Cubic Fluorite-Type CaH2 with a Small Bandgap

Article abstract of DOI:10.1021/jacs.7b05746

A cubic variant of CaH₂ adopting a fluorite-type crystal structure was synthesized by cationic substitution with La or Y, yielding the first alkaline earth hydride-based with fluorite-type framework. The material has a bandgap of ～2.5 eV (greenish yellow in color), which is much smaller than that of orthorhombic PbCl₂-type CaH₂ (4.4 eV) and is, in fact, the smallest among alkaline or alkaline earth metal hydrides reported to date. Analysis of the density functional theory band structure of cubic-CaH₂ indicates that its conduction band minimum is formed mainly by the interaction between the Ca 3d e_g orbitals around the crystallographic cavity defined by cubes of H^- ions. The use of such cavities in the creation of low-lying conduction band minima by semiconductors is extremely rare, and has similarities to inorganic electrides.

Full text of DOI:10.1021/jacs.7b05746

Communication
pubs.acs.org/JACS
Cubic Fluorite-Type CaH with a Small Bandgap
2
,
†
†,‡
§,¶
Kazutaka Ikeda, Toshiya Otomo,§,¶
§,¶
Hiroshi Mizoguchi,* †,‡,⊥SangWon Park, Takashi Honda,
,
and Hideo Hosono*
†Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan
‡
Laboratory for Materials Research, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan
§
Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
¶
J-PARC Center, KEK, Tokai, 319-1106, Japan
⊥
ACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
*
S Supporting Information
we encounter much more unusual structural trends in salt-like
hydrides. As shown in Table S1, MH adopts rocksalt-type
6
ABSTRACT: A cubic variant of CaH adopting a fluorite-
2
type crystal structure was synthesized by cationic
substitution with La or Y, yielding the first alkaline earth
hydride-based with fluorite-type framework. The material
has a bandgap of ∼2.5 eV (greenish yellow in color),
crystal structure for M = Li−Cs, as could be expected from the
large ionicities of these compounds. However, the crystal
structures appearing for MH2 phases do not follow the
tendency seen for ionic compounds, e.g., fluorides. Like
MgF , MgH takes a rutile-type structure, but MH (M = Ca,
which is much smaller than that of orthorhombic PbCl -
2
2
2
2
type CaH (4.4 eV) and is, in fact, the smallest among
2
Sr, or Ba) adopts the PbCl -type (cotunnite-type) rather than
2
alkaline or alkaline earth metal hydrides reported to date.
Analysis of the density functional theory band structure of
the fluorite-type preferred by the corresponding fluorides.
Meanwhile, ScH , YH , or LnH (Ln: lanthanide ion)
2
2
2
cubic-CaH indicates that its conduction band minimum is
2
crystallizes in fluorite-type structures, whereas low oxidation
formed mainly by the interaction between the Ca 3d eg
orbitals around the crystallographic cavity defined by cubes
2+
state including Ln does not appear in fluorides generally.
Among these curious hydrides, CaH plays an important role as
−
2
of H ions. The use of such cavities in the creation of low-
it lies in boundary region between rutile-type, fluorite-type, and
lying conduction band minima by semiconductors is
extremely rare, and has similarities to inorganic electrides.
PbCl -type structure. The PbCl -type CaH structure observed
2
2
2
under ambient conditions, which we will denote as o-CaH (α-
2
CaH ), seems to capture a brief intermediate point as the
2
−
relative size of H ion shrinks drastically across the series from
−
n inorganic solids, the hydride ion, H might be expected to
−
MgH to CaH due to the size flexibility of the H ion. Its role
2
2
−
I
behave like the F ion, due to their of equal charge and
as a boundary phase is further highlighted by the discovery that
it undergoes a pressure-induced structural phase transition from
1
,2
−
−
similar ionic sizes. Similar to F , H commonly forms salt-
like ionic compounds with positive cations such as alkali-metal
ions in spite of its relatively small electron affinity. However,
they differ in the extent to which simple radius ratio
considerations can rationalize their structural chemistries, as is
7
PbCl -type to InNi -type at ∼15 GPa, as well as the existence
2
2
high temperature phase (β-CaH ) over the 1053−1256 K
whose crystal structure has not been confirmed experimentally.
2
8
Curiously, the fluorite-type structure that is anticipated to be a
summarized for MX and MX (X = F or H) compounds in
Table S1 in the Supporting Information. Generally, the crystal
low pressure phase relative to the PbCl -type has not yet been
2
2
3
observed for CaH . Finding conditions that fluorite-type CaH
2
2
structures of MF compounds, as well as the corresponding
oxides, or oxyfluorides with large ionicities, shift systematically
could be stabilized would thus be interesting from the point of
view of understanding the structural trends within this system.
2
from the rutile-type, to the fluorite- and PbCl -types, depending
on the cation/anion radius ratio. The anionic coordination
Furthermore, new polymorphs of CaH could be of interest for
catalytic applications, as Ru-loaded o-CaH2 has attracted
2
2
3
number of cation increases from 6 to 8 or 9. Fluorite-type
attention recently as an efficient catalyst for ammonia
9
structure appears for MF with M = Ca, Sr, or Ba. The high-
synthesis. Here we report Ca1−x
M
x
H2+x (M = Y or La, x =
2
pressure phases of MF compounds can be similarly under-
∼0.15), with fluorite-type structures. These phases are the first
alkaline earth metal hydrides adopting fluorite-type structure.
This cubic hydride exhibits a bandgap (2.5 eV) much smaller
2
stood when one factors in the larger compressibility of the
anions over the cations. Under pressure, the structures tend to
4
than that of the original o-CaH (4.4 eV), and is, in fact, the
shift as though the cation has a larger relative size. For
2
smallest among alkali or alkaline earth hydrides reported to
example, CaF transforms from the fluorite-type to the PbCl -
2
2
5
type at 9.5 GPa. Even more striking is the formation of the
InNi -type crystal structure with 11 coordination number of
Received: June 3, 2017
2
indium-type positions for X at extreme pressures. In contrast,
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b05746
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
4
date. A chemical bonding analysis through density functional
theory (DFT) band structure calculations will trace this unique
bandgap to the dominate role that tiny crystallographic cavity
spaces defined by H-cubes play in their conduction band
minima. This bonding situation arises from Ca−Ca covalent
interactions around the cavity centers, that is, Ca multicenter
bonding states stemming from the Ca 3d orbitals with relatively
deep binding energy.
dense packing than c-CaH . Thus, we can conclude that the
2
stability of c-CaH relative to o-CaH will be greatest at low
2
2
pressures.
Figure 2a−c shows calculated band structure of hypothetical
c-CaH along with isostructural ScH and TiH (using the
2
2
2
We began our investigation by exploring the phase stability
of two polymorphisms of CaH by theoretical methods: o-
2
CaH and the hypothetical cubic phase with fluorite-type
2
structure we denote as c-CaH (Figure 1). The two structures
2
Figure 2. Electronic band structures of MH hydrides with fluorite-
2
11
type structures. (a) Hypothetical c-CaH , (b) ScH , and (c) TiH .
2
2
2
The Brillouin zone for the cubic F-lattice, along with labels for the
special k-points, is shown in Figure S1 of the Supporting Information.
In the band diagrams, the contribution from M 3d e orbitals are
highlighted with a fat-band representation in red. Schematic orbital
g
Figure 1. Crystal structures of CaH . (a) Orthorhombic phase (PbCl -
type) and (b) a hypothetical cubic fluorite-type phase, overlaid with a
2
2
interactions at the points labeled 1−4 in the band diagram of CaH are
2
3
D electron density isosurface map of the DFT band structure in
shown in panel d. The gray, green, or blue orbitals belong to the ions
−4
−3
region from ECBM to ECBM + 0.1 eV at 1.2 × 10 Å . (c) 2D cross
section along the (110) plane of the partial electron density map
displayed as an isosurface in panel b.
at z = 0, z = 1/2, or z = 1/4 (or 3/4), respectively.
11
experimentally determined crystal structures ) for comparison.
In these panels, fatbands are used to highlight the orbital
bear a close relationship suggesting that their energy difference
could be small. In these two structures, the Ca ion sublattices
adopt the two simplest variants of close-packing the hexagonal
closest packed (hcp) and cubic closest packed arrangements,
respectively, in fact correspond to the β- and α-polymorphs of
elemental Ca. These two arrangements offer different
contributions from the Metal 3d e , the most likely contributor
g
to the conduction band. The band structure of these three
hydrides is very similar to that we presented for the
12
isostructural LnH2 in a previous report, except for the
position of Fermi energy (E ). To understand the specific
F
chemical bonding appearing in c-CaH , we focus on four key
2
interstitial spaces for the H ions to occupy. In o-CaH , there
are two crystallographic sites for hydrogen, H1 and H2, which
points labeled in Figure 2a, the orbital character for each of
which is shown in Figure 2d. The occupied states are based on
the H 1s, ranging from the H 1s−Ca 4s σ bonding state
indicated as point 1 (Γ, −6.5 eV) to the H 1s−H 1s σ*
antibonding state comprising the valence band maximum
(VBM) at point 2 (Γ, 0 eV), respectively. The full bandwidth
related to the occupied H 1s states, ∼6.5 eV, derives mainly on
the H 1s−H 1s interaction, as well as stabilization by
interactions with the Ca 4s. Though both the maximum and
minimum of the valence band occur at the Γ point, the
conduction band minimum (CBM) is located at the X point
(point 4), indicating that c-CaH2 is an indirect type
semiconductor with the calculated bandgap 0.2 eV. At the X
2
are coordinated with four and five Ca ions, respectively. Each
2+
−
Ca ion is then bound to nine H ions. In c-CaH , on the
2
other hand, the H atoms occupy the tetrahedral holes of the Ca
sublattice (sites we label as H due to their T site symmetry)
t
d
This arrangement creates a H sublattice simple cubic structure,
half of whose cubic holes are empty, leaving a tiny cavity site, V,
with O site symmetry at the center of the fcc unit cell. To
h
compare relative energies of these polymorphs, DFT periodic
calculations including structural relaxation were performed.
(
see Table S2 in the Supporting Information). The total energy
(E ) values indicate that o-CaH is slightly stable rather than c-
tot
2
CaH2 at 0 K. As MH2 compounds of alkaline-earth are
generally considered to be ionic compounds, the lattice energy
values were also calculated using MADEL for comparison.
The results suggest that electrostatic interactions favor o-CaH2.
This idea is supported by the larger bond valence sum of the Ca
point, contributions from the Ca 3d are evident near E . Ca 3d
F
t −H 1s σ interaction happens in the valence band at point 3
2g
10
(X, −2.9 eV). The remaining Ca 3d orbitals, the e set, form
g
unique multicenter bond through the cavity surrounded by
−
eight H anions (point 4, +0.2 eV, CBM). At this point, Ca−Ca
ions in o-CaH , indicating the electrostatic effect of ionic
interactions can give rise to a low-lying bonding state without
interference with H 1s, due to the restrictions of orbital
symmetry. To better see the character of the band near the
CBM, electron density maps in the energy region from ECBM to
ECBM +0.1 eV are shown in Figure 1b,c. Notably, the
2
packing, that is, atomic density, is a crucial factor for
competition between these crystal structures. These findings
agree well with the tendency of pressure-induced phase
transitions in ionic systems, which point to o-CaH being a
2
B
DOI: 10.1021/jacs.7b05746
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
accumulation of electron density from the Ca 3d orbital at the
y2
X point, together with other Ca 3d e orbitals at the equivalent
g
k-points, results in the appearance of the electron density peak
at the cavity site. This can be regarded as being based on
multicenter bonding states involving six Ca ions. This
electronic structure makes a sharp contrast with most
compound semiconductors such as those of the III-V and II-
VI types. In these more typical cases, the CBM states are mainly
focused on the constituent ions (especially on the cations).
Cavity spaces rarely play roles in the CBM or other
electronically active states. Instead, the shape of the CBM
electron density is very similar to that of CBM of the
1
3
antifluorite-type Mg Si semiconductor, or the states near the
2
Figure 3. (a) Diffuse reflectance spectra of o-CaH , c-Ca(F H ) (0
12
2
1−x
x 2
E of the LaH electride. Here, we emphasize the similarity
F
2
<
x < 0.6), and c-(Ca0.84La0.16)H2.12. (b) Estimated bandgaps. Those of
with electride compounds. In inorganic electrides, the crystallo-
graphic interspaces are occupied by anionic electrons, which
have high chemical reactivity owing to their small work
Ca0.84La0.16H2.12 and o-CaH are shown by a green dashed line and
blue square, respectively. (c) Photo of o-CaH and Ca0.84La0.16H2.12
pellets.
2
2
1
4
functions. We can find systematic variation of the band
structure from going from c-CaH to MH fluorite-type phases
cationic compositions measured by EPMA agreed well with
nominal ones. Figure S4 in the Supporting Information shows
the thermal desorption spectrum (TDS) of the Ca0.84La0.16Hx
powder, which yielded the chemical composition
Ca0.84La0.16H2.12, which is consistent with an assignment of an
approximately +3 the valence state to the La. The XRD pattern
of the cubic phases are similar to that of α-Ca with fcc-type
structure. Thus, the positions of H ions were determined via
neutron powder diffraction (NPD) analysis, in which the total
H-content is fixed to the value from the TDS measurement.
Results of the Rietveld structure refinements for
2
2
with M = Sc, Ti, or V (see Table S1). Following the general
periodic trends, we expect that M moves to right in the periodic
table, its 3d orbital energy level will deepen, while its band
filling will increase. At the same time, the cationic size of the M
site will decrease. All of these effects are evident in Figure 2,
where the 3d orbital band energies drops from Ca to Sc, and to
Ti. Simultaneously, the decrease of lattice sizes due to the
variation of cationic size enhances the interaction between H
ions, resulting in the increase of the VB width of H 1s. As
combined result of these effects, the VBM, that is, H 1s−H 1s
σ* antibonding state starts to overlap energetically with the
CBM, leading to the indirect bandgap closing. Simultaneously,
Ca0.84La0.16H2.12 are given in Figure 4, Tables S3 and S4, and
the E climbs to higher bands as the electron count rises.
F
On the basis of these calculations, we attempted the synthesis
of CaH by ionic substitution via solid state reactions, and the
2
details are given in the Supporting Information. In our first
experimental approach, we attempted anionic substitution of
−
CaF by H ion. The powder X-ray diffraction (XRD) patterns
2
in Figure S2a in the Supporting Information confirm the
formation of the cubic phases, Ca(F H ) , as reported in the
1
−x x 2
1
5
literature. The systematic shift of lattice constants evident in
Figure S2b supports the formation of solid solution in the
range, 0 < x < 0.6. These lattice constants almost agree with the
value obtained from the DFT geometrical optimization of c-
Figure 4. Rietveld refinement results for the structure Ca0.84La0.16H2.12
at 300 K from TOF-NPD data. Observed (gray dots), calculated
(black line), and difference profiles are presented. The vertical bars at
the bottom show the calculated positions of the Bragg reflections of
CaH (5.433 Å). All of the samples appeared white in color and
2
were insulating. Figure 3a shows the diffuse reflectance spectra
of the solid solutions along with spectra of some related phases.
As shown in Figure 3b, the absorption edge of the solid
Ca0.84La0.16H2.12
.
−
solution is shifted drastically to lower energy by H -
substitution, with the bandgap reached getting as low as 3.5
CIF file in the Supporting Information. The best fitting was
obtained using the model based on fluorite-type structure, in
which H partially at the V site with 16% occupancy. In this way,
eV for Ca(F H ) . This decrease in bandgap originates
0.4 0.6 2
mainly from the shallower VB energy position of the filled H 1s
level relative to that of the F 2p. Notably, the estimated
bandgap of Ca(F H ) (3.5 eV) is not only smaller than that
3
+
the excess H atoms needed to balance the charges of the La
cations are accommodated by the V sites. The formation of
these c-Ca1−xM H compounds by cationic substitution
0.4 0.6 2
of CaF but also that of o-CaH (4.4 eV), which agrees well
2
2
x
2+x
with the tendency revealed by DFT calculations in Table S2.
reminds us yttria stabilized zirconia (YSZ). In contrast with
This reflects the differences in electronic structure achievable
white color of o-CaH , the obtained M-substituted cubic phases
2
through the polymorphism of CaH , especially, in the CB
appear yellow in color (Figure 3c) and are insulating. The
2
electronic structure derived from the Ca 4s/3d orbitals.
diffuse reflectance spectrum is presented in Figure 3a,b
In our efforts to synthesize c-CaH , we also tried cationic
alongside those measured for c-Ca(F1−xH ) Notably, the
2
x 2
substitution of o-CaH using MH (M = Y or La). As shown in
estimated bandgap of 2.5 eV is much smaller than that of
Ca(F H ) (3.5 eV), and agrees well with the apparent color.
2
3
Figure S3 of the Supporting Information, the powder XRD
patterns for the Ca0.87Y0.13H and Ca0.84La0.16H compounds
0.4 0.6 2
The calculated bandgap of c-CaH from DFT, 0.2 eV (Table
x
x
2
obtained can be indexed with cubic F-centered lattices. The
S2) is far smaller than this experimental value, which is
C
DOI: 10.1021/jacs.7b05746
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
1
6
attributed to the well-known bandgap problem of DFT. As far
as we know, c-Ca1−xM H has the smallest bandgap among
(11) (a) Venturini, E. L.; Morosin, B. Phys. Lett. A 1977, 61, 326−
28. (b) Irving, P. E.; Beevers, C. J. Metall. Trans. 1971, 2, 613−615.
3
(
x
2+x
12) Mizoguchi, H.; Okunaka, M.; Kitano, M.; Matsuishi, S.;
the hydrides of alkali and alkaline-earth metals. As shown in
Figure 3a, the optical spectra of all the cubic phases stand up
slowly around bandgaps. This feature originates from the
indirect type transition, which is shown in the band structure of
Figure 2a.
In conclusion, CaH -based having fluorite-type structures
were synthesized by partial substitution of Ca with La . The
obtained yellow insulating sample has bandgap of ∼2.5 eV,
which is much smaller than that of o-CaH (4.4 eV). Analysis of
band structure by DFT calculations indicates that Ca 3d
orbitals play an important role near E in c-CaH . The CBM
Yokoyama, T.; Hosono, H. Inorg. Chem. 2016, 55, 8833−8838.
13) Mizoguchi, H.; Muraba, Y.; Fredrickson, D. C.; Matsuishi, S.;
Kamiya, T.; Hosono, H. Angew. Chem., Int. Ed. 2017, 56, 10135−
0139.
14) (a) Dye, J. L. Acc. Chem. Res. 2009, 42, 1564−1572.
(
1
(
2
(b) Matsuishi, S.; Toda, Y.; Miyakawa, M.; Hayashi, K.; Kamiya, T.;
Hirano, M.; Tanaka, I.; Hosono, H. Science 2003, 301, 626−629.
(c) Miao, M.; Hoffmann, R. J. Am. Chem. Soc. 2015, 137, 3631−3637.
(15) (a) Brice, J. F.; Courtois, A.; Aubry, J. J. Solid State Chem. 1978,
2
+
3+
2
2
4, 381−387. (b) Leveque, R.; Zanne, M.; Vergnat-Grandjean, D.;
Brice, J. F. J. Solid State Chem. 1980, 33, 233−243.
16) Jones, R. O.; Gunnarsson, O. Rev. Mod. Phys. 1989, 61, 689−
46.
F
2
(
7
state is formed mainly by the interaction between Ca 3d eg
around crystallographic cavity spaces.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b05746.
■
*
S
Geometry optimization for orthorhombic and cubic
CaH , Brillouin zone for cubic F-lattice, powder XRD
2
patterns for Ca(F1−xH ) and Ca M H (M = Y or
x 2
1−x
x
2+y
La), and TDS spectrum for Ca0.84La0.16H2+y (PDF)
Data for Ca0.84La0.16H2.12 (CIF)
AUTHOR INFORMATION
Corresponding Authors
mizoguchi@mces.titech.ac.jp
hosono@lucid.msl.titech.ac.jp
■
*
*
ORCID
Hiroshi Mizoguchi: 0000-0002-0992-7449
Takashi Honda: 0000-0003-4121-8957
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This study was supported by the JSPS KAKENHI Grant (No.
7H06153); by the JST ACCEL project; by the Neutron
■
1
Scattering Program Advisory Committee of IMSS, KEK
Proposal No. 2014S06). We are grateful to Drs. D. C.
(
Fredrickson, S. Matsuishi, and M. Kitano (Tokyo Institute of
Technology) for useful discussion.
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DOI: 10.1021/jacs.7b05746
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX