.413. The measured pressure range of 5.0_–_7.1 GPa was higher than the critical pressure; hence, the H composition of the fcc hydride increased along the

high-composition boundary of the miscibility gap with decreasing temperature below ~600 K, as shown in Fig. 5. For the hcp hydride, a miscibility gap has been theoretically predicted38, but

has not been experimentally confirmed. The observed monotonic decrease in the _x–T_ curve for the hcp hydride that is shown in Fig. 5 implies that the critical pressure was above ~6 GPa.

Consequently, the H composition decreased with decreasing temperature along the low-composition boundary of the miscibility gap. METHODS X-RAY DIFFRACTION The starting material was

reagent-grade pure iron flakes (purity: 99.9%) with a lateral particle size <100 μm and a thickness <20 μm. The flakes were mixed with BN powder (purity: 99% and grain size: >10 μm)

at a volume ratio of 2:3 and compacted into a disc that was 0.5 mm in diameter and 0.2 mm in height. The sample disc was loaded along with a compacted AlH3 disc, which served as an internal

H source, into a sleeve made of pyrolytic BN. This sleeve was placed into a NaCl capsule that was surrounded by a cylindrical graphite heater. The cell assembly was performed in the air.

High pressures and temperatures were generated using a cubic-type multi-anvil press. The internal H source decomposed into fluid H2 and Al metal upon heating above 800 K. The fluid H2

reacted with the Fe specimen to form FeH_x_ in the NaCl capsule. The temperature was monitored using Pt/Pt_–_13%Rh thermocouples with an uncertainty of less than 20 K. _In situ_ X-ray

diffraction measurements were conducted using synchrotron radiation at the BL14B1 beamline of SPring-8. Details of the high-pressure generation, the hydrogenation cell, and the _in situ_

synchrotron-radiation X-ray diffraction technique are described elsewhere26. NEUTRON DIFFRACTION The cell assembly for the high-pressure neutron diffraction measurements was essentially the

same as that used for X-ray diffraction. A compacted Fe disc, 3 mm in diameter and 2.5 mm in height, was prepared by pressing Fe flakes in a piston-cylinder-type mold. The Fe specimen was

placed at the center of a NaCl capsule (5.5 mm in diameter and 8 mm in height) and AlD3 (isotopic purity: 96 atom% D) pellets, which served as an internal D source, was placed above and

below the Fe specimen. The NaCl capsule was inserted into a cylindrical graphite heater and embedded in a pressure-transmitting medium made of MgO (17-mm edge cube). The cell assembly was

performed in the air. _In situ_ neutron diffraction measurements were conducted using the pulsed neutron source at the BL 11 (PLANET) beamline of J-PARC27. The collected diffraction profiles

were refined using Z-Rietveld software (version 0.9.42.2)29. In the refinement, H atoms that were included as an impurity at four atom% were assumed to randomly occupy the D atom sites. For

simplicity, the site occupancies of the H atoms and the H composition are notated as gD and x, respectively. The cell assembly and the high-pressure apparatus that was used for the neutron

diffraction experiments are described in detail elsewhere3. DATA AVAILABILITY All data supporting the findings of this study are available within the paper and Methods. The crystallographic

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hydrogen pressures: Thermodynamical calculations. _Acta Metall. Mater._ 40, 2327–2336 (1992). Article  CAS  Google Scholar  Download references ACKNOWLEDGEMENTS Neutron diffraction

experiments were performed under proposal no. 2014B0017 at J-PARC. Synchrotron X-ray diffraction experiments were performed under proposal numbers 2015A3602 and 2016B3651 at SPring-8. This

work was performed under the Inter-university Cooperative Research Program of the Institute for Materials Research, Tohoku University, proposal numbers 16K0075 and 17K0018, and supported

partially by the Grants-in-aid for Scientific Research, grant numbers 24241032, 25220911 and 18H05224 of Japan Society for the Promotion of Science. We thank T. Yagi and H. Kagi for their

polite discussions. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology, 1-1-1,

Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5148, Japan Akihiko Machida & Hiroyuki Saitoh * J-PARC Center, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki, 319-1195, Japan Takanori Hattori 

& Asami Sano-Furukawa * Neutron Science and Technology Center, Comprehensive Research Organization for Science and Society, Shirakata, Tokai, Naka, Ibaraki, 319-1106, Japan Ken-ichi

Funakoshi * Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan Toyoto Sato & Shin-ichi Orimo * WPI-Advanced Institute for Materials

Research (AIMR), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, 980-8577, Japan Shin-ichi Orimo * Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo,

113-0033, Japan Katsutoshi Aoki Authors * Akihiko Machida View author publications You can also search for this author inPubMed Google Scholar * Hiroyuki Saitoh View author publications You

can also search for this author inPubMed Google Scholar * Takanori Hattori View author publications You can also search for this author inPubMed Google Scholar * Asami Sano-Furukawa View

author publications You can also search for this author inPubMed Google Scholar * Ken-ichi Funakoshi View author publications You can also search for this author inPubMed Google Scholar *

Toyoto Sato View author publications You can also search for this author inPubMed Google Scholar * Shin-ichi Orimo View author publications You can also search for this author inPubMed 

Google Scholar * Katsutoshi Aoki View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS A.M., H.S., T.H., A.S.-F. and K.F. performed the

high-pressure neutron diffraction experiments. H.S. performed the high-pressure X-ray diffraction experiments. T.S. and S.O. prepared AlD3 and AlH3. A.M. and K.A. analyzed the neutron and

X-ray diffraction data. A.M. and K.A. wrote the manuscript. K.A. directed this study. CORRESPONDING AUTHORS Correspondence to Akihiko Machida or Katsutoshi Aoki. ETHICS DECLARATIONS

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THIS ARTICLE CITE THIS ARTICLE Machida, A., Saitoh, H., Hattori, T. _et al._ Hexagonal Close-packed Iron Hydride behind the Conventional Phase Diagram. _Sci Rep_ 9, 12290 (2019).

https://doi.org/10.1038/s41598-019-48817-7 Download citation * Received: 21 February 2019 * Accepted: 12 August 2019 * Published: 23 August 2019 * DOI:

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