YHO, an Air-Stable Ionic Hydride

Article abstract of DOI:10.1021/acs.inorgchem.9b02308

Metal hydride oxides are an emerging field in solid-state research. While some lanthanide hydride oxides (LnHO) were known, YHO has only been found in thin films so far. Yttrium hydride oxide, YHO, can be synthesized as bulk samples by a reaction of Y₂O₃ with hydrides (YH₃, CaH₂), by a reaction of YH₃ with CaO, or by a metathesis of YOF with LiH or NaH. X-ray and neutron powder diffraction reveal an anti-LiMgN type structure for YHO (Pnma, a = 7.5367(3) ?, b = 3.7578(2) ?, and c = 5.3249(3) ?) and YDO (Pnma, a = 7.5309(3) ?, b = 3.75349(13) ?, and c = 5.3192(2) ?); in other words, a distorted fluorite type with ordered hydride and oxide anions was observed. Bond lengths (average 2.267 ? (Y-O), 2.352 ? (Y-H), 2.363 ? (Y-D), >2.4 ? (H-H and D-D), >2.6 ? (H-O and D-O), and >2.8 ? (O-O)) and quantum-mechanical calculations on density functional theory level (band gap 2.8 eV) suggest yttrium hydride oxide to be a semiconductor and to have considerable ionic bonding character. Nonetheless, YHO exhibits a surprising stability in air. An in situ X-ray diffraction experiment shows that decomposition of YHO to Y₂O₃ starts at only above 500 K and is still not complete after 14 h of heating to a final temperature of 1000 K. YHO hydrolyzes in water very slowly. The inertness of YHO in air is very beneficial for its potential use as a functional material.

Full text of DOI:10.1021/acs.inorgchem.9b02308

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
YHO, an Air-Stable Ionic Hydride
Nicolas Zapp, Henry Auer, and Holger Kohlmann*
Inorganic Chemistry, Leipzig University, Leipzig 04109, Germany
S
* Supporting Information
ABSTRACT: Metal hydride oxides are an emerging field in
solid-state research. While some lanthanide hydride oxides
(LnHO) were known, YHO has only been found in thin films so
far. Yttrium hydride oxide, YHO, can be synthesized as bulk
samples by a reaction of Y2O3 with hydrides (YH3, CaH2), by a
reaction of YH3 with CaO, or by a metathesis of YOF with LiH
or NaH. X-ray and neutron powder diffraction reveal an anti-
LiMgN type structure for YHO (Pnma, a = 7.5367(3) Å, b =
3.7578(2) Å, and c = 5.3249(3) Å) and YDO (Pnma, a =
7.5309(3) Å, b = 3.75349(13) Å, and c = 5.3192(2) Å); in other
words, a distorted fluorite type with ordered hydride and oxide
anions was observed. Bond lengths (average 2.267 Å (Y−O),
2.352 Å (Y−H), 2.363 Å (Y−D), >2.4 Å (H−H and D−D), >2.6 Å (H−O and D−O), and >2.8 Å (O−O)) and quantum-
mechanical calculations on density functional theory level (band gap 2.8 eV) suggest yttrium hydride oxide to be a
semiconductor and to have considerable ionic bonding character. Nonetheless, YHO exhibits a surprising stability in air. An in
situ X-ray diffraction experiment shows that decomposition of YHO to Y2O3 starts at only above 500 K and is still not complete
after 14 h of heating to a final temperature of 1000 K. YHO hydrolyzes in water very slowly. The inertness of YHO in air is very
beneficial for its potential use as a functional material.
of the smaller ones seem to be fixed at 1:1. YHO was observed
only in thin films,18 and its crystal structure was discussed on
the basis of quantum-mechanical calculations.19 The large
lanthanide ions (La−Nd) crystallize in a fluorite type
superstructure with a space group of P4/nmm, in which the
anions are ordered (LaHO structure type).14,20−22 The
lanthanum, cerium, and praseodymium compounds are
hygroscopic and react quickly with humid air. While LaHO
and PrHO can be stored in dry air, CeHO reacts pyrophori-
cally to form CeO2. No report is given on the air stability of
NdHO.
In our study, we aimed to expand the known REHO series
toward yttrium and investigate its thermal resistivity toward air
at elevated temperatures by in situ powder X-ray diffraction
(PXRD). Furthermore, YHO can form from yttria Y2O3 and
has a similar diffraction pattern. As yttria is an important
material for luminophors,23 spectral filtering,24 and quantum
information processing, for example,25 this study provides an
important insight into its chemistry under reducing conditions.
INTRODUCTION
■
Hydrogen-containing functional materials are usually con-
textualized with hydrogen storage concepts. Apart from this,
the employment of hydrides in catalysts,1,2 battery materi-
als,3−5 and phosphors6 is becoming more important. LiLa2HO3
is a prominent example, as it was the first material to show
pure hydride ion H− conduction.7 It is a hydride oxide
(sometimes also called oxyhydride and not to be confused with
hydroxides), a term generalizing substances whose anionic
sublattices are composed of both hydrogen and oxygen. This
class of compounds is the least developed in comparison to the
group of mixed anionic oxides, which are an emerging field for
the development of new materials.8 Other recent examples for
hydride oxides are LaSrCoH0 . 7 O3 ,9 SrVO2 H,10
11
12
Sr3Co2H0.84O4.33
,
and BaTiHxO3−x
,
which are promising
candidates in the design of hydrogen-containing functional
materials, as some of them are usually inert to air. This is an
outstanding behavior, as ionic hydrides such as lithium hydride
(LiH) and calcium hydride (CaH2) decompose quickly in air.
Hydride oxides with the general formula REHO (RE: rare
earth element) were first reported in the 1960s13 and studied
more intensively during recent years. They crystallize in two
different structure types. The fluorite structure type with space
EXPERIMENTAL SECTION
■
Four different methods were employed for the synthesis of yttrium
hydride oxide YHO (Table 1) using (a) yttria Y2O3 and yttrium
hydride YH3, (b) yttria and calcium hydride CaH2, (c) yttrium
hydride and calcium oxide CaO, and (d) yttrium oxide fluoride YOF
and alkaline hydride AH (A = Li or Na). Unless otherwise stated, all
group Fm3
̅
m is known for La−Pr and Sm−Er.2,13−17 It exhibits
a face centered cubic lattice of RE atoms, whose tetrahedral
voids are occupied by H and O in a disordered fashion. For
SmHO, GdHO, and HoHO, long-term stability in air at room
temperature was shown.16,17 Variable H/O ratios were
reported for the large lanthanides (La−Pr), whereas the ratios
Received: July 30, 2019
© XXXX American Chemical Society
A
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before starting the measurement that lasted 15 min. In total, each
temperature step lasted 30 min.
Table 1. Overview of the Different Synthesis Strategies for
YHO
a
PXRD data were analyzed by the Rietveld technique using the
software Topas version 5 (Bruker AXS) and full profile analysis. The
background was fitted using a Chebychev fifth order polynomial.
PND data were analyzed with FullProf using a pseudo-Voigt reflection
profile. A polynomial of third (YDO) and fifth (YHO) order
described the background. In both PXRD and PND, zero-point error,
scaling factor, lattice, profile and atomic parameters, and isotropic
displacement factors Biso were refined if the phase ratio exceeded 10
wt %; otherwise only the former four parameters were refined. The
label
reaction scheme
yield/wt %
a
b
c
Y2O3 + YH3 → 3YHO
83
85
95
81
72
Y2O3 + CaH2 → 2YHO + CaO
YH3 + CaO → YHO + CaH2
YOF + NaH → YHO + NaF
YOF + LiH → YHO + LiF
d
a
Experiments were conducted in fused silica ampules under an argon
B
iso values from PXRD data were corrected by an offset value (overall
atmosphere at 975 K < T < 1175 K (a, b, and c) or in aluminum
crucibles under a hydrogen atmosphere and 725 K (d). The yield
refers to the weight ratio of yttrium-containing phases, which were
determined by a Rietveld analysis.
B). It was refined from the diamond reflections using a literature value
for diamond (Biso(C) = 0.142 Å2)29 and fixed afterward. The H
content of YHO obtained from Y2O3 and YH3 was determined by the
carrier gas hot extraction method on four specimens using a VARIO
EL microanalyzer (Elementar Analysensysteme GmbH).
The density functional theory (DFT) calculations were performed
with the Vienna ab initio simulation package (VASP)30,31 version 5.4.4
using projector-augmented wave pseudopotentials (PAWs)32 from
the VASP database. In case of the rare earth elements, PAWs
simulating trivalent ions were employed for RE = Ce−Lu (RE_3),
and for yttrium, the s- and p-levels of the fourth principal quantum
number were treated as semicore states (Y_sv). For lanthanum,
hydrogen, and oxygen, the standard PAWs were used (La, H, O). The
exchange correlation potential was approximated with the GGA-PBE
samples were handled in an argon filled glovebox, in which the oxygen
and water content were kept below 1 ppm. Yttria (Aldrich, 99.99%)
was dried prior to use at 1373 K for 6 h26 and transferred while hot
into the glovebox. Yttrium hydride or yttrium deuteride was obtained
from metallic yttrium (Smart Elements, 99.99%, stored under argon)
and hydrogen (Air Liquide, 99.9%) or deuterium (Air Liquide,
̈
99.8%) at 2−5 MPa and 573 K in Inconel (Bohler L718V) autoclaves.
Yttrium oxide fluoride was obtained from yttria and a slight excess
(1.1 equiv) of powdered polytetrafluoroethylene (Alfa Aesar) at 823
K in air.27 Calcium hydride was obtained from calcium (Alfa Aesar,
99.5%) and hydrogen at 6 MPa and 675 K; calcium oxide was
prepared from calcium carbonate at 1275 K and transferred while hot
into the glovebox. Syntheses (a), (b), and (c) were conducted in
fused silica glass ampules, and synthesis (d) was conducted in sealed
aluminum crucibles. For synthesis (a), 1−2 g of 1:2 molar mixtures of
yttria:yttrium hydride or deuteride, respectively, was annealed at 1173
K for 48 h with a heating rate of 50 K h−1. For synthesis (b), 0.4 g of a
1:4 molar mixture of yttria:calcium hydride was annealed at 1173 K
for 48 h with a heating rate of 50 K h−1.16 For synthesis (c), 0.1 g of a
1:1 molar mixture of yttrium hydride:calcium oxide was heated at a
rate of 25 K h−1 to 973 K and annealed for 48 h. For synthesis (d), 35
mg (NaH) and 30 mg (LiH) of 1:1.3 molar mixtures of yttrium oxide
fluoride:sodium hydride (Aldrich, 95%) and 1:1 molar mixtures of
yttrium oxide fluoride:lithium hydride were transferred into a
differential scanning calorimeter (DSC, Q1000, TA Instruments)
with an attached pressure chamber and heated to 723 K at 1 MPa
hydrogen gas with a rate of 1 K min−1; then, the mixture was annealed
for 2 h and cooled at the same rate to 300 K. After 0.5 h, the
procedure was repeated. To investigate the air stability of YHO at
room temperature, a sample was prepared via synthesis (a) for
powder X-ray diffraction (PXRD) in Guinier geometry (for details,
see below) with exposure to air while collecting PXRD patterns in
regular intervals. The stability toward liquid water was investigated by
grinding a sample of (a) under deionized water for 1 min before
preparing a Guinier PXRD sample.
method,33 and the Brillouin zone was integrated using the tetrahedron
34
̈
method with Blochl corrections. The k-grid was generated
automatically and Γ-centered, ensuring a k-point density of 0.03
Å−1. The cutoff energy was set to 800 eV. Structure optimizations
were conducted with full degrees of freedom (cell volume, cell shape,
and atomic positions) using a conjugate gradient algorithm,
converging forces to 10 μeV Å−1 and electronic energies to 1 μeV.
Thus, obtained energies were used to calculate free reaction
enthalpies ΔrG. The energies of RE2O3 and REH3 stem from
̅ ̅
structures with space groups Ia3 (bixbyite structure type) and P3c1
(YD3 structure type), respectively, which exhibited smaller values than
other modifications. The optimized structures were used as input for
density of states (DOS) calculations. Here, the k-grid was enhanced
to a k-point density of 0.01 Å−1, and the DOS were calculated with a
resolution in energy of 1 meV.
RESULTS AND DISCUSSION
■
The reaction of yttrium oxide and yttrium hydride produced a
gray powder. As the latter decomposes at elevated temper-
atures,35 an excess of it ensured high yield. However, small
amounts of yttrium hydride (5−20 wt %) remained in the
sample. The sample’s color is thus a mixture of the dark
colored yttrium hydride and the reaction products. PXRD
analysis showed a new phase with a reflection pattern similar to
Y2O3 or LaHO as well as reflections belonging to fluorite type
YHO (Figure S1 in the Supporting Information). The latter
has only been reported in thin films.18 Indexing the remaining
reflections yielded a primitive orthorhombic cell with a = 7.537
Å, b = 7.516 Å, and c = 5.325 Å. A model in P21mn, derived
from LaHO in P4/nmm via Pmmn, explained all the reflections
of the PXRD pattern. The hydrogen content of the sample,
determined by CHN elemental analysis, suggested the
composition of YHO (exp.: 1.220(5) wt %, calc.: 1.14 wt
%). The other synthesis approaches (Table 1, b−d) yielded
orthorhombic YHO, whose lattice parameters are not
significantly different (Figures S3−S6). Thus, YHO seems to
exhibit a fixed composition. The DSC experiments of the
reaction of YOF with LiH and NaH show broad exothermic
signals at mild conditions (ca. 600 K; Figures S9 and S10). To
determine the atomic positions of hydrogen in YHO, neutron
diffraction data were collected on a deuterated sample of YDO
Powder neutron diffraction (PND) data of YDO and YHO
obtained from route (a) were collected on the E9 high resolution
diffractometer at Helmholtz Zentrum Berlin (HZB) at 298(2) K
using a vanadium sample holder of 6 mm diameter.28 All powder X-
ray diffraction experiments were conducted with Cu Kα1 radiation
from curved Ge(111) monochromators at T = 296(2) K. Most
specimens were measured in Debye−Scherrer geometry on a Stoe
STADI-P diffractometer. Samples were mixed with powdered
diamond and filled into 0.3 mm glass capillaries (WJM Glas/Muller
̈
GmbH, Berlin) prior to fusing. Experiments in Guinier geometry were
conducted on a Huber G670 diffractometer. The powders were
dispersed in Apiezon grease and placed either on top of one or
between two Kapton sheets (for noninert or inert measurements,
respectively). In situ data were collected on a Rigaku Smart Lab
diffractometer using an Anton Paar HTK1200 reaction chamber and
Bragg−Brentano geometry. The sample (YHO, obtained from route
(a)) was heated in 25 K steps from 300 to 1075 K with a rate of 5 K
min−1 in air. At each step, the temperature was equilibrated for 10 min
B
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2
(D = H, Figure 1). D has a significantly smaller incoherent
subgroup relationship between the YDO and LaHO structures,
but of said crystallographic relationship, there is an indirect
scattering length compared to H (bi(1H) = 25.3 fm, bi(D) =
1
̈
one via the aristotype CaF2 in Fm3
Figure S12). Similar structures (i.e., bixbyite type Y2O3 (Ia3
̅
m (cf. Barnighausen tree in
),
̅
̅
YOF type (R3m), and LaOF type (P4/nmm)) are also found in
different branches of the fluorite type symmetry tree.38 The
most related structure is that of LaOF, which is obtained from
a klassengleiche transformation of the Pt2Si type, which is in
turn an aristotype of YDO.
The coordination polyhedron of yttrium atoms in YDO is a
distorted tetragonal prism; deuterium and oxygen atoms are
both coordinated in a distorted tetrahedral fashion (Figure 2).
Figure 2. Coordination polyhedra in the crystal structure of YDO.
Figure 1. Rietveld refinement of the crystal structure of YDO (top)
and YHO (bottom) obtained from synthesis route (a) (Table 1) on
the basis of PND data at 298(2) K (E9, HZB, Germany, λ =
1.79820(2) Å). Top: Rwp = 7.7%, GooF = 1.6. Bragg markers denote
from top to bottom YDO Pnma (68.2(7) wt %, RBragg = 4.2%, a =
Table 3. Atomic Distances in YDO (Pnma) and YHO
(Pnma) up to 3 Å on the Basis of Powder Neutron
Diffraction Data at 298(2) K
7.5309(3) Å, b = 3.75349(13) Å, c = 5.3192(2) Å), YDO Fm3
̅
m
distance/Å
(12.3(3) wt %, a = 5.28501(14) Å), and YD2 Fm3m (19.4(4) wt %, a
̅
= 5.19688(8) Å). Bottom: Rwp = 1.6%, GooF = 2.1. Bragg markers
denote from top to bottom YHO Pnma (71(1) wt %, RBragg = 11.4%, a
atom 1
Y
atom 2
D/H
quantity
YDO
YHO
1
2
1
2.308(6)
2.355(3)
2.425(6)
2.307(8)
2.372(6)
2.377(7)
= 7.5367(3) Å, b = 3.7578(2) Å, c = 5.3249(3) Å), YHO Fm3
̅
m
(10.7(6) wt %, a = 5.30227 Å), and YH2 Fm3m (18.2(8) wt %, a =
̅
5.2081(2) Å). Red: measurement. Black: calculated. Blue: difference.
An offset of 104 counts was added to the difference curve of YHO.
Rietveld refinements of PXRD data of both structures are provided in
Figures S1 and S2 in the Supporting Information.
a
a
2.363
2.352
Y
O
1
2
1
2.249(6)
2.257(3)
2.292(6)
2.217(5)
2.273(3)
2.314(5)
a
a
2.266
2.268
4.0 fm),36 which results in a lower background. Starting from
the anion positions derived from LaHO, O and D sites were
successfully allocated and positions refined. Furthermore, the
1:1:1 composition was confirmed by the refinement. A
subsequent symmetry search resulted in a smaller cell (b′ =
b/2) with space group Pnma and an isotypical structure to
LiMgN37 (Table 2). YDO is thus the first representative of the
LiMgN antitype. There is no direct crystallographic group−
D/H
D/H
D/H
O
2
1
1
2
2.408(3)
2.676(6)
2.684(6)
2.696(4)
2.440(4)
2.633(11)
2.727(11)
2.676(4)
a
a
2.685
2.679
O
O
2
2.866(4)
2.879(3)
a
Mean values are given in italics.
The Y−D and Y−O bond lengths (Table 3) agree with those
in trigonal YD3 (Y−D: 2.13−2.52 Å)39 and bixbyite type Y2O3
(Y−O: 2.24−2.33 Å).40 The large D−O bond distances
illustrate the anionic nature of D and prove that YDO is a
hydride oxide and not a hydroxide (D−O distance in YOOD:
0.959 Å).41 The D−D bond distance is slightly smaller than
that of O−O due to the higher Coulomb repulsion of O2−
compared to D−. In comparison with LaHO, the yttrium
polyhedra show a significant tilt toward a (5.0(1)°, cf. Figure 3;
LaHO: 2.803(3)°,20 NdHO: 4.049(2)°).22 This results from
the smaller size of yttrium compared to lanthanum ions (r(Y3+)
= 1.159 Å, r(La3+) = 1.300 Å),42 as the tilt enables the anions
to better shield the cationic charges. It furthermore demands
Table 2. Crystal Structure Parameters of YDO and YHO on
the Basis of Powder Neutron Diffraction Data at 298(2) K
a
2
atom Wyckoff label
x
y
z
Biso/Å
Y
4c
4c
4c
•
*
•
*
•
*
0.3756(7)
0.3734(5)
0.6434(5)
0.6442(4)
0.0996(5)
0.1031(6)
1/4
1/4
1/4
1/4
1/4
1/4
0.2806(5)
0.2782(5)
0.4852(7)
0.4828(8)
0.5156(8)
0.508(2)
0.20(3)
0.52(4)
0.24(6)
0.51(6)
1.67(9)
2.29(9)
O
D
H
a
YDO: •, Pnma, a = 7.5309(3) Å, b = 3.75349(13) Å, c = 5.3192(2)
Å. YHO: *, Pnma, a = 7.5367(3) Å, b = 3.7578(2) Å, c = 5.3249(3) Å.
C
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F− and O2−, this segregation results in an inhomogeneous
shielding of the cationic charges. As fluorine is highly
electronegative, it can compensate this, contrasting hydrogen,
which has only a medium electronegativity. Therefore, the
anions in hydride oxides REHO will surround the cations more
homogeneously, which is why they adopt different structure
types than oxide fluorides REOF. A similar anionic ordering
scheme to REHO is found in lithium hydride nitride Li4NH.45
The driving force for the formation of anionically ordered or
disordered REHO compounds was attributed in the literature
to the bonding behavior of oxygen: as the RE−O bond length
increases with the RE radius, an ordered modification is
preferred for larger RE cations because they enable shorter
RE−O bond distances.2 In YDO, a similar driving force might
be present, as the ordering also enables a shorter RE−O
distance (Figure S14). Thus, the anti-LiMgN structure type
might be suitable for other small rare earth elements as well.
The crystal structure of YHO, as refined with PND data
(Figure 1, Table 2), is very similar to that of YDO. The major
differences are the lattice parameters and the thermal
displacement factors (Table 2), which are caused by the
isotopic effect of deuterium with a doubled mass as compared
to protium. The high RBragg value results from the inferior
signal-to-noise ratio, which is due to the high incoherent
Figure 3. Crystal structures of YDO (left) and LaHO (right).20
the cations to shift from another in the c direction, which is
incompatible with the symmetry in P4/nmm. Here, half of the
lanthanum atoms, La3, are situated on a mirror plane.
The second major difference between YDO and LaHO is
the ordering scheme of the anions in the polyhedra
surrounding the yttrium and lanthanum atoms, respectively.
From the seven ways to distribute two atomic species on the
vertices of a cube (Figure 4), the one with m symmetry is
1
scattering length of H. Further details of the crystal structure
investigations may be obtained from FIZ Karlsruhe, Eggen-
stein-Leopoldshafen 76344, Germany (Fax: + 49-7247-808-
666. E-mail: crysdata@fiz-karlsruhe.de) on quoting the
deposition number CSD-1903936 (YDO) and CSD-1903938
(YHO).
DFT calculations of the anti-LiMgN and the LaHO
structure type for REHO (RE = Y, La−Lu) show a structure
directing effect of the cation radius. The free reaction enthalpy
of the reaction as a function of the ionic radii r (Figure S16)
shows a stabilization of the anti-LiMgN over the LaHO
structure type with decreasing r. The offset of yttrium and
lanthanum in the plot is due to the different pseudopotentials,
which did not contain f states. The difference in the value of
YHO to a previous publication of ours17 stems from an
updated pseudopotential of yttrium. A calculation of the DOS
confirmed the ionic character of YHO (Figure S17). In
comparison with that of Y2O3 (Figure S18), the smaller band
gap (Egap(YHO) = 2.8 eV, Egap(Y2O3) = 4.1 eV), which results
from the hydrogen states near the Fermi level, and the higher
dispersion is evident. Therefore, YHO seems to be a
semiconductor. It should be noted that DFT calculations are
known to underestimate band gaps.46 The experimental Egap of
Y2O3, for example, is about 5 eV.47
To evaluate the air stability of YHO, an in situ PXRD
experiment was conducted from 300 to 1075 K in air (Figure
5). Orthorhombic YHO starts to decompose only at above 500
K, and even at 1025 K, a significant amount is still present.
Fluorite type YHO is observed up to 600 K, and YH2 is
observed until 725 K. Bixbyite type Y2O3 is the decomposition
product of each phase (2YHO + O2 → Y2O3 + H2O). The
reaction mechanism of YHO to Y2O3 needs further evaluation,
as at least one intermediate phase is observed with a reflection
pattern similar to Y2O3. This disturbed the refinement of Y2O3,
Figure 4. The ordering scheme for anions X and Y in fluorite type
related compounds MXY has seven possible rotationally distinct two-
coloring of the eight vertices of a cube. The respective point
symmetries given below are often lowered in compounds MXY by the
distortion of the cubes. Ordering schemes for different anionic ratios
are provided in Figure S15.
̅
found in YDO, and those with 43m, 4mm, and mmm
symmetries are found in LaHO. They result from the ordering
of hydrogen (deuterium) and oxygen atoms and the tendency
of similar ions to occupy symmetrically equivalent positions;
the latter is equivalent to Pauling’s rule of parsimony.43 Only
specific Wyckoff positions are compatible with these require-
ments (i.e., 4c in YDO and 8i in LaHO), causing different
coordination environments. In comparison with oxide fluorides
REOF, which show ordering schemes with symmetry 3m (YOF
particularly at 800
50 K, which is why the phase ratios
type R3
̅
m) and 4mm (LaOF type P4/nmm),44 the tendency of
derived here show inconsistencies. The corresponding
intensity maximum is a result of interfering reflections of all
these phases. The total duration of the experiment, 16 h,
further illustrates the remarkable stability of YHO in air. A
hydrogen (deuterium) atoms to occupy neighboring positions
to oxygen atoms is prominent; this is in contrast to fluorine
atoms, which tend to segregate. Due to the different charges of
D
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can still be observed at 1025 K after 14 h of heating.
Furthermore, YHO hydrolyzes only slowly in liquid water.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02308.
PXRD analysis of YHO obtained from Y2O3, YH3, and
YOF; PXRD data of YHO stored in air; PXRD data of
YHO treated with liquid water; DSC curves of YOF +
AH (A = Li, Na); full in situ diffraction plot;
̈
Barnighausen tree; comparison of the unit cells of
orthorhombic YDO, cubic REHO, and tetragonal
LaHO; correlation of normalized cell volume and
polyhedral volumes of O and H with cationic radius;
ordering schemes for MXxY2−x (0 < x < 2) compounds;
DFT results; and DOS plots of YHO and Y2O3 (PDF)
Figure 5. In situ X-ray diffraction data and phase analysis of the
thermal decomposition of YHO in air (heating rate: 5 K min−1). Top:
part of diffraction data (λ: CuKα1; square root of intensity). Bottom:
phase ratios from Rietveld refinement. Error bars are smaller than the
displayed symbols. The full diffraction plot is shown in Figure S11.
Accession Codes
CCDC 1903936 and 1903938 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by e-
mailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
similar thermal stability of an ionic hydride was only observed
for the hydride silicate LiSr2(SiO4)H.48 At room temperature,
YHO did not react with air in 20 days. Only the YH2 phase
showed a decrease in intensity; the decomposition product is
probably amorphous (Figure S7). Treatment of YHO with
liquid water did not produce any gas bubbles or a change in
color. The Rietveld refinement showed a decrease in the phase
ratio of YHO toward YH2 (85.8(1)% to 75.7(2)%), which
indicates a slow hydrolysis to a probably amorphous product
(cf. Figure S8).
For thin films, the hydrolysis of YH2 led to the formation of
isotypic YHxOy.18 After treatment with water, a group of
reflections between those of YH2 and YHO appears, indicating
a variable composition of cubic YHO at mild conditions.
Treatment with mineralic acids (pH <1) leads to a
decomposition of YHO under formation of hydrogen gas.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: holger.kohlmann@uni-leipzig.de. Phone: +49 341 97
36201.
ORCID
Holger Kohlmann: 0000-0002-8873-525X
Author Contributions
This manuscript was written through contributions of all
authors.
Funding
This work was supported by funds from Leipzig University via
̈
a Ph.D. scholarship (Doktorandenforderplatz) and the
Deutsche Forschungsgemeinschaft (DFG, Grant Ko1803/12-
1).
CONCLUSION
■
We reported on the synthesis of orthorhombic yttrium hydride
oxide, YHO, a new representative of the REHO compound
class, and we analyzed its crystal structure by powder neutron
and powder X-ray diffraction. It is the first example of the anti-
LiMgN structure type with space group Pnma, and it is thus
related to the LaHO, YOF, LaOF, and bixbyite structure types
via the fluorite aristotype, in which most rare earth hydride
oxides crystallize. Additionally, a second modification of YHO
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is dedicated to Dr. Robert Haberkorn on the
occasion of his 65th birthday. We thank Alrik Dorn (Leipzig
University) for help with the synthesis, Christian Pflug
(Leipzig University) for providing educts, Manuela Roßberg
(Leipzig University) for conducting the elemental analysis, and
Harald Krautscheid (Leipzig University) for granting access to
an X-ray powder diffractometer. The Ph.D. scholarship from
̅
(fluorite type, Fm3m) was observed in a bulk material for the
first time. Orthorhombic YHO can be obtained by different
synthesis routes, starting from Y2O3, YH3, or YOF, with
varying yields. DFT calculations confirmed its ionic character.
These calculations predicted a semiconducting behavior and
the possibility of other small rare earth elements to crystallize
in this structure type. The stability of YHO in air and moisture,
as observed in situ at elevated temperatures and ex situ at room
temperature, is outstanding for an ionic hydride, and this
stability marks YHO’s significance in the search for functional
materials containing anionic hydrogen-like catalysts or
phosphors, as it is stable toward oxygen up to 500 K and
̈
Leipzig University (Doktorandenforderplatz) is greatly appre-
ciated. We thank the Helmholtz-Zentrum Berlin for providing
beamtime and Dr. Alexandra Franz (Helmholtz-Zentrum
Berlin) and Anton Werwein (Leipzig University) for carrying
out the neutron diffraction experiments. Computations for this
work were done with resources of the Leipzig University
Computing Centre. We thank A. Rost (Leipzig University) for
his support as the software administrator.
E
DOI: 10.1021/acs.inorgchem.9b02308
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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High-Pressure Polymorphs of LaHO with Anion Coordination
Reversal. J. Am. Chem. Soc. 2019, 141, 8717−8720.
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