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First-Principles Calculations on the Crystal/Electronic Structure and Phase Stability of H-Doped SrFeO 2

Yuji Kurauchi, Hideyuki Kamisaka, Tsukasa Katayama, Akira Chikamatsu, and Tetsuya Hasegawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12863 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

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The Journal of Physical Chemistry

First-Principles Calculations on the Crystal/Electronic Structure and Phase Stability of H-Doped SrFeO2

Yuji Kurauchi,† Hideyuki Kamisaka,*,† Tsukasa Katayama,‡ Akira Chikamatsu,† and Tetsuya Hasegawa†,§

Corresponding Author TEL/FAX: +81-3-5841-4354 E-mail: [email protected]

†

Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku,

Tokyo, 113-0033, Japan ‡

Institute for Innovative Research, School of Materials and Chemical Technology, Tokyo

Institute of Technology, Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan §

Kanagawa Academy of Science and Technology (KAST), Kawasaki, Kanagawa, 213-0012,

Japan

1 ACS Paragon Plus Environment


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Abstract It has been recently reported that H-doping of SrFeO2 induces an insulator-to-metal transition but generates only a small amount of carrier electrons. In order to investigate the crystal structure of H-doped SrFeO2 (SFOH) and the origin of its peculiar transport properties, we performed DFT-based first-principles calculations. Through structural sampling and total energy calculations, we showed that the doped hydrogen atoms exist in hydride form (H−). Incorporation of the hydride drastically changed the valence state and/or the d-band configuration of the Fe ions adjacent to the doped hydride, resulting in a metallic density of states in specific hydride configurations. Thermodynamic analysis revealed that formation of an insulating phase with O-site hydride was energetically preferable, but metallic phases with O-site hydride and/or interstitial hydride could also be present at reaction temperature, suggesting that SFOH is a mixture of an insulating matrix and metallic domains. This two-phase model accounted for the observed low carrier density as well as the metallic transport properties.


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Introduction The recent development of topochemical reactions using solid-state reductants, such as CaH2 and NaH, at modestly high temperatures of 200 < T < 600 °C has presented methods to introduce large amounts of oxygen vacancies and/or hydrogen into transition metal oxides without changing their cation frameworks.1 This reaction method has provided us with a new research field involving the modification of the physical properties of existing transition metal oxides. Particularly, the effect of hydrogenation has been discussed for a series of transition metal oxides with perovskite-type structures.2–10 In these compounds, the hydrogen atoms were characterized as hydrides at lattice oxygen sites (HO−) by X-ray/neutron Rietveld refinements and/or thermal gravimetric analysis. In the titanium oxides ATiO3−xHx (where A = Ca, Sr, Ba, or Eu), the HO− doping generates one free electron per H atom (O2− → HO− + e−) and causes insulator-to-metal transitions.2–5,11 The incorporation of hydride into O-sites can also induce antiferromagnetic (AFM) orderings of B-site cation spins at high Néel temperatures of TN > 300 K, as seen in SrCrO2H and Sr2CoO3H0.7.8–9 The AFM state of Sr2CoO3H0.7 is due to the strong σ-bonding of Co(eg)-H(1s)-Co(eg),9,12 whereas the stability of the AFM state in SrCrO2H is suggested to come from the original Cr(t2g)-O(2p) π-type



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interaction enhanced by the hydride substitution.13 Reactions with solid-state reductants can also modify the structures of transition metal oxides through the introduction of a large amount of oxygen vacancies.1 A typical example is the synthesis of SrFeO2 by reacting the precursor SrFeO3−δ with CaH2.14 SrFeO2 has a so-called infinite-layer structure (space group: P4/mmm) with corner-sharing FeO2 square nets separated by Sr ions (Fig. 1), and bulk SrFeO2 is reported to be an AFM insulator with a Néel temperature of 473 K.14 On the other hand, Katayama et al. performed a similar CaH2 treatment of SrFeO3−x thin films and found that the obtained film contains 0.24 hydrogen atoms per formula unit (4 × 1021 cm−3), possibly due to the more efficient diffusion of hydrogen.15 The SrFeOxH0.24 (SFOH) film exhibited metallic conduction, suggesting carrier doping by HO− or H+ inserted between FeO2 sheets. However, the microscopic location of the hydrogen atoms is unknown because neither Rietveld refinements nor thermal gravimetric analyses are applicable to thin films due to their small volume and the relatively large background signals from substrates. Furthermore, the carrier electron density per hydrogen is as low as ~0.1% (3.1 × 1018 cm−3),15 suggesting the presence of electron trapping states.


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Figure 1. The crystal structure of SrFeO2. Solid lines enclose a primitive cell.

In this study, we conducted first-principles density functional theory (DFT) calculations to investigate the crystal and electronic structures of SFOH, and analyzed its phase stability based on thermodynamic analysis. Our results indicated the incorporation of hydride anions, and led us to conclude that SFOH consists of an insulating matrix and metallic domains.

Method All our present calculations were performed for periodic model unit cells using a plane wave basis set as implemented in the VASP software package.16 For the DFT functional, the hybrid functional proposed by Heyd-Scuseria-Ernzerhof (HSE)17 was adopted with the range separation parameter µ = 0.2 Å−1 (HSE06). The effect of core electrons was incorporated using the projector augmented wave approach (PAW).18–19 The cut-off energy was 600 eV for



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valence electronic states, composed of 4s24p65s2 electrons in Sr, 3p63d64s2 in Fe, 2s22p4 of O, and 1s1 in H. The Brillouin zone integration was carried out according to the Monkhorst– Pack k-point scheme,20 with the density of the mesh point being ca. 5000 points Å−3. We assumed that the SFOH maintains the G-type q = (1/2, 1/2, 1/2) antiferromagnetic order,14 and thus used the spin-polarized method. Auxiliary utilities, Bader charge analysis,21 Phonopy,22 and VESTA23 were used for population analysis of the valence electrons, calculation of the phonons, and visualization of the crystal structures, respectively. The phonon calculation implemented in Phonopy is based on the finite displacement method in the harmonic approximation. We employed a displacement of 0.01 Å. It is worth discussing our choice of the HSE functional. Reliable prediction of electron trapping/conducting character requires a careful choice of DFT functional. Conventional semi-local functionals tend not to reproduce the electron trapping states well because of their self-interaction error (SIE). This error results in the spurious repulsion of an electron by itself, resulting in unphysical delocalization of the electrons. In the hybrid functional, the problem of SIE is remedied by inclusion of the exact exchange term, as in the case of the Hartree– Fock approximation. Our calculation with the HSE functional well reproduced the lattice constants of


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pristine SrFeO2,14 and the electronic structure reported in literature using the GGA+U method (Fig. S1).24–25 The HSE functional successfully described the divalent (d6) iron with an anomalous high-spin (S = 2) state having one minority-spin electron in the dz2 orbital. The order of d-bands was also consistent with that reported in the previous works using the GGA+U method. Determination of the optimal locations and chemical states of the hydrogen atoms was carried out for a 2 × 2 × 2 super cell of the primitive SrFeO2 unit. Two hydrogen atoms were added to the cell, giving a theoretical hydrogen concentration of 0.25 per formula unit (f.u.−1), which is close to the experimental value of 0.24 f.u.−1.15 We assumed that the hydrogen atoms are either positioned at interstitial sites (Hi) or substitute lattice oxygen atoms (HO). Three combinations of Hi and HO were considered for these two hydrogen atoms: two interstitial sites ([2Hi]), two oxygen sites ([2HO]), and their combination ([HiHO]). For each combination, relative spatial orientations of the two hydrogen atoms were varied. Ionic positions and lattice constants were optimized for all considered unit cells until the largest residual force on an atom and the pressure on the cell became < 0.01 eV Å−1 and < 0.1 GPa, respectively.

Results and Discussion



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Figure 2 shows the crystal structures with the two lowest total energy values for the three anion compositions [2Hi], [HiHO], and [2HO], where [X]1st and [X]2nd denote the structures with the lowest and the second lowest total energy, respectively. As seen from the figure, interstitial hydrogen atoms Hi prefer to be located at the apical sites that were originally occupied by oxygen in the initial perovskite structure rather than to make O–Hi hydroxide bonds. Except for the [HiHO] group, the hydrogen atoms are aligned in a one-dimensional manner with little distortion of the FeO2 square network in the stable structures.

Figure 2. The crystal structures with the lowest total energy values. Fe atoms are distinguished by whether they are adjacent to H (green) or not (orange). In the [HiHO] structures, Fe2 atoms with apical hydrogen are surrounded by broken circles.


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Table 1 summarizes characteristics of the structures shown in Fig. 2 and undoped SrFeO2 for comparison. The atomic charge states and the magnetic moment values of Fe and H were evaluated by Bader charge analysis21 and summation of spin polarization in PAW spheres, respectively. The Bader charge and magnetic moment of high-spin Fe2+ in undoped SrFeO2 were estimated to be +1.27 e and 3.62 µB, respectively. Compared with its nominal values, +2 e and 4 µB, the calculated values are generally underestimated due to spread of electronic population. The charge and magnetic moment of Fe in the H-doped systems depend on its position. The Fe atoms not adjacent to hydrogen (Fe1 in Fig. 2) have almost the same charge and magnetic moment values as those in undoped SrFeO2. Contrary, the values of Fe adjacent to hydrogen (Fe2 in Fig. 2) differ between [2Hi], [HiHO], and [2HO] structures, implying changes in valence states and d electron configurations caused by the neighboring hydrogen. The Bader charges of hydrogen are in the range of −0.49 e to −0.56 e for Hi atoms and −0.63 e to −0.66 e for HO atoms. Those in CaH2 and H2O as references were evaluated to be −0.72 e and +1.00 e, respectively, and thus we can conclude that the hydrogen in SrFeO2 exists as hydride (H−).



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Table 1. Average Bader charge and magnetic moment of Fe/H, and Fe–O/Fe–H bond length in the structures in Fig. 2 and undoped SrFeO2 cell Undoped Fe Fe–O [2Hi]1st Fe1 Fe2 Hi Fe1–O Fe2–O Fe2–Hi [HiHO]1st Fe1 Fe2 Hi HO Fe1–O Fe2–O Fe2–Hi Fe2–HO [2HO]1st Fe1 Fe2 HO Fe1–O Fe2–O Fe2–HO

Bader charge [e] +1.27

magnetic moment [µB]

bond length [Å]

cell

Bader charge [e]

magnetic moment [µB]

bond length [Å]

3.62 2.00

+1.21 +1.66 −0.56

+1.25 +1.21 −0.55 −0.63

+1.24 +0.67 −0.66

1.99 2.00 1.76

[2Hi]2nd Fe1 Fe2 Hi Fe1–O Fe2–O Fe2–Hi

2.00 2.00 1.74 2.06

[HiHO]2nd Fe1 Fe2 Hi HO Fe1–O Fe2–O Fe2–Hi Fe2–HO

2.00 1.93 2.00

[2HO]2nd Fe1 Fe2 HO Fe1–O Fe2–O Fe2–HO

3.61 3.94 0.00

3.62 3.55 0.00 0.00

3.62 3.10 0.00

+1.27 +1.47 −0.49

3.62 3.71 0.00 1.99 2.01 1.72

+1.22 +1.23 −0.53 −0.63

3.59 3.59 0.00 0.00 2.00 2.00 1.72 2.07

+1.20 +0.99 −0.64

3.63 3.40 0.00 2.00 1.96 2.02

The Fe–anion bond lengths in the ab-plane (Fe–O and Fe–HO) are around 2.0 Å, which is almost the same as that of undoped SrFeO2. On the other hand, the out-of-plane Fe–Hi bond lengths are significantly shorter than those in the ab-plane, 1.74–1.76 Å. As a result,


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H-doped SrFeO2 has anisotropic Fe–H bonds, which make an obvious difference in lattice vibrations. Figure 3 shows the calculated phonon density of states (DOS) for the [X]1st (X = 2Hi, HiHO, 2HO) structures. Because hydrogen has a much smaller mass than the other constituent atoms, the vibrational motion of hydrogen atoms has a high frequency above 22 THz, which is isolated from the other motions. In the calculated phonon DOSs for [2Hi] and [2HO], interstitial hydrogen (Hi) and O-site hydrogen (HO) exhibit significant differences. In particular, the frequency of stretching (along the Fe–H bond) modes of Hi is ca. 50 THz (Fig. 3a) and much higher than that of HO, which is ca. 30 THz (Fig. 3c). This difference in frequency can be explained in terms of the Fe–H bond distance: Hi–Fe has a shorter bond length of r(Fe–Hi) = 1.7 Å than Ho–Fe, r(Fe–HO) = 2.0 Å. The phonon DOS of [HiHO] shows peaks from both Hi and HO, whose frequencies are almost the same as those in [2Hi] and [2HO] (Fig. 3b). This suggests that the vibration of each hydrogen atom is almost independent.



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Figure 3. Phonon DOS for the [X]1st structures in Fig. 2. Peaks above 22 THz come from the vibration of hydrogen atoms. S and B stand for stretching and bending modes, respectively. Peaks corresponding to the [HiHO]1st structure are shown by broken lines.

The electron population analysis (Table 1) suggested changes in the valence state and d electron configurations in Fe adjacent to hydrogen (Fe2 in Fig. 2). The chemical formulae of the [2Hi] and [2HO] structures are SrFeO2H−0.25 and SrFeO1.75H−0.25, respectively. According to the limitation of valence neutrality, the iron in the [2Hi] and [2HO] cells should be oxidized and reduced, respectively, from the divalent Fe2+. We assessed the valence state of iron by calculating the partial density of states (PDOS) for Fe-3d electrons. Because the 3d bands are well localized, we can easily determine the electron occupations of the states and estimate the nominal charges of atoms more clearly than with Bader’s analysis. Figure 4 shows the PDOS for minority-spin dzx and dz2 components of Fe2 along with the PDOS for H-1s electrons. Because the d-electron configuration of high-spin Fe2+ (d6) ions in the pristine SrFeO2 is 12 ACS Paragon Plus Environment

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(xy)↑(yz, zx)↑↑(x2−y2)↑(z2)↑↓,18 the minority-spin dyz/dzx-band is empty and the dz2-band is occupied (Fig. 4a). All iron atoms not adjacent to a hydrogen (Fe1 in Fig. 2) are in a similar divalent state, which is consistent with the results of population analysis in Table 1. Conversely, the electronic state of iron adjacent to the hydrogen atoms (Fe2 in Fig. 2) in [2Hi] and [2HO] deviates from the divalent state. The dz2-band of Fe2 in the [2Hi] cells releases its electron and becomes partially filled (nominally Fe2.5+) in [2Hi]2nd, or even empty (Fe3+) in [2Hi]1st (Figs. 4b-c). In contrast, the dzx-band in the [2HO] cells accommodates an additional electron and the band becomes partially filled (Fe1.5+) in [2HO]2nd or totally filled (Fe+) in [2HO]1st (Figs. 4d-e). We note that the degeneracy of the dyz and dzx orbitals in undoped SrFeO2 is broken in Fe2 atoms in the [2HO] structures because of different bond lengths between Fe2–O and Fe2–HO (Table 1). Only the dzx orbital accommodates electrons whose lobe is directed along the Fe2–HO bond (Fig. 2) and thus receives relatively smaller Coulomb repulsion from O-2p electrons in the atom. The nominal magnetic moment on Fe2 for all respective cases can be estimated as 5 µB (Fe3+), 4.5 µB (Fe2.5+), 3.5 µB (Fe1.5+), and 3 µB (Fe+). Table 1 confirms that the decrease of Bader charges and magnetic moments of Fe2 in [2HO] and [2Hi] structures is in this order. In conclusion, oxidation or reduction of iron occurs only for the nearest neighbors to the hydrogen, and the charge state of iron depends on



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the configurations of the hydrogen atoms.

Figure 4. PDOS for minority-spin dzx and dz2 electrons of iron and 1s electrons of hydrogen atoms. (a) Undoped SrFeO2. (b, c) Fe2 and H in [2Hi]1st and [2Hi]2nd. (d, e) Fe2 and H in [2HO]1st and [2HO]2nd. Vertical broken lines denote the Fermi energy. The x, y, and z coordinates are the same as those in Fig. 2.

The PDOS of H-1s electrons spreads over the valence band and few states in the


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conduction band (Fig. 4), which supports our conclusion based on the Bader charge analysis (Table 1) that the chemical state of hydrogen is hydride. The distribution of H-1s states in the [2Hi] structures is similar to that of Fe-3dz2 (Figs. 4b-c), which indicates hybridization between them. This hybridization originates from orbital overlap in the Fe2–Hi bond along the z axis (Fig. 2). In contrast, PDOSs of Hi-1s and dzx in [2Hi] (Figs. 4b-c), and those of HO-1s and the d orbitals in [2HO] (Figs. 4d-e) are not correlated, reflecting their negligibly weak interactions. The electron occupation in the dzx and dz2 band governs the electronic conduction properties of [2Hi] and [2HO]. Figures 5a-b and 5e-f show the total DOS for the [2Hi]1st/2nd and [2HO]1st/2nd structures, respectively. Model cells with the partially filled d-band, [2Hi]2nd and [2HO]2nd, show “metallic” DOS with finite density of states at the Fermi energy (EF) (Figs. 5b, 5f). Meanwhile, other models with integer occupation numbers, [2Hi]1st and [2HO]1st, exhibit an insulating feature characterized by a gap at EF (Figs. 5a, 5e).



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Figure 5. Total DOS for the structures in Fig. 2. (a) [2Hi]1st. (b) [2Hi]2nd. (c) [HiHO]1st. (d) [HiHO]2nd. (e) [2HO]1st. (f) [2HO]2nd. [2Hi] and [2HO] have symmetric DOS with respect to spins, and thus only one spin direction is depicted. Vertical broken lines denote the Fermi energy.

All the iron ions in [HiHO] are divalent, as expected from the composition SrFeO1.875H−0.25. However, the d-electron configuration of Fe2 with apical hydride (surrounded by broken circles in Fig. 2) is different from that in pristine SrFeO2; the minority spin occupied not the dz2 orbital but the dzx orbital. Because the fully occupied 1s orbital of the apical hydride has a large overlap with the dz2 orbital immediately below, the occupation of dz2 should be suppressed by Coulomb repulsion. Figures 5c and 5d show the total DOSs for [HiHO]1st and [HiHO]2nd, respectively. The band of the destabilized dz2 orbital is located in the conduction band in the total DOS. In case of [HiHO]2nd, however, the bottom of the dz2 16 ACS Paragon Plus Environment

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band traverses the Fermi energy, resulting in the metallic DOS (Fig. 5d). Table 2 summarizes the nominal charge state of iron in each model, and indicates whether the compound is metallic or insulating. Because the incorporated hydrogen affects the charge state of adjacent iron atoms, the over-all electronic band structure is sensitive to the hydrogen configuration.

Table 2. The nominal charge states of iron and conductivities of the structures shown in Fig. 2. cell

charge states of Fe

conductivity

[2Hi]1st

Fe2+ 75% : Fe3+ 25%

insulator

[2Hi]2nd

Fe2+ 50% : Fe2.5+ 50%

metal

1st

2+

[HiHO]

Fe

[HiHO]2nd

Fe2+

[2HO]1st

Fe2+ 75% : Fe+ 25%

[2HO]

2nd

2+

insulator metal

1.5+

Fe 50 % : Fe

50 %

insulator metal

Next, we examined the phase stability of SFOH by comparing the change in the Gibbs free energy, ∆G = Gproduct–Greactant, associated with the reactions forming SFOH from SrFeO2.5 and CaH2, as illustrated in Fig. 6. The ∆G value for undoped SrFeO2 is also presented for comparison. Although the details of the H-doping process with CaH2 are still a matter of debate, the formation of CaO and H2 species as its byproduct has been suggested in several reports.26–27 We employed the following reaction formula:



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SrFeO2.5 + 0.75 CaH2 → SrFeO2−δH0.25 + (0.5+δ) CaO + (0.375+δ) H2 + (0.25−δ) CaH2. where the change of the Gibbs energy is denoted as ∆G[X] (X = 2Hi, 2HO, HiHO, or Undoped). The free energy of each species was estimated using the total energy obtained by DFT calculation. Lattice vibrations were not included in the present analysis, because their contributions to ∆G are anticipated to be negligibly small. The free energy of the produced H2 gas was evaluated by counting both its translational and rotational motions at T = 520 K. The partial pressure of the emitted H2 gas (PH2) was assumed to be from 1 Pa to 104 Pa. The hydrogenation reaction was performed using a sealed grass tube containing large excess CaH2. Under this circumstance, oxygen molecules immediately react with CaH2 and will be eliminated from the system. Therefore, we can regard that the partial pressure of O2 gas is zero.

Figure 6. A schematic energy diagram of the hydrogenation reaction for H-doped SrFeO2. The chemical formulae of reactants and products is shown for three types of H-doped SrFeO2 and undoped SrFeO2. The change of Gibbs energy though the reaction (∆G[X]) is defined as the energy differences between reactants and products.


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Figure 7 shows the calculated ∆G[X] as a function of PH2, where the solid and dashed lines in the figure represent ∆G[X]1st and ∆G[X]2nd, respectively. Notably, ∆G[2HO]1st (blue solid line) is the lowest under H2 pressures up to 104 Pa, followed by the second group, ∆G[2HO]2nd, ∆G[HiHO]1st, and ∆G[HiHO]2nd, which have higher energies by 8–17 kJ mol−1, corresponding 1000–2000 K. The ∆G[2Hi] values are much higher than ∆G[2HO]1st. These indicate that the [2HO] and [HiHO] structures are dominant in SFOH. This preference can be rationalized in terms of the formation of CaO and H2. The exothermic conversion of CaH2 to CaO is essential to the negative ∆G values. According to Fig. 6, the formation of [2HO] and [HiHO] structures produces larger amounts of CaO and H2, and thus releases more heat.

Figure 7. Calculated ∆G[X] values as a function of the partial pressure of H2. For X = 2Hi, 2HO, and HiHO, solid (dashed) lines correspond to the formation of the [X]1st ([X]2nd) structures in Fig. 2.

From Table 2 and Figure 7, it is expected that the SFOH films contain large amounts of Fe+ and Fe1.5+. Such anomalously reduced states of transition metals have been found in transition metal oxides prepared by topochemical methods, including Co+ and Co2+ in 19 ACS Paragon Plus Environment


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LaSrCoO3.5−x and Ni+ in LaNiO2.29–30 Mössbauer spectroscopy of F-doped SrFeO2 (Sr1−δFeO2−xFx) revealed the presence of monovalent Fe.31 However, the Fe-2p XPS spectrum of the SFOH film resembles that of Fe2+ with a similar binding energy,15 although the XPS spectrum of the Fe(I) compound is not well established. Thus, we do not exclude the possibility that the actual amount of Fe+ is less than that evaluated in Table 2 due to kinetic effects. For example, if the energy barrier for a hydrogen atom to substitute an O-site oxygen is much higher than that of the insertion process, the formation of [2HO] structures is suppressed. In this case, the secondary stable [HiHO] structures with Fe2+ would be the primary component in the SFOH, which is consistent with the XPS results. As seen from Fig. 7, the energy differences between [2HO]1st, [2HO]2nd, [HiHO]1st, and [HiHO]2nd are less than 20 kJ mol−1, which is comparable with the thermal energy around the reaction temperature (240–280 °C),15 suggesting that these four phases coexist in the SFOH films obtained by the hydridation of SrFeO3−δ with CaH2. Here we propose a two-phase model, in which SFOH is composed of an insulating primary phase, [2HO]1st/[HiHO]1st, and a metallic secondary phase, [2HO]2nd/[HiHO]2nd. The ratio of these two phases can be estimated from the difference in ∆G and the reaction temperature of T = 520 K as [2HO]1st/[HiHO]1st (insulating) : [2HO]2nd/[HiHO]2nd (metallic) ~ 0.85 : 0.15. Formation of a connected network


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of metallic clusters in the insulating matrix could account not only for the high electric conduction but also for the small amount of carrier electrons observed in the experiment.15

Conclusion We proposed the crystal and electronic structures of H-doped SrFeO2 considering the stability of various phases with different hydrogen configurations, including interstitial hydrogen filling the missing apical oxygen site, and hydrogen substituting the lattice oxygen site. The doped hydrogen was found to exist as hydride regardless of its spatial location in the crystal. The doped hydrogen drastically changed the energy position and occupation state of the minor-spin dzx and dz2 bands of the adjacent iron atoms. As a result, either an insulating electronic band structure with a finite bandgap or a metallic band structure with fine density of state at the Fermi level was presented, depending on the arrangement of the incorporated hydride ions. Thermodynamic analysis revealed that an insulating SFOH phase with O-site hydride was preferably formed, but less stable metallic phases could also coexist. Percolation of the metallic domains explains the metallic transport and low carrier density experimentally observed.



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Acknowledgement The computations were partially performed using Research Center for Computational Science, Okazaki, Japan. This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency and a Grant-in-Aid for Scientific Research (No. 16H06441) from the Japan Society for the Promotion of Science (JSPS).

References (1) Yamamoto, T.; Kageyama, H. Hydride Reductions of Transition Metal Oxides. Chem. Lett. 2013, 42, 946–953. (2) Kobayashi, Y.; Hernandez, O. J.; Sakaguchi, T.; Yajima, T.; Roisnel, T.; Tsujimoto, Y.; Morita, M.; Noda, Y.; Mogami, Y.; Kitada, A.; et al. An Oxyhydride of BaTiO3 Exhibiting Hydride Exchange and Electronic Conductivity. Nat. Mater. 2012, 11, 507–511. (3) Yajima, T.; Kitada, A.; Kobayashi, Y.; Sakaguchi, T.; Bouilly, G.; Kasahara, S.; Terashima, T.; Takano, M.; Kageyama, H. Epitaxial Thin Films of ATiO3−xHx (A = Ba, Sr, Ca) with Metallic Conductivity. J. Am. Chem. Soc. 2012, 134, 8782–8785.


Page 22 of 37

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4) Sakaguchi, T.; Kobayashi, Y.; Yajima, T.; Ohkura, M.; Tassel, C.; Takeiri, F.; Mitsuoka, S.; Ohkubo, H.; Yamamoto, T.; Kim, J. E.; et al. Oxyhydrides of (Ca,Sr,Ba)TiO3 Perovskite Solid Solutions. Inorg. Chem. 2012, 51, 11371–11376. (5) Yamamoto, T.; Yoshii, R.; Bouilly, G.; Kobayashi, Y.; Fujita, K.; Kususe, Y.; Matsushita, Y.; Tanaka, K.; Kageyama, H. An Antiferro-to-Ferromagnetic Transition in EuTiO3−xHx Induced by Hydride Substitution. Inorg. Chem. 2015, 54, 1501–1507. (6) Romero, F. D.; Leach, A.; Möller, J. S.; Foronda, F.; Blundell, S. J.; Hayward, M. A. Strontium Vanadium Oxide–Hydrides: “Square-Planar” Two-Electron Phases. Angew. Chem. Int. Ed. 2014, 53, 7556–7559. (7) Bang, J.; Matsuishi, S.; Hiraka, H.; Fujisaki, F.; Otomo, T.; Maki, S.; Yamaura, J.; Kumai, R.; Murakami, Y.; Hosono, H. Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4−xHx. J. Am. Chem. Soc. 2014, 136, 7221–7224. (8) Tassel, C.; Goto, Y.; Kuno, Y.; Hester, J.; Green, M.; Kobayashi, Y.; Kageyama, H. Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetic-Transition Temperature. Angew. Chem. 2014, 126, 1–5. (9) Hayward, M. A.; Cussen, E. J.; Claridge, J. B.; Bieringer, M.; Rosseinsky, M. J.; Kiely, C. J.; Blundell, S. J.; Marshall, I. M.; Pratt, F. L. The Hydride Anion in an Extended Transition



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Metal Oxide Array: LaSrCoO3H0.7. Science 2002, 295, 1882–1884. (10) Helps, R. M.; Rees, N. H.; Hayward, M. A. Sr3Co2O4.33H0.84: An Extended Transition Metal Oxide-Hydride. Inorg. Chem. 2010, 49, 11062–11068. (11) Zhang, J.; Gou, G.; Pan, B.; Study of Phase Stability and Hydride Diffusion Mechanism of BaTiO3 Oxyhydride from First-Principles. J. Phys. Chem. C 2014, 118, 17254–17259. (12) Bridges, C. A.; Darling, G. R.; Hayward, M. A.; Rosseinsky, M. J. Electronic Structure, Magnetic Ordering, and Formation Pathway of the Transition Metal Oxide Hydride LaSrCoO3H0.7. J. Am. Chem. Soc. 2005, 127, 5996–6011. (13) Liu, K.; Hou, Y.; Gong, X.; Xiang, H.; Orbital Delocalization and Enhancement of Magnetic Interactions in Perovskite Oxyhydrides. Sci. Rep. 2016, 6, 19653. (14) Tsujimoto, Y.; Tassel, C.; Hayashi, N.; Watanabe, T.; Kageyama, H.; Yoshimura, K.; Takano, M.; Ceretti, M.; Ritter, C.; Paulus, W. Infinite-layer Iron Oxide with a Square-planar Coordination. Nature 2007, 450, 1062–1065. (15) Katayama, T.; Chikamatsu, A.; Hirose, Y.; Kumigashira, H.; Fukumura, T.; Hasegawa, T. Metallic Conductivity in Infinite-layer Strontium Iron Oxide Thin Films Reduced by Calcium Hydride. J. Phys. D: Appl. Phys. 2014, 47, 135304. (16) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-energy


Page 24 of 37

Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Calculations using A Plane-wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (17) Heyd, J.; Scuseria, G. E.; Ernzerhof, M.; Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207−8215. (18) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. Phys. Rev. B 1999, 59, 1758−1775. (19) Blöchl, P. E. Projector Augmented-wave Method. Phys. Rev. 1994, 50, 17953−17979. (20) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin- zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (21) Henkelman, G.; Arnaldsson, A.; Jonsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (22) Togo, A.; Tanaka, I. First Principles Phonon Calculations in Materials Science. Scr. Mater. 2015, 108, 1−5. (23) Momma, K.; Izumi, F. VESTA 3 for Three-dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (24) Xiang, H. J.; Wei, S.-H.; Whangbo, M.-H. Origin of the Structural and Magnetic Anomalies of the Layered Compound SrFeO2: A Density Functional Investigation. Phys. Rev. Lett. 2008, 100, 167207.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

(25) Pruneda, J. M.; Íñiguez, J.; Canadell, E.; Kageyama, H.; Takano, M. Structural and Electronic Properties of SrFeO2 from First Principles. Phys. Rev. B 2008, 78, 115101. (26) Franklin, A. D.; Campbell, R. B. Low Temperature Reduction of Iron Oxides. J. Phys. Chem. 1955, 59, 65−67. (27) Kobayashi, Y.; Li, Z.; Hirai, K.; Tassel, C.; Loyer, F.; Ichikawa, N.; Abe, N.; Yamamoto, T.; Shimakawa, Y.; Yoshimura, K.; et al. Gas Phase Contributions to Topochemical Hydride Reduction Reactions. J. Solid State Chem. 2013, 207, 190−193. (28) Chikamatsu, A.; Matsuyama, T.; Hirose, Y.; Kumigashira, H.; Oshima, M.; Hasegawa, T. Investigation of Electronic States of Infinite-layer SrFeO2 Epitaxial Thin Films by X-ray Photoemission and Absorption Spectroscopies. J. Electron Spectroscopy and Related Phenom. 2012, 184, 547−550. (29) Hayward, M. A.; Green, M. A.; Rosseinsky, M. J.; Sloan, J.; Sodium Hydride as a Powerful

Reducing

Agent

for

Topotactic

Oxide

Deintercalation:

Synthesis

and

Characterization of the Nickel(I) Oxide LaNiO2. J. Am. Chem. Soc. 1999, 121, 8843−8854. (30) Hayward, M. A.; Rosseinsky, M. J. Anion Vacancy Distribution and Magnetism in the New Reduced Layered Co(II)/Co(I) Phase LaSrCoO3.5−x. Chem. Mater. 2000, 12, 2182−2195.


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(31) Li, B.; Woods, J.; Siewenie, J.; Hah, H.-Y.; Johnson, J. A.; Johnson, C. E.; Louca, D. The Magnetic and Crystal Structures of Sr1–δFeO2–xFx, a new oxyfluoride. Chem. Commun.

2016, 52, 2386−2389.



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