Article

Geometrical Frustration of B-H Bonds in
Layered Hydrogen Borides Accessible by Soft
Chemistry
Satoshi Tominaka, Ryota
Ishibiki, Asahi Fujino, ..., Iwao
Matsuda, Hideo Hosono,
Takahiro Kondo
tominaka.satoshi@nims.go.jp (S.T.)
takahiro@ims.tsukuba.ac.jp (T.K.)

HIGHLIGHTSThe structure of a layeredhydrogen boride via a soft
chemical route is determined
The structure consists of B-H-B
bridging bonds and B-H terminal
bonds
The local structure induces
macroscopic amorphous natureby geometrical frustration
The layered hydrogen boride iselectrically conductive

A layered hydrogen boride synthesized via a soft chemical route consists of threecenter, two-electron (3c-2e) B-H-B bridging bonds as well as ordinary two-center,two-electron (2c-2e) B-H terminal bonds. The local chemical units form a layered
structure, which is amorphous due to the geometrical frustration of terminal B-H
bonds. The layered hydrogen boride is electrically conductive.

Tominaka et al., Chem 6, 406–418
February 13, 2020 ª 2019 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2019.11.006

Article

Geometrical Frustration of B-H Bonds
in Layered Hydrogen Borides
Accessible by Soft Chemistry
Satoshi Tominaka,1,2,* Ryota Ishibiki,3 Asahi Fujino,3 Kohsaku Kawakami,1 Koji Ohara,2Takuya Masuda,4 Iwao Matsuda,5 Hideo Hosono,6 and Takahiro Kondo6,7,8,*

SUMMARY

The Bigger Picture

Hydrogen borides adopt a variety of structures because of the electron-deficient character of boron. Recently, we reported the synthesis of a layeredhydrogen boride via a soft chemical route. Here, we ascertain the atomic arrangements in the layered hydrogen boride by using pair-distribution functions:the material dominantly consists of a corrugated B network decorated withthree-center, two-electron B-H-B bridging bonds as well as ordinary two-center,two-electron B-H terminal bonds. The material is locally ordered but amorphousby diffractometry. This discrepancy can be accounted for by geometrical frustration caused by the positions of terminal B-H bonds located on one of two
equivalent B atoms in the B-H-B bridging bonds. This material is electricallyconductive (0.13 S cm 1 below 10 C) rather than ion conductive, and its B-H-B
bonds are cleaved by the adsorption of molecules; this dynamic chemical nature
originates from the frustrated structure and leads to unique hydrogen boride
functionalities.

Layered hydrogen borides were
investigated computationally, anda variety of structures have been
proposed. Although thosesimulated energetically favored
structures are crystalline, thesynthesized material isamorphous. Is there any sciencebehind the amorphous nature?
Or, is the material simply
disordered? Thus, we analyzed
the structure fully. The structure
consists of three-center, twoelectron (3c-2e) B-H-B bridging
bonds as well as ordinary twocenter, two-electron (2c-2e) B-H
terminal bonds. This unique local
structure forms a layered structure
without long-range orders
because of geometrical frustrationof terminal B-H bonds. The local
structure dominates not only themacroscopic structure but also theelectrical conductivity uponadsorption of molecules. Such a
dynamic chemical nature is unique
as a two-dimensional material andwill position hydrogen boride
nanosheets as potential sensing
materials and catalysts.

INTRODUCTION
Boron is an element that is located between metals and non-metals on the periodic
table and is known to exhibit intermediate, semimetallic physical properties. Boron
has a 2s22p1 electronic configuration of valence electrons, which results in as manyas 16 bulk allotropes because of the ‘‘geometrical frustration’’ associated with theelectron-deficient character of boron.1–5 This variety of structures originates fromelectronic configurations that are likely to form molecules or clusters such as B12 icosahedra. Boron adopts structures, such as borophene, which is a single-atomic twodimensional (2D) layer of boron that is analogous to graphene,6–9 boron nanotubes,3
which are analogous to carbon nanotubes, and borospherene, which is analogous tofullerene.10 Some of these boron nanostructures are known to have higher conductivities than those of bulk structures (10 4–10 7 S cm 1),6 for example, boron nanotubes are known to exhibit high metallic conductivities that exceed 102 S cm 1.6
Carbon allotropes and boron allotropes are to some extent analogous, but their hydrides clearly highlight the differences between boron and carbon. Hydrocarbons
are common organic compounds on the Earth and are generally non-conductive
because of their fully occupied valence orbitals. Typical boron hydrides are discrete
molecules (boranes), such as diborane (B2H6) and decaborane (B10H14), or anions,such as borohydride (BH4 ). These molecular boranes have been synthesized andinvestigated for decades, whereas solid-state boron hydrides with extended structures have been investigated little, although they are expected to be conductive

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because of the electron-deficient nature of boron. In these years, computational
studies suggested that some 2D hydrogen borides (HBs) should exhibit unique electronic properties (as summarized in Figures S1–S7).11–15 Because of the electronic
configuration of boron, a variety of 2D boron hydride structures are energeticallyfavorable, as corroborated computationally (cf. Table S1).11 Because of the fickleness of boron, existing syntheses of 2D boron hydrides do not provide homogeneous and highly pure materials, especially when chemical routes are used.
In 2017, we successfully synthesized one such 2D material in its powder form by thechemical ion exchange of magnesium (Mg) cations in magnesium diboride (MgB2)
for protons in an organic solvent.16 Through careful analysis, we concluded thatthe hexagonal boron networks of the original MgB2 were retained, and proton exchange provided a hexagonal HB (h-HB) structure (cf. Figure S1).17 Note that thismaterial is referred to as ‘‘boride’’ (rather than a ‘‘hydride’’) because the electronic
states of its boron atoms are similar to those in MgB2.16 In slightly more detail,this h-HB has a H/B ratio of 1.0 and a layered structure with 6-fold local symmetry
and three-center, two-electron (3c-2e) B-H-B bridging bonds. The material is amorphous by X-ray diffractometry (XRD) and exhibits short-range orders of within 6 Å;
hence, we considered that the 2-fold symmetry of the hydrogen positions might
result in the formation of domains with three different orientations in the 6-fold symmetry of the boron network. Although we proposed a structural model that accounted for the experimental data, the details were not sufficiently clear becauseof slight oxidization of the sample prepared using our early synthetic method. Moreover, we were concerned that the structure might have changed by oxidation. Suchsoft chemical methods can produce metastable structures that are often not predicted computationally, and the observed structure might have formed throughoxidation of such a metastable structure. Thus, the synthesis and analysis of the unoxidized layered HB is of great importance for achieving a better understanding of
this system.
Here, we ascertain the structure of chemically synthesized layered HB and discuss
how local ordering on the basis of chemical bond formation results in macroscopic
disorder. We found that geometrical frustration accounts for the amorphous stateof the layered HB and refer to this structure as ‘‘frustrated hydrogen boride’’(f-HB). To determine the structure, we improved our Schlenk method and obtained
a pure HB sample by a proton-exchange reaction in which MgB2 crystals were
immersed in a methanol (or acetonitrile) solution containing a sulfonated polystyrene as a proton-exchange polymer, under N2, as reported previously (details areprovided in the Supplemental Information, Figures S8 and S9).16 The structure was
fully analyzed using pair-distribution functions (PDFs), XRD, infrared spectroscopy,
and quantum chemical calculations. The structure itself is very interesting in viewof its geometrical frustration, which tends to form amorphous products. We also
investigated the electrical conductivity of the HB sample.

1International Center for Materials

Nanoarchitectonics, National Institute forMaterials Science, Tsukuba, Ibaraki 305-0044,
Japan
2Research and Utilization Division, Japan

Synchrotron Radiation Research Institute, 1-1-1
Kouto, Sayo-gun, Hyogo 679-5198, Japan
3Graduate School of Pure and Applied Sciences,

University of Tsukuba, Tsukuba, Ibaraki 305-8573,
Japan
4Research Center for Advanced Measurement

and Characterization, National Institute forMaterials Science, Tsukuba, Ibaraki 305-0044,
Japan
5Institute for Solid State Physics, University ofTokyo, Kashiwa, Chiba 277-8581, Japan
6Materials Research Center for Element Strategy,

RESULTS AND DISCUSSION
Chemical Synthesis of Oxide-free HBBlack MgB2 powder was treated by a proton-exchange process. The color of the material changed from black to yellow during the 3-day ion-exchange treatment, and a paleyellow powder was recovered (Figure 1A). The HB sample was subjected to scanningelectron microscopy, as shown in Figure 1A. The HB sample forms sheets, some ofwhich are folded and wrinkled, which indicates that these sheets are flexible. We further
evaluated the thickness of the HB sample by atomic force microscopy (AFM) under Ar

Tokyo Institute of Technology, Yokohama,
Kanagawa 226-8503, Japan
7Department of Materials Science and Tsukuba

Research Center for Energy Materials Science,
Faculty of Pure and Applied Sciences, Universityof Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
8Lead Contact*Correspondence:
tominaka.satoshi@nims.go.jp (S.T.),
takahiro@ims.tsukuba.ac.jp (T.K.)
https://doi.org/10.1016/j.chempr.2019.11.006

Chem 6, 406–418, February 13, 2020407

Figure 1. HB Sheets Synthesized by Our Improved Method(A) Scanning electron microscopy image. Scale bar, 10 mm. The inset shows a photographic image of an HB sample pelletized for conductivity
experiments.
(B) AFM images with line profiles for HB on a mica surface. Scale bar, 0.4 mm.
(C and D) Core-level Mg 2p (C) and B 1s X-ray photoelectron spectra (D).

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Figure 2. Structural Analysis of the Soft Chemically Synthesized Layered HB(A) X-ray diffraction pattern with the simulated pattern for f-HB.
(B and C) X-ray pair-distribution functions collected at 6  C with those simulated for h-HB (B) andf-HB (C). The yellow curve in the bottom panel is the total-electron PDF simulated for charge
densities above 0.8 electrons per Å 3 , which corresponds to the charge density maps shown in
Figure 4. The insets show the primary components that form layered HB structures: B6 hexagonalrings (B: blue balls) and H atoms (white balls).

without exposing the sample to air (Figure 1B). Here, to evaluate its non-aggregatedstructure, HB sheets were prepared on a mica surface by dropping an acetonitrile suspension, followed by spin coating and drying in a glove box. We found that thinner(1 nm) and smaller sheets were located at the rotational center of the spin-coated surface (Figure 1B). The thickness corresponds to a few monolayers of HB sheets. Note thatthicker (R 3 nm) and larger sheets were found at surrounding regions with increasing
distance from the rotational center of the spin-coated surface (Figure 1B).
Elemental analysis indicated that H/B = 1.0, and no other elements, including Mg,
were detected (Figure S10), as reported previously.16 The improved synthesis
method and purity of the product were also confirmed by the core-level Mg 2p
and B 1s photoelectron spectra (Figures 1C and 1D), which show only a single
peak for negatively charged boron at 187.5 eV and no oxide component. The
absence of an oxidized phase was also confirmed by synchrotron powder XRD, inwhich Bragg peaks assigned to boric acid crystals were absent (Figure 2A).

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Chemical Bonds in the Layered HB
The material is non-crystalline, as evidenced by XRD, which exhibited a broad diffraction pattern (Figure 2A); the non-crystallinity was also confirmed by the X-ray PDF ofthe material (Figure 2B), which clearly reveals only short-range orders that are within6 Å. The fact that the product is amorphous does not mean that the material has
no order. We uncovered details, from chemical bonds to overall structures, in a
step-by-step fashion. PDFs were obtained from X-ray total scattering experiments(Figure S11) and are useful for discussing bond distances and atomic arrangements.
Note that the PDF obtained for this oxide-free sample is different to the one reported
previously.16 The previously reported data were calculated by subtracting the total
scattering signal of the more-oxidized sample from that of the less-oxidized sample;
the data shown in Figure 2B are without such subtraction treatment. Thus, the unoxidized phase in the previously analyzed sample might have a slightly different and/or
non-dominant structure in the sample. Consequently, here, we carefully analyzed
the unoxidized phase by using the data obtained for the pure sample.
The main PDF peak (blue dots in Figure 2B) is assigned to the B-B distance, at 1.79 Å
(second peak), which is longer than a typical B-B single bond (1.70 Å)18 and close tothat in MgB2 (1.78 Å, Inorganic Crystal Structure Database [ICSD] no. 93925).19 Thisbond length is also close to that in B2H6 (1.75–1.79 Å, ICSD no. 15598)20,21 or otherB2H6 derivatives with B-H-B bonds (1.82 Å),22 supporting the formation of 3c-2e
bridging bonds at least partially with little stress in terms of B-B bond distances.
The peak, located at 1.40 Å, is assigned to a B-H distance and is far longer than aterminal B-H bond (1.18 Å, bond order = 1),21 which also indicates that layered
HB is composed of 3c-2e bridging B-H-B bonds (>1.3 Å, bond order = 0.5) (Figure 2B). When looking at the computationally studied layered HB structures and theirsimulated PDFs (Figures S1–S7), the formation of 3c-2e bridging bonds is reasonablebecause the B-H/B-B atomic-pair ratio (namely the ratio of the first PDF peak ataround 1.3 Å to the second at around 1.7 Å) of the experimental PDF (Figure 2B)
is apparently larger than those in structures devoid of 3c-2e bridging bonds; thisis further discussed below together with curve-fitting data.
The Fourier transform infrared (FTIR) spectrum confirms the presence of B-H-B
bridging bonds, as evidenced by the presence of a broad peak at around
1,400 cm 1 (Figures 3A and 3C) (details of infrared [IR] peak assignments are available in the Supplemental Information, Figures S12–S14). The position of this IR peakis dependent on the B-B distance in the bridging bond (Figure S14) of the hexagonalboron network, and the peak position is well matched to the B-B distance (1.79 nm)
obtained by PDF analysis. The additional peak at 2,500 cm 1 is assigned to terminal
B-H stretching; however, as discussed above, the PDF does not exhibit a peak assigned to typical terminal B-H groups (1.18 Å). Thus, we conclude that the terminal
B-H groups are in unique environments, which is further discussed below. In addition
to these terminal B-H groups, there is broad absorption behavior above>3,000 cm 1, which is probably assignable to water impurity. Note that this HB material reacted little with water in our hydrolysis experiments. The details of thisobserved stability to hydrolysis will be reported in the future, but this stability is
consistent with the fact that boric acid (oxidized B) was not detected by FTIR spectroscopy, and no oxidized boron was detected by photoelectron spectroscopy.
Local Geometric Frustration Dominates the Macroscopic Amorphous Natureof f-HBThe HB structure was further analyzed by curve fitting the PDF data. The h-HB structure (Figure 2B, top and Figure S1) was chosen as the initial structural model because

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Figure 3. Functional Groups in f-HB as Analyzed by FTIR Spectroscopy(A) B-H-B vibrations in the bridging bonds in hexagonal B networks are found at around 1,300 cm 1by quantum chemical calculations. The peak positions depend on the B-B distances in the bridging
bonds (1.65, 1.68, 1.75, 1.78, 1.79, 1.83, 1.86, and 1.90 Å, from right to left).
(B) Simulated IR spectrum for the f-HB structure solved by PDF analysis. The wavenumbers havebeen multiplied by 1.13. 23 The insets highlight the major vibrational modes.
(C) Attenuated total reflectance FTIR spectrum (experimental data). The asterisks indicate peaks
assigned to boric acid (according to a National Institute of Standards and Technology database).

it contains hexagonal B networks and B-H-B bridging bonds, both of which were
determined to exist in the synthesized sample. The initial model was gradually
altered by changing the atomic coordinates under the bond-length constraints byleast-squares or Bayesian optimization in order to obtain PDF data that resembled
the experimental data; both optimization methods resulted in the same atomic
arrangement via an in-house program (details are available in the Supplemental
Information, Figures S15 and S16). A unit cell model with a = 5.164 Å, b =4.147 Å, c = 2.843 Å, and b = 103.3 (P21 space group) accounts for the experimental
PDF (Figure 2C, red curve) and FTIR (Figure 3B) data well, with the exception of theamorphous nature shown by XRD (Figure 2A). The formation energy calculated byquantum chemical calculations is 4.48 eV per atom (structure obtained from PDF
analysis without geometry optimization, Table S2), which is slightly (by 0.5 eV
per atom) smaller than those of typical layered HBs (Table S1, calculated using
norm-conserving pseudopotentials with dispersion corrections). The derived structural model of f-HB is displayed in Figure 4.
The HB structure is composed of corrugated layers of B atoms (Figure 4C) that areformed by hexagonal boron networks of chair-type B6 rings, which means that theoriginal hexagonal boron network of MgB2 has been retained. These chair-type B6rings are reasonable because the PDF peaks simulated using the h-HB structure (Figure 2B, top), in which the B6 rings are flat, do not match the experimental data atlonger distances. B-H-B bridging bonds are located at the ridge lines of the corrugated layers. As opposed to the B2H6-type double B-H-B bridging bonds found in
the h-HB model, this structure has a single B-H-B bridging bond per B2 unit, which

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411

Figure 4. Structural Model of f-HB(A) Primary unit structure composed of a B-H-B three-center, two-electron (3c-2e) bridging bond.
Additional H atoms can be located at two possible terminal positions (HT ) as well as the bridge
site (H B).
(B) The bridging bonds decorate ridge lines of the hexagonal boron network composed of chairtype B6 rings. The terminal B-H bonds are located on the left-hand side in this model.
(C) Corrugated networks shown along different directions. The color maps (left) are charge density
maps (valence electrons only) simulated by quantum chemical calculations. The isosurfaces show
charge densities at 0.8 (yellow) and 1.2 (red) electrons per Å 3 , as illustrated using the VESTA
program. 24 The blue and white balls show the core positions of B and H atoms, respectively.

is probably due to the step-by-step hydrogenation during the ion-exchange process
to form a single B-H-B bridging bond first, which corrugates the B hexagonal layer
and then blocks the formation of other B-H-B bridging bonds by steric hindrance.
This bridging bonds are themselves symmetric (B-HB-B environments, C2v point
group; Figure 4A), but the remaining electron per B2 unit forms a terminal B-H groupat one of the equivalent terminal positions (HT in Figure 4A), resulting in asymmetric
B-HB-B-HT or HT-B-HB-B environments (Cs point group, Figure 4B). As discussed
above, this terminal B-H bond is longer than typical B-H bonds (i.e., both B-H bonds
in HT-B-HB show a peak at around 1.4 Å in Figure 2B), suggestive of weak bonding
between the B and H atoms. To discuss the nature of this chemical bonding, we performed Bader charge analysis to understand the partial charges on the various
atoms. This charge analysis (Table S3) revealed that the H atoms are negatively
charged (partial charges of 0.66 (HT) and 1.38 (HB)), whereas the B atoms are positively charged. Bader analysis (Figure S17) also showed that the charges in the B-B

412

Chem 6, 406–418, February 13, 2020bonds are distributed equally between the B atoms (i.e., indicative of covalentbonding), whereas the charges in the B-H bonds peak around the H cores, andthe electrons were distributed along the bonds toward the B atoms (i.e., indicativeof covalent bonding with ionic bonding nature). The terminal B-H groups might
be expected to provide ionic conduction; however, these H atoms are not macroscopically mobile (electronic conduction appears to dominate over ionic conduction) as observed by AC impedance spectroscopy (below).
The structure is in the P21 space group, which implies an ordered arrangement of terminal B-H groups, but the lack of long-range ordering indicates disorders in the arrangements of terminal B-H groups. The different B-H arrangements (B-HB-B-HT and
HT-B-HB-B) are equivalent but result in different local distortions of the B layer. Thus,
we conclude that the non-crystalline nature of the material originates from such arrangements of H atoms, i.e., the geometrical frustration of terminal B-H groups. This,in turn, means that the local chemical nature of the terminal B-H moiety governs themacroscopic structural and chemical order.
Delocalized Electrons in f-HB Enable Electrical Conduction
Boron has the [He]2s22p1 electronic configuration, and its proportion of valence
electrons is significant and detectable by X-ray scattering. The broad feature below
1 Å in the PDF of HB is not due to termination rippling (i.e., noise resulting from theFourier transformation), rather it is associated with valence-electron scattering.25 To
confirm this, we first calculated the valence-electron charge density (Figure 4C) in
the f-HB structure and then simulated the PDF by using the results from these calculations. For this total-electron PDF (TE-PDF) simulation, we used the in-house program described in the Supplemental Information (Figures S18 and S19).
The simulation clearly reproduces the feature below 1 Å (Figure 2C, yellow curve).
The peak is mainly associated with pairs of core electrons on B atoms and the valence
electrons. The feature at around 0.7 Å is mainly associated with scattering pairs of B
core electrons and electrons along the B-B bonds. The TE-PDF does not show a clear
peak assigned to the B-H distance, which should be located at around 1.4 Å, asshown by the blue curve, suggesting that electron delocalization around the H atoms
has been overestimated in the quantum chemical calculations. Because these calculations implicitly assume a ground state for the crystalline domain using periodic
boundary conditions, the calculations might overestimate electron delocalization.
This, in turn, suggests that the material is in a metastable state, as also indicatedby the observation that PDFs for the structures optimized by quantum chemical calculations do not fit the experimental data well (Figure S19).
Although electron delocalization might be overestimated, the zero band gap (Figure S20) obtained by the quantum chemical calculations for the structure determined by the PDF fitting motivated us to evaluate its conductivity. We acquired
AC impedance spectra of f-HB in order to discuss its electronic and/or ionic conductivities and to support the electronic structure described above. A powder sample
was pelletized under Ar in a glovebox and then mounted in an airtight cell. Figure 5A
displays representative raw impedance data that exhibit behavior typical of an electronic conductor, rather than that of an ionic conductor given that the phase angles
are 0 over the entire frequency range (Figures S21–S24). We also confirmed electrical conduction by acquiring current-voltage curves (Figures 5B and S25), whichshow ohmic behavior over the entire temperature range (from 0 C to 50 C). This,in turn, means that ionic conduction is, at most, lower than electron conduction
and therefore not observable.

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413

Figure 5. Conductivity of f-HB(A) AC impedance spectrum acquired at 10  C. The spike at 50 Hz is AC noise.
(B) Current-voltage curves acquired at various temperatures between 0  C and 50  C.
(C) Temperature-dependent resistivity obtained from temperature-controlled AC impedance
measurements.

Figure 5C shows the temperature-dependent conductivity of the sample (obtainedby impedance spectroscopy). At temperatures below 10 C (Figure 5C), the conductivity is fascinatingly high for an HB, at 0.13 S cm 1 (0 C–10 C). The activation energy
for conduction was calculated to be 0.10 eV (Figure S24), which indicates that conduction is metallic rather than semiconductive in nature. Metallic conductivity is rarein 2D materials, and only a few compounds such as some of the transition-metal dichalcogenides26,27 and 2D metal-organic frameworks28 are known. Our observationis consistent with the zero band gap simulated by quantum chemical calculations forthe derived f-HB (Figure S20).
Local Chemistry Governs the Macroscopic Nature
The last part of this paper focuses on the chemical nature of f-HB. We observed a dramatic increase in resistance above 30 C during our conductivity experiments (Figures
S21 and S25). This metal-to-insulator transition is found to be reversible, displays an
obvious hysteresis, and is reproducible among different samples. It is noteworthy thatthe impedance phase is still 0 at 1 Hz (Figure S22). This phenomenon is not associatedwith our experimental setup, including electrode contact, as confirmed by control experiments using an Au pellet (Figure S23). The transition was still observed (Figure S21) after
the f-HB pellet was heated at 150 C, where weakly bound hydrogen atoms are removed(Figure S26). Hence, the transition is not caused by dehydrogenation.
Temperature-modulated differential scanning calorimetry (DSC) (Figure S27) wasused to obtain an understanding of the mechanism associated with the observed
dramatic change in impedance; however, this technique did not reveal any peaks
or prominent baseline shifts in the relevant temperature range, indicating that theimpendence transition is not accompanied by a phase transition, in which enthalpy,
entropy, and heat capacity changes. Temperature-controlled FTIR spectroscopy(Figure S12), XRD (Figure S28), XPS, and solid-state NMR spectroscopy failed todetect any obvious changes in the layered HB structure. Temperature-controlled
PDFs reveal that peaks assignable to B-H and B-B distances are sharper and more
intense at the elevated temperature (Figure S11), whereas the broad feature assignable to the valence electrons located below 1 Å is slightly less intense, which meansthat electrons are localized around atomic cores at the elevated temperatures.
A similar transition without an obvious structural change was observed in a 2D cobalt
dithiolene metal-organic framework, which exhibits a transition to a metallic state atlower temperatures that is probably associated with interlayer states.28 To investigate the impact of interlayer states, we exposed the HB material to ethanol, which

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decreased the intensity of the IR peak assigned to the B-H-B bonds and increased
the intensity of the terminal B-H stretching peak (Figure S29). This suggests thatthe B-H-B bonds dissociate to form terminal B-H bonds by the adsorption of ethanol,
which in turn means that the chemical groups in f-HB are flexible and/or dynamic andrespond to chemical adsorption.
Careful inspection of the conductivity data reveal that conductivity as well as transition temperature depends slightly on the sample (Figure S21) (note that more samples were measured than shown here) and that residual solvent used in the chemical
synthesis is considered to have an impact. Temperature cycling and reassembly of
the cell successfully removed the transition (Figure S30); thus, we conclude thatthe reversible transition is associated with desorption and adsorption of impurities.
Temperature cycling between 0 C and 50 C (switched every 1 h) for 5 days resulted
in the ordering of local structure, as evidenced by the PDFs (Figure S31). The PDF
exhibited a peak at 3.0 Å, which is assigned to the second-nearest neighboring
B-B distances of the hexagonal B networks or the third-nearest neighboring HB-B
distances, which also resembles the behavior of the simulated PDF well. The overall
PDF became more similar to that simulated for the P21 structure by optimizing the H
positions, yet with a non-crystalline nature (Figure S31).
All of these observations indicate that local B-H chemical bonds in f-HB govern its
macroscopic structure as well as electronic nature; f-HB is an electrically conductive
solid-state material with local functional groups that respond to chemical adsorption, which we believe will open up the unique functionalities of HB materials.

EXPERIMENTAL PROCEDURES
Preparation of HB Sheets
MgB2 powder (60 mg, 99%, Rare Metallic Co. Ltd., Tokyo, Japan) was added to 200 mLof methanol (99.8%, Wako Pure Chemical Industries Ltd., Osaka, Japan) or acetonitrile(99.5%, Wako PureChemical Industries Ltd., Osaka, Japan), followed by ultrasonication for 30 min. The prepared suspension was then added to a methanol (or acetonitrile) suspension (100 mL) of the ion-exchange resin (30 mL, Amberlite IR120B
hydrogen form, Organo Corp., Tokyo, Japan) under nitrogen at room temperature.
After stirring at 250 rpm for 3 days, the supernatant, or filtrate after filtration througha 1.0 or 0.2 mm pore filter (Omnipore Membrane Filters, Merck Millipore, Billerica, MA),
was evaporated on an oil bath at 343 K under vacuum to yield the product as a yellow
powder. Note that methanol was used for the samples shown in this main text (PDF, IR,and conductivity experiments), and acetonitrile was used for some of the samples
shown in the Supplemental Information. Both sets of samples adopt the same structure, as confirmed by their PDFs and IR spectra (Figure S12). Because we found thatthe dynamic structure changed upon ethanol adsorption, trace amounts of residual
solvents are likely to be important for reproducibility; accordingly, the information provided herein has been written as carefully as possible.
Conductivity Measurements
The HB powder sample was pelletized into a 1-cm-diameter, 620-mm-thick disk at
0.31 GPa under Ar and assembled in a stainless steel cell (split test cell, MTI Corporation). The cell was placed in a temperature-controlled chamber (BioChamber
LS-5N, Nippon Brower), which operates as a Peltier temperature controller and a
Faraday cage for reducing noise. Impedance spectra were obtained by the AC
impedance technique (MTZ-35, BioLogic; input impedance = 10 MU) in the
104–1 Hz frequency range at an AC voltage of 10 mV. The spectrum was collected
every 0.5 C in the 0 C–10 C or 0 C–50 C temperature range for several cycles to

Chem 6, 406–418, February 13, 2020415

confirm data stability. Current-voltage curves were obtained using an electrochemical measurement system (HZ-7000, Hokuto Denko; Input impedance R1 TU) every
1 C in the 0 C–10 C or 0 C–50 C temperature range for several cycles to confirm
data stability (30 min per point). As contact materials, we tested SS304 stainless steel
plates as well as copper foil (99.96%, 0.1 mm thick, work function = 4.51 eV) andconfirm that identical data were obtained.
X-Ray Structural Analysis
X-ray total scattering data used to generate PDFs were collected using a flat panel detector on the BL08W beamline (l = 0.10713 Å) at the SPring-8 facility.29 The samples weresealed in 1.0-mm-diameter Hilgenberg quartz capillaries under Ar. The collected 2D
data were converted into scattering patterns by using the PIXIA program (ver. 8.01),30
in which all pixels were assumed to be cylindrical absorbers; solid angles, penetration
depths, and polarization effects were corrected for each pixel. The scattering data
were further corrected using the MaterialsPDF program30 by subtracting Compton scattering intensities and then normalized using scattering form factors calculated for theH1B1 composition by using atomic scattering factors. The obtained structure function,
S(Q), was converted into the reduced PDF, G(r), by Fourier transformation (the maximumof the moment transfer of scattering particle [Qmax] = 20.4 Å 1) with the revised Lorch
function (D = 0.9).31 PDFs were simulated using the PDFfit2 program32 (Qmax = 20.0
Å 1, diameter = 30.0 Å, and the isotropic atomic displacement parameter [Uiso] =0.002 Å2). In these simulations, we simply assumed all the electrons of each atom are
localized at the center of the atom, with a distribution simulated using the atomic
displacement parameter. PDF simulation under bond-length constraints and without periodic boundary conditions as well as curve fitting with Bayesian optimization were performed using the in-house program detailed in the Supplemental Information. The TEPDF was simulated for a model containing the calculated charge densities (valence electrons) and core electrons for the B atoms. The scattering factor for an electron was
assumed to be 1.0. Because X-ray scattering factors depend on scattering vectors, andfully delocalized electrons are hard to observe in general scattering experiments, weused electron densities above a cutoff value (0.8 electrons per Å3) and simulated at interatomic distances above 0.2 Å. The unit cell was divided into 72 3 60 3 50 cells.
Quantum Chemical Calculations
Spectroscopic data were simulated using density functional theory (DFT) within the
CASTEP program.33 The generalized gradient approximation (GGA) and the PerdewBurke-Ernzerhof (PBE) exchange-correlation functional,34 with norm-conserving pseudopotentials,35 which are based on the kinetic energy optimization scheme (details areavailable in the CASTEP manual),36 and a 720.0 eV energy cutoff were used. This functional was chosen for its ability to closely simulate the experimental IR spectrum ofdiborane (although only trivial differences exist between the major functionals). The
convergence thresholds are energy = 5 3 10 6 eV per atom, maximum force = 0.02
eV per Å, and maximum displacement = 0.003 Å. For the dispersion corrections thattake van der Waals interactions into consideration, a semiempirical correction method
as represented by the Tkatchenko-Scheffler (TS) scheme was used (sR = 0.94 andd = 20.0).37 In addition to DFT calculations using norm-conserving pseudopotentials(the CASTEP program), we used the Quantum ESPRESSO program (PWscf program)38
with projector augmented wave (PAW) pseudopotentials (energy cutoff, 25.0 Ry andcharge cutoff, 225.0 Ry) and the PBE exchange-correlation functionals.

DATA AND CODE AVAILABILITY
The codes for analyzing PDFs (TE-PDF and Bayesian optimization) will be available
from the National Institute for Materials Science (Satoshi Tominaka).

416

Chem 6, 406–418, February 13, 2020

SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2019.11.006.

ACKNOWLEDGMENTS
We thank N. Tsuji and K. Nakada (Japan Synchrotron Radiation Research Institute[JASRI]) for help with the synchrotron experiments at SPring-8 and D. Umeyama(NIMS) for the help with FTIR spectroscopy under N2. We thank Professor Y. Majima(Tokyo Institute of Technology) for the useful comments regarding the conductivity
experiments. We thank Mr. K. Matsushita (NIMS) for AFM support. We thank Professor B. Slater (University College London) for the useful discussion. This work waspartly supported by JSPS KAKENHI (18H03874, 18K05192, 19H02551, and
19H05046), Japan; the MEXT Element Strategy Initiative to Form Core ResearchCenter and Murata Science Foundation, Japan; and KUMAGAI, Ogasawara, andSamco foundations for the Promotion of Science & Engineering, Japan. The synchrotron radiation experiments were performed on the BL08W beamline (PDF) at theSPring-8 facility with the approval of the JASRI (proposal nos. 2017A1207 and2018A1304) and on the BL5S2 beamline (XRD) of the AichiSR facility, Japan (proposal no. 201804032). We would like to thank Editage (www.editage.com) for English language editing.

AUTHOR CONTRIBUTIONS
Conceptualization, S.T. and T.K.; Sample Preparation, R.I. and T.K.; Structural Analysis, PDF Simulation & Programming, Conductivity Experiments, Quantum Chemical
Calculations, and IR Spectroscopy, S.T.; XPS and Scanning Electron Microscopy, R.I.
and T.K.; AFM, T.M., R.I., and T.K.; Calorimetry, S.T. and K.K.; X-Ray Diffraction, S.T.
and K.O.; Ethanol Adsorption in IR, A.F. and T.K.; TDS, A.F., H.H., and T.K.; Writing –Draft, S.T. and T.K.; Discussion of Results, S.T., R.I., A.F., K.K., K.O., T.M., I.M., H.H.,
and T.K.

DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: July 29, 2019
Revised: October 14, 2019
Accepted: November 12, 2019
Published: December 9, 2019

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