The negative Knight shift in Ba-Ti oxyhydride: An indication of the

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The negative Knight shift in Ba-Ti oxyhydride: An indication of the multiple hydrogen occupation Tai Misaki, Itaru Oikawa, and Hitoshi Takamura Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01434 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Chemistry of Materials

The negative Knight shift in Ba-Ti oxyhydride: An indication of the multiple hydrogen occupation

Tai Misaki, Itaru Oikawa, Hitoshi Takamura* Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan

1 ACS Paragon Plus Environment

Chemistry of Materials 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

Abstract

Multiple hydrogen occupation in Ba-Ti oxyhydride is demonstrated by a negative Knight shift in the 1H

magic-angle spinning (MAS) NMR spectra combined with hydrogen concentration and X-ray

photoelectron spectroscopic analysis. A negative Knight shift, an indicator of the interaction between

conduction band electrons and a probe nucleus, is observed in the 1H MAS NMR signal of Ba-Ti

oxyhydrides at room temperature for the first time. This Knight shift indicates multiple hydrogen

occupation. This was confirmed by the X-ray photoelectron spectra of Ba-Ti oxyhydrides and the hydrogen

concentrations. The presence of hydrogen species with different bonding characteristics indicates a

flexible hydrogen configuration in Ba-Ti oxyhydrides, from an ionic state hydride-ion with single

occupation to partially anionic state hydrogen atoms with multiple occupation. It also indicates the

potential to modify the electronic structure and physical properties of Ba-Ti oxyhydrides by manipulating

their hydrogen configuration.


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Introduction

Mixed anion compounds have been investigated in a wide variety of fields because of their

photocatalytic activity, ferroelectricity, colossal magnetoresistance and oxygen reduction reaction

activity.1-5 These properties arise from differences in the electronic structure, electronegativity and the

ionic radius of different anions in the sublattice. Among mixed anion compounds, oxyhydrides with

hydride ions (H-) and oxide ions in the anion sub-lattice have been widely synthesized and investigated.

Unique properties not found in other mixed anion compounds are expected because of the lower charge

density and the mass of the hydride ions, and also the differences in the symmetry of the electron orbital to

those of the oxide ions.6 Owing to the low charge density, they are expected to be more mobile and have a

lower activation energy. Unlike the O 2p orbital, the spherical symmetry of H 1s orbital influences the

hybridization of orbitals between the hydride ion and cations, as suggested in the corner-shared VO2 sheets

in SrVO2H.7

Since the introduction of the hydride ion into oxide ion sites requires a highly reducing

condition, only a few oxyhydrides containing compositional amounts of hydride ions have been reported to

date.8-11 Oxyhydrides containing transition metals such as LaSrCoO3H0.7, ATiO3-xHy (A = Ba, Ca, Sr) and

Srn+1VnO2n+1Hn (n = 1, 2) have been reported to be successfully synthesized by topochemical reaction

methods.7,12,13 In addition, high pressure synthesis is known to be an alternate route to synthesize transition

metal oxyhydrides as demonstrated in SrCrO2H, LaSrMnO3.3H0.7 and LaSr3NiRuO4H4.14-16



Among transition metal oxyhydrides, Ba-Ti oxyhydride, BaTiO3-xHy, is first synthesized by

Kobayashi et al. via topochemical reaction of BaTiO3 and CaH2.17 This material shows high hydride ion

solubility up to x = 0.6, high electronic conductivity and hydride ion diffusivity. The reduction of BaTiO3

accompanied by the incorporation of hydride ions induces electronic conductivity and a change in color

from white to blue due to the decrease in the oxidation state of Ti. DFT calculations show the electronic

conductivity of this material is derived from the partial occupation of electrons at Ti 3d orbital in the

conduction band, which indicates metallicity in the material.18 In addition, this oxyhydride exhibits

diffusivity and the surface exchange of the hydride ion with deuterium gas at 400°C.17 While this exchange

behavior indicates the fast diffusion of hydride ion, its mechanism is still under discussion.19

Since the incorporation of hydrogen has an immense influence on the electronic structure of

Ba-Ti oxyhydrides, the hydrogen configuration is important with respect to electrical conductivity,

diffusion properties and surface kinetics. A single state of the hydride ion in the anion site (H-O) is

clarified by neutron diffraction and 1H magic-angle spinning (MAS) nuclear magnetic resonance (NMR)

spectroscopy;17 however, other hydrogen configurations, i.e. interstitial proton, and the multiple hydrogen

occupation have been suggested.20,21 In BaTiO3, protons in the interstitial-site bonded with the oxide-ion

have been reported to be introduced by applying high H2 pressure up to 32.4 MPa at 100°C.20 Density

functional theory (DFT) calculations indicate that hydrogen diffusion occurs in the form of an interstitial

proton replacing the original hydride ions in the anion site, or oxygen-vacancy mediated mechanism.18,19,22


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Other DFT calculation carried out by Iwazaki et al. reported that two hydride ions occupy one anion site to

form a multiple-hydrogen occupation ((2H-)O) site in SrTiO3.21 The (2H-)O site is expected to be formed by

trapping two hydride ions at the oxygen vacancy when Fermi level is close to the conduction-band

minimum.. This was supported by the positive muon spin relaxation (G+SR) study reported by Ito et al.

which revealed that the (2H-)O site can be formed as a excited configuration by this trapping mechanism.23

However, existence of the multiple occupation site as a stable site and their hydrogen state has yet to be

clarified experimentally.

Despite the huge influence on the electronic structure and physical properties of Ba-Ti

oxyhydrides, their hydrogen configurations, including their occupation site, hydrogen state and bonding

character remain poorly understood. To elucidate the hydrogen configurations and their role in the lattice,

perovskite-type Ba-Ti oxyhydrides with different hydrogen concentrations are synthesized and

investigated by means of crystal structure, defect concentration, hydrogen gas evolution behavior,

electronic state and the chemical environment of hydrogen nucleus. In this study, 1H and 2H NMR

spectroscopy are introduced to analyze the hydrogen configurations in the material owing to sensitive

characteristics in the chemical state and the electric structure in the vicinity of the nuclei.

Experimental

Ba-Ti oxyhydrides were prepared by the topochemical reaction method from BaTiO3 (99%,

Sigma-Aldrich) and CaH2 (99.9%, Sigma-Aldrich) according to a method outlined in a previous report.17



BaTiO3 and CaH2 were mixed in the molar ratio of 1:3 and pelletized in an Ar-filled grove box. The

pelletized mixture was sealed in a quartz tube under vacuum and annealed at 420°C for 12, 24, and 48 h.

After annealing, the sample was crushed in air, and then washed with an NH4Cl/methanol solution (0.1 M)

and distilled water to remove residual Ca compounds (CaH2 and CaO). The solution was removed by

centrifuging. After removing the solution, the sample was dried at 150°C in a vacuum and Ba-Ti

oxyhydride in powder form was prepared. In addition to the oxyhydrides, Ba-Ti oxydeuterides were

synthesized from BaTiO3 and CaD2 by the same procedure. CaD2 was synthesized by annealing Ca metal

(99%, Sigma-Aldrich) under D2 pressure of 3 MPa at 500°C for 5 h. To investigate the dynamics of

hydrogen species in the oxyhydride, deuterium exchange was carried out under a D2 pressure of 0.7 MPa at

326°C for 12 h.

The crystalline phase and lattice parameters were determined by X-ray diffraction (XRD)

measurements using D8 ADVANCE (Bruker). Lattice constants were determined using TOPAS4 software.

The microstructure of the Ba-Ti oxyhydride powder was investigated by scanning electron microscopy

(SEM) using JSM-6360LA (JEOL). The amount of oxygen defects was evaluated by a thermogravimetric

analysis (TGA) using Pyris 1 TGA (PerkinElmer). The hydrogen content was evaluated by the inert gas

fusion technique using ONH836 (LECO). This technique combines the fusion of the sample and the

detection of the exhaust gases to evaluate ppm levels of hydrogen in the sample.24,25 Hydrogen gas

evolution behavior was investigated using a quadrupole mass spectrometer (QMG422, Pfeiffer Vaccume).


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The sample was heated from room temperature to 800°C at a heating rate of 10°C/min. in an Ar flow of

100 ml/min. X-ray photoelectron spectroscopy (XPS) analyses were carried out using a VG Theta Probe

(Thermo Fisher Scientific Inc.). The X-ray source was monochromated Al-K radiation (1486.68 eV). The

photoelectron detection angle was between 11.875° and 68.125° normal to the sample surface. The binding

energy of the spectra was calibrated using the C 1s line from the adventitious carbon at 285.00 eV.

The local structure of the hydrogen nuclei was investigated using 1H and 2H MAS NMR

spectroscopy at room temperature. 1H MAS NMR measurements were carried out using JNM-ECA300

(JEOL) at a magnetic field strength of 7.05 T with a Larmor frequency of 300.17 MHz. The sample was

packed into a 4 mm zirconia rotor in an Ar-filled grove box. The spinning speed was 15 kHz for all

spectra. The spectra were recorded using a DEPTH pulse sequence in order to suppress the probe

background.26 The spin-lattice relaxation time (T1) was obtained by a saturation recovery method.

Tetramethylsilane was used as a chemical shift reference set at 0 ppm for all 1H NMR spectra. 2H MAS

NMR spectroscopy was also carried out using JNM-ECA300 with a Larmor frequency of 46.08 MHz. The

sample was packed into 4 mm zirconia rotor in an Ar-filled glovebox and span at 15 kHz. The spectra were

recorded using a single pulse sequence, and a 2H signal from heavy water was set as a chemical shift

reference at 0 ppm.

Results and Discussion

The influence of the reducing treatment time on the phases present in Ba-Ti oxyhydrides was



evaluated. After reduction by CaH2, the color of the sample changed from white to light or deep blue. Fig.1

(a) shows the XRD patterns of Ba-Ti oxyhydrides reduced at 420°C for 12, 24 and 48 h. All samples

exhibited a single phase perovskite-type structure. The cell volume contracted more than BaTiO3 in the

sample reduced for 12 h, and volume expansion occurred as the reducing time was increased to 24 and 48

h (Supplementary Fig. S1). While contraction is the result of the structural transformation into a higher

symmetry phase, expansion can be attributed to the reduction expansion due to the increase in the

hydrogen concentration over the reduction time period. Single phase Ba-Ti oxyhydride was confirmed by

SEM images: the microstructure is homogeneous and there is no distinct difference in the particle size of

the three samples (Supplementary Fig. S2).

The amount of oxygen defects and hydrogen concentration were measured by TGA and inert gas

fusion techniques, respectively. The quantity of oxygen defects, which is the total number of missing oxide

ions in the anion sites (x+y in the formula BaTiO3-xHy), was estimated from the weight gain due to

oxidation of the sample (Fig. 1 (b)). The extent of hydrogen concentration was determined by detecting the

quantity of hydrogen gas exhausted from the fusion of the sample. The evaluated amounts of hydrogen by

the inert gas fusion technique is provided in the Supplementary Fig. S3. Table.1 summarizes the amount of

oxygen defects and hydrogen concentration in the samples reduced at 420°C for 12, 24 and 48 h. Both

values increased with increased reducing time, and the hydrogen concentration was twice that of the

amount of oxygen defect. This result differs from the reported trends of oxygen defects and hydrogen


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concentration in Ba-Ti oxyhydride where the hydrogen concentration estimated from gas analysis was

equal to the amount of oxygen defects.17 It should be acknowledged that the presence of indistinguishable

adsorbed water or other hydrogen species can result in overestimations of the hydrogen concentration

using the inert gas fusion technique. In this study, however, all samples were dried and treated in an

Ar-filled grove box before the measurement, and even in the sample with small hydrogen concentration

where the influence of adsorbed water is expected to be relatively large, the ratio of hydrogen

concentration to oxygen defect sites is approximately 2:1. This implies that excess hydrogen expected

from the compensation of oxygen defects was introduced into the Ba-Ti oxyhydrides. The influence of

adsorbed water and impurities to the amounts of hydrogen and oxygen defect are discussed later with the

results of 1H MAS NMR which indicates the presence of impurities.

Different hydrogen behavior was also indicated in the hydrogen gas evolution analysis.

The

hydrogen gas evolution behavior of Ba-Ti oxyhydride shown in Fig. 2 indicates that gas evolution occurs

in several steps: evolution began at 450°C and peaked at 500°C and 600°C for all samples. The increase in

gas evolution with increasing hydrogen concentration is consistent with the results of the inert gas fusion

technique. In the case of Ba-Ti oxyhydride with the H-O in the anion site, only a single peak was observed

at around 450°C.17 The multi-step gas evolution behavior observed in this study with peaks at higher

temperatures indicates that the hydrogen configuration of Ba-Ti oxyhydride is different from the H-O sites.

It also indicates several other hydrogen configurations with more thermodynamically stable hydrogen



configurations.

The influence of hydrogen incorporation to the electronic state of Ti was clarified by XPS. Fig.

3 shows the Ti 2p X-ray photoelectron spectra of BaTiO3 and Ba-Ti oxyhydrides with different reducing

times. The Ti 2p spectrum of BaTiO3 shows typical peaks of Ti 2p1/2 at 465.5 eV and Ti 2p3/2 at around

459.7 eV. A slight shift in Ti 2p3/2 to higher binding energy is observed in the sample reduced for 12 h.

This shift is presumably correlated to the contraction of the lattice. It has been reported that the shift to

higher binding energy is related to the decrease in the interatomic distance of M-Ti (M = Ti, Ca, Sr, Ba) in

TiO2 and MTiO3.27 The XRD result of the 12 h sample indicates a decrease in the lattice constant. This

contraction causes a decrease in the atomic distance of Ba-Ti, which may result in a change in the bonding

character of Ti ion. In addition to the peaks observed in BaTiO3, a shoulder at the lower binding energy

side of the Ti 2p3/2 peak is observed in the spectra. This shoulder can be attributed to the reduction of the

oxidation state of Ti accompanied by the incorporation of hydrogen. The increase in the component of

reduced Ti ions as the hydrogen concentration increases implies that hydrogen incorporation is

compensated by the reduction of Ti4+.

In order to clarify the hydrogen configurations, 1H MAS NMR measurements were carried out.

Fig. 4 (a) shows the

1H

MAS NMR spectra of the Ba-Ti oxyhydrides with different hydrogen

concentrations. Three peaks were observed for all samples, with Peaks 1 and 2 located at 4.7 and 1.1 ppm,

respectively. The range for Peak 3, from -3.3 to -18.5 ppm, indicates a negative shift as the hydrogen


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concentration increases. The increasing intensity of peaks 1 and 3 with increasing hydrogen concentration

suggests these peaks represent 1H signals from the sample, while peak 2 is assigned to the signal from

residual Ca(OH)2, which is reported to appear at 1.0 ppm.28 The peak area, chemical shift and full width at

half maxima (FWHM) of peak 1 and peak 2 are shown in the Supplementary Fig. S4. The assignment of

peak 1 is discussed later in this paper with the 1H and 2H NMR results of Ba-Ti oxydeuteride. The 1H MAS

NMR spectra reveal the presence of Ca(OH)2 as an impurity. Therefore, it should be noted that the

influence of Ca(OH)2 to the amounts of hydrogen and oxygen defects needs to be taken into account. A

rough estimate of the amount of Ca(OH)2 from the peak area of 1H MAS NMR spectrum is that it is below

11% for all the samples (11% for the 12 h sample, 7% for the 24 h sample and 4% for the 48 h sample). In

accordance with this estimation, the extent of overestimation of the hydrogen contents and underestimation

of the oxygen defect concentration is likely less than 11% of the hydride ions in the anion sites. The

minor contribution of the Ca(OH)2 impurity is also confirmed by the high temperature XRD of the 48h sample. The absence of the phases related to CaO and Ca(OH)2 when the temperature is increased to 800°C (Supplementary Fig. S5) indicates that the amount of Ca(OH)2 is below the detection limit of the XRD measurement. Therefore, while the quantities of hydrogen and oxygen defects evaluated from the inert gas fusion technique and TGA are acknowledged to be insufficient to

completely eliminate the influence of the impurity, the estimated amounts of Ca(OH)2 indicate that both

quantities are dominated by the hydride ions in the sample.



The peak area, chemical shift and FWHM of peak 3 is shown in Fig.4 (b) as a function of

hydrogen concentration. The peak area increased with increasing hydrogen concentration. This trend is in

good agreement with the hydrogen concentration estimated from the inert gas fusion technique. The

full-width at half maximum also increases with increasing hydrogen concentration. This can be explained

by the stronger dipolar-dipolar interaction as the hydrogen concentration increases.

The chemical shift shows a negative shift with increasing hydrogen concentration. Hayashi et al.

reported that the chemical shift of typical 1H NMR spectra is located in the range of 20 to -5 ppm.29

However, the chemical shift of peak 3, at -18.5 ppm, fell outside this range in the sample with hydrogen

concentration of 0.38 mol H/ f.u. (formula unit).

This negative shift can be attributed to the Knight shift

due to the interaction between nuclear magnetic moment and conduction electrons. Both the change in the

sample color from white to blue and the XPS spectra indicate the presence of conduction electrons from

the introduction of hydrogen. These electrons can interact with the magnetic moment of the hydrogen

nucleus. A negative Knight shift is observed in the case of diamagnetism of the conduction electrons or a

core polarization interaction. Since contribution from the diamagnetism of conduction electrons to the

Knight shift is generally negligible, the negative shift can be attributed to the core polarization interaction.

This shift has been observed for transition metal hydrides, i.e. TiH2 and ZrH2.30,31 While it is possible

that TiH2- is an origin of the negative Knight shift, this was ruled out by comparing the peak position. The peak position of TiH2- ( = 0 – 1.0) has been reported to be in the range of -60 to -150 ppm which is more


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negative than that of peak 3.30

The negative Knight shift of the peak 3 can be explained by the transferred core polarization

interaction, which appeared between proton and conduction band electrons of d orbital. In the case of

Ba-Ti oxyhydride, conduction bands consist of 3d orbitals of the Ti ion. The Knight shift caused by the

core polarization interaction can be defined by the following equation;

K CP

where

B

2

B

H hfd N e

(1)

is the bohr magneton, Hdhf is the hyperfine field, and Ne is the density of the conduction electron

at the nuclear site. From this equation, the negative shift can be observed when the density of the

conduction electron increases with increasing hydrogen concentration. Because this shift has not been

reported in any previous studies on Ba-Ti oxyhydrides,17 it can be concluded that the hydrogen

configurations in the Ba-Ti oxyhydride of the present study is different from the H-O site in the previous

study.

The increase in the conduction electrons with increasing hydrogen concentration also affects the

1H

spin-lattice relaxation time T1. The square root of spin-lattice relaxation rate T1-1/2 increases with

increasing hydrogen concentration (Supplementary Fig. S5). When the spin-lattice relaxation is determined

by the core-polarization interaction between conduction electrons and proton, a square root of the

spin-lattice relaxation rate is linearly increase with the increasing density of the d-electron state at the

Fermi level. While it is difficult to clearly confirm this dependence only from three different hydrogen



concentrations, the increase trend of the square root spin-lattice relaxation rate indicates an increase in

conduction electrons with increasing hydrogen concentration and the interaction between the hydrogen

nuclei and the conduction electrons.

By considering the negative Knight shift and 1H spin-lattice relaxation rate of peak 3, this peak

can be assigned to the different hydrogen environment from the reported hydride ion singly occupying the

anion site, and to explain the present results, this hydrogen environment can be 1H in the multiple

hydrogen occupation site. As mentioned above, the negative Knight shift of the peak is due to the core

polarization derived from spin polarization of 3d electrons in the conduction band. However, no previous

studies have discussed a negative Knight shift in the peak from hydride ions in the H-O sites (Fig. 5 (a)).

This can be explained by the two hydrogen occupation in the anion site. This occupation model is

predicted in SrTiO3 by DFT calculations and indicated by the positive muon relaxation spectroscopy of

Ba-Ti oxyhydride at low temperatures.21,23 The results of oxygen defects and hydrogen concentration in

this study are consistent with this occupation model. In addition to these observations, a negative Knight

shift is an indicator of the two hydrogen occupation sites. When two hydrogen atoms occupy an anion site,

the hybridization of the Ti 3d orbital and H 1s orbital takes place due to the symmetry of the orbitals (Fig.

5 (b)). The conduction band minimum in perovskite-type Ba-Ti oxyhydrides consist of a t2g orbital derived

from the ligand field splitting of Ti 3d electrons. The t2g orbital of Ti spreads toward the space between

two adjacent anion sites with no electron density in the direction of the anion site. The incorporation of


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hydrogen into the lattice results in a rise in the Fermi level and the partial occupation of the conduction

band with electrons from hydrogen. Consequently, the hybridization of the Ti t2g orbital and H 1s orbital

occurs when two hydrogen atoms occupy the anion site because hydrogen in this site is placed off-center

from the anion site and the orbitals overlap. This hybridization causes a transferred core-polarization

interaction between the spin polarized Ti 3d electron and H 1s electron, producing a hyperfine field at the

hydrogen position under an external magnetic field (Fig. 5 (c)). The produced hyperfine field is in the

opposite direction to the external field and results in a negative shift in the 1H NMR signal. In conclusion,

the negative Knight shift in the 1H signal is due to the transferred core-polarization interaction of Ti 3d

electron in the conduction band derived from the hybridization of Ti 3d and H 1s orbitals.

The presence of Knight shift in the multiple hydrogen occupation site indicates excess Ti 3d

electrons in the conduction band. When two hydrogen atoms occupy the anion site as a negatively charged

hydride ion, the net charges derived from the lack of oxide-ion are likely compensated by these two

hydride ions. However, in this scenario, no negative Knight shift would be observed due to the lack of

electron in the conduction band. This inconsistency is explained by the presence of the oxygen vacancy

and/or the single hydride ion in the anion site, or the hydrogen state of the multiple occupation site. When

the oxygen vacancy and/or the single hydride ion occupy the anion site in addition to the two hydrides, the

conduction band is filled with the excess electrons which results in the Knight shift of the hydrides in the

multiple occupation site. Another explanation is that the hydrogen state at the multiple occupation site can



be between the ionic state hydride ion and atomic state hydrogen (H0) with a positively charged proton

screened by electrons to compensate the charge. The atomic state hydrogen is commonly found in

transition metal hydrides, such as TiH2 and ZrH2, showing a negative Knight shift. In this hydrogen state,

the proton occupies the interstitial site and is screened by the delocalized electrons derived from the

exchange interaction between the metal and the hydrogen atom.32-34 When the hydrogen atoms occupy the

double occupation site, the bonding orbital between Ti 3d and H 1s is created and the electrons in the

conduction band occupy this orbital. The hydrogen state in the double occupation site may be determined

by the distribution of the conduction electrons in the bonding orbital between Ti and H. If the electrons

were fully localized in the vicinity of the hydrogen atoms, the hydrogen state would be negatively charged

hydride ions. However, when the electrons are delocalized in the bonding orbital between Ti 3d and H 1s,

the conduction band is occupied by these electrons and the transferred core-polarization interaction occurs.

To confirm the 1H signal in the multiple occupation sites and the influence from other

hydrogen-containing species, Ba-Ti oxydeuteride was synthesized (Supplementary Fig. S7 (a)), and 1H and

2H

MAS NMR measurements were carried out. Two peaks were observed at 5.1 and 1.0 ppm in the 1H

MAS NMR spectrum of the oxydeuteride (Supplementary Fig. S7 (b)). Because these 1H signals did not

originate from the oxydeuteride, the peak at 1.0 ppm is assigned to the residual Ca(OH)2 as in the 1H NMR

spectra of the oxyhydrides. Meanwhile, the peak at 5.1 ppm is originated from adsorbed water. Because

the signal from adsorbed water was observed at 5.1 ppm in the oxydeuteride, and the 1H signal from the


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hydride ion in the single occupation sites has been reported to appear at 4.4 ppm with an intensity

proportional to the hydrogen concentration,17 the origin of the 1H signal observed at 4.7 ppm in the

oxyhydrides (peak 1 in Fig. 4 (a)) may have been either adsorbed water or the hydride ion in the single

occupation sites.

Due to the poor resolution of the 1H NMR spectra, it is difficult to confidently conclude

the assignment of the peak 1 at 4.7 ppm in terms of the chemical shift. If peak 1 can be assigned to the

signal from adsorbed water, the influence of adsorbed water to the amounts of hydrogen in the results of the inert gas fusion analysis is estimated to be less than 9% (6% for the 12h sample and 9% for the 24h and 48h sample) of the amounts of hydrogen in the double occupation sites, which is further evidence that the results of the inert gas fusion analysis are dominated by the hydrogens in the oxyhydride samples. As observed in the 1H NMR spectra of the oxyhydrides, a negative Knight shift is also

confirmed in a 2H signal from the oxydeuteride. The 2H MAS NMR spectrum of the same oxydeuteride is

shown in Fig. 4 (c). Both sharp and broad peaks were observed in the 2H NMR spectrum. Since the broad

peak at -53.8 ppm shows a negative shift similar to that of the 1H MAS NMR spectra of the oxyhydride,

this peak is assigned to deuterium in the double occupation sites. Considering the narrow line width

compared to the peak at -53.8 ppm, the sharp peak at 1.3 ppm is presumably that of deuterium in the single

occupation sites.

The line width of the 2H signals at -53.8 ppm is derived from the spatially asymmetric



Page 18 of 35

environment of deuterium atoms in the double occupation site due to the quadrupole interaction. The

FWHM of the 2H signal assigned to the multiple occupation site is 84 ppm, which is nearly 4 times larger

than that of the 1H signal from the same site. Because the magnetogyric ratio

of 2H is approximately 15%

of 1H, the much weaker dipole-dipole interaction results in a narrower line width of 2H signals than that for

the 1H signals. However, in the present results, the FWHM of the 2H signal from the multiple occupation

site is larger than the 1H signal. This is because the line width of the peak is determined by the quadrupole

interaction originating from the strong electric field gradient at the 2H site in the spatially asymmetric

environment of the multiple occupation site (Fig. 5 (b)). Meanwhile, the 2H signal at 1.0 ppm appeared in

the narrow line width due to the spatially symmetric environment of 2H atom in the anion site (Fig. 5 (a)).

From the 2H MAS NMR spectrum, the presence of the two hydrogen configuration, both the single and the

double occupation sites, in the oxydeuteride is highly likely. It should be noted, however, that the sharp

peak attributed to the single occupation site in the 2H NMR spectrum was not observed in the 1H NMR

spectra of the oxyhydride. This mismatch may be attributed to the closeness of the peaks for adsorbed

water and Ca(OH)2: the sharp component is inseparable from these peaks.

To clarify the dynamics of hydrogen species in the Ba-Ti oxyhydride, deuterium exchange

behavior was investigated by placing the oxyhydride in D2 flow under 0.7 MPa at 326°C. The 2H MAS

NMR spectra of Ba-Ti oxyhydrides before and after deuterium exchange is shown in Fig. 4 (d). Peaks are

only observed in the sample after deuterium exchange. This suggests that the hydrogen is exchangeable.


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The chemical shift and line shape indicate the sharp and broad peaks can be assigned to the single

occupation site and the double occupation site, respectively. However, after the deuterium exchange of the

Ba-Ti oxyhydride, the intensity of the broad peak is lower than that for the oxydeuteride (Fig. 4 (c)), which

suggests a slower exchange at the double occupation sites than at the single occupation sites. Kobayashi et

al. reported the fast exchange of the hydride-ion at the single occupation site with deuterium.17 The 2H

NMR spectrum after deuterium exchange shows the preferential exchange of deuterium at the single

occupation sites and is consistent with Kobayashi et al.’s findings. The deuterium exchange behavior of

the oxyhydride indicates the mobility of the hydrogen atom differs in the two hydrogen configuration at

the single occupation sites and the double occupation sites.

The present study suggests that the hydrogen configuration in the Ba-Ti oxyhydrides is flexible

and that besides the single hydrogen occupation at the anion site, the double hydrogen occupation is also

possible. The preference of these two hydrogen occupation seems to be correlated to the temperature and

the reducing condition of the heat treatment. The temperature of heat treatment in the present study is

lower than that in the preceding study. In addition, the particle size and surface condition of BaTiO3 and

CaH2 as well as atmosphere during the topochemical reaction may affect the hydrogen configuration.

Further investigation is needed to clarify the origin of the hydrogen configurations and their influence on

the physical properties, and the investigation on the direct evidence of the doubly occupied hydride ions,

i.e. by observing H-D coupling in NMR, is our future challenge. The hydrogen configuration and



concentration affect the electronic structure, bonding state and the apparent diffusivity of the hydride ion.

It is expected that it will be possible to control the multi-functions of this material by changing the

hydrogen configuration; i.e. electronic conductivity, ionic conductivity, surface reactivity and reducing

ability.

Conclusions

In conclusion, the double hydrogen occupation in Ba-Ti oxyhydride is indicated by the hydrogen

concentration twice as large as the oxygen defects concentration and the negative Knight shift in the 1H

and 2H MAS NMR spectra of the oxyhydrides and oxydeuteride. The hydrogen concentration of the

oxyhydrides with different reducing times was evaluated as 0.19 mol% H/ f.u. for 12 h, 0.29 mol% H/ f.u.

for 24 h and 0.38 mol% H/ f.u. for 48h. All of these values are twice those for the oxygen defect

concentrations estimated from a thermogravimetric analysis. From the 1H MAS NMR spectra of the

oxyhydrides with different hydrogen concentrations, a 1H signal with a negative Knight shift was

observed, indicating that the hydrogen in the oxyhydrides interacts with the electrons in the conduction

band. These two results differ significantly from those reported in the literature thus for hydride ions

occupying the single occupation sites. That is, the hydrogen configurations, the occupation sites and the

bonding nature of hydrogen atoms, in the present study of Ba-Ti oxyhydride indicate that two hydrogen

atoms occupy a single oxide ion site. In the case of the single occupation sites, where the oxide-ion site is

occupied with one hydride ion, no interaction between conduction band electrons and the hydrogen


Page 20 of 35

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nucleus occurs due to the difference in the orbital symmetry. This explains the absence of the negative

Knight shift observed in the 1H and 2H NMR spectra of the single occupation site in the Ba-Ti oxyhydride.

Meanwhile, a negative Knight shift is observed in the double occupation sites because the transferred

core-polarization interaction between d electrons in the conduction band and hydrogen atom takes place

due to the hybridization between the t2g orbital of Ti and the 1s orbital of hydrogen. It is therefore,

reasonable to assume that the negative Knight shift seen in the 1H and 2H NMR signals can be originated

from the double occupation sites.

Supporting Information

Detailed structural characterization, evaluation of hydrogen concentration, 1H NMR peak parameters and

1H

spin-lattice relaxation rate, influence of impurities, synthesis and characterization of Ba-Ti

oxydeuteride.

Data availability

The data that supports the findings of this study are available from the corresponding author upon

reasonable request.

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Acknowledgement This work was supported in part by JSPS KAKENHI Grant Number JP18H03832.

Author contributions

T.M. and I.O. performed the experiments and H.T. supervised and analyzed the work.

Competing interests

The authors declare no competing interests.


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ACS Paragon Plus Environment

1.6

-12

H2 (m/e = 2) ion current / ×10

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

A


Page 28 of 35

Reduced at 420°C for 48 h Reduced at 420°C for 24 h Reduced at 420°C for 12 h

1.4 1.2 450 1.0 0.8 0.6 0.4

200

400

600

Temperature / °C Fig.2


800

Page 29 of 35

Spectrum Background

Reduced at 420&C for 48 h Ti4+

Intensity (a.u.)

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


Reduced Ti ion 420&C for 24 h

420&C for 12 h

Ti 2p3/2

Ti 2p1/2

BaTiO3

475

470

465 460 455 Binding energy / eV Fig.3


450

445



Page 30 of 35

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Page 32 of 35

Table

Reducing time

Oxygen defect / mol / f.u. .

Hydrogen concentration / mol / f.u.

48 h

0.19

0.38

24 h

0.14

0.29

12 h

0.10

0.19


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Figure captions

Fig. 1 Characterization of crystalline phase and oxygen defects in Ba-Ti oxyhydride. (a) XRD patterns of Ba-Ti oxyhydrides reduced at 420°C for 48, 24 and 12 h. The inlets show the appearance of the samples. (b) Weight change during heating to 800°C. Fig. 2 Hydrogen gas evolution behavior. Evolution gas of Ba-Ti oxyhydrides reduced for different time is monitored by quadrupole mass spectrometer by heating the sample up to 800°C. Fig. 3 Electronic state of Ti ion measured by XPS. Ti 2p XPS spectra of pure BaTiO3 and Ba-Ti oxyhydrides reduced for 12, 24 and 48 h. The spectrum is shown in the dotted line. Deconvoluted peaks are indicated by the solid red and blue lines. The background intensity is indicated by the solid black line. Fig. 4 Local structure of 1H and 2H in Ba-Ti oxyhydride and oxydeuteride measured by MAS NMR spectroscopy. (a) 1H MAS NMR spectra of Ba-Ti oxyhydride powder with hydrogen concentrations of 0.38, 0.29 and 0.19 mol H / f.u. (b) Hydrogen concentration dependence of peak area, chemical shift and full-width at half of maximum of peak 3 of 1H MAS NMR spectra. (c) 2H MAS NMR spectrum of Ba-Ti oxydeuteride reduced at 420°C for 48 h. (d) 2H MAS NMR spectrum of Ba-Ti oxyhydride before and after deuterium exchange under a deuterium gas pressure of 0.7 MPa at 326°C. Fig. 5 Schematic view of the hydrogen configuration in Ba-Ti oxyhydride and the transferred



core-polarization interaction. (a) Hydride ion in the anion site. (b) Multiple hydrogen occupation sites. (c) Transferred core-polarization interaction between a Ti 3d conduction electron and H 1s electron due to hybridization of the Ti 3d orbital and H 1s orbital. Table Caption

Table1 Oxygen defects and hydrogen concentration of Ba-Ti oxyhydrides estimated from TGA and the

inert gas fusion technique.


Page 34 of 35

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ToC graphic


The negative Knight shift in Ba-Ti oxyhydride: An indication of the

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