Structural Distortion in Perovskite Type KCaH3–xFx (0.54 ≤ x ≤ 3)

Article abstract of DOI:10.1002/zaac.201900309

Representatives of the solid solution series KCaH_3–xF_x were synthesized by solid state reactions from binary metal hydrides and fluorides. Crystal structures were analyzed by Rietveld refinement based on X-ray powder diffraction. The degree of substitution was determined by refinement of site occupancy factors as well as elemental analysis for hydrogen. Three sections of x in KCaH_3–xF_x can be distinguished. For x < 0.54 no hydride fluoride exists, i.e. there is no hydride of the composition KCaH₃ and the solid solution starts only at x = 0.54. The tetragonal SrTiO₃ type structure with partial ordering of hydrogen and fluorine atoms is found for 0.54 ≤ x ≤ 1.7. Both anion positions show mixed occupation with some preference of hydrogen atoms for 8h and fluorine atoms for 4a sites (I4/mcm, SrTiO₃ type). For fluorine-rich compounds a solid solution with orthorhombic GdFeO₃ type structure (Pnma) and a perfectly statistical distribution of hydrogen and fluorine atoms is found (1.8 ≤ x ≤ 3). Interatomic distances resulting from the structure refinements are in the range of typical K–H, K–F, Ca–H, and Ca–F distances for mainly ionic compounds.

Full text of DOI:10.1002/zaac.201900309

Journal of Inorganic and General Chemistry
ARTICLE
DOI: 10.1002/zaac.201900309
Zeitschrift für anorganische und allgemeine Chemie
Structural Distortion in Perovskite Type KCaH3–xFx (0.54 Յ x Յ 3)
Christian Pflug[a] and Holger Kohlmann*[a]
Dedicated to Prof. Dr. Hans-Jörg Deiseroth on the Occasion of his 75th Birthday
Abstract. Representatives of the solid solution series KCaH3–xFx were
synthesized by solid state reactions from binary metal hydrides and anion positions show mixed occupation with some preference of
fluorides. Crystal structures were analyzed by Rietveld refinement hydrogen atoms for 8h and fluorine atoms for 4a sites (I4/mcm, SrTiO3
based on X-ray powder diffraction. The degree of substitution was type). For fluorine-rich compounds a solid solution with orthorhombic
determined by refinement of site occupancy factors as well as elemen- GdFeO3 type structure (Pnma) and a perfectly statistical distribution
tal analysis for hydrogen. Three sections of x in KCaH3–xFx can be of hydrogen and fluorine atoms is found (1.8 Յ x Յ 3). Interatomic
of hydrogen and fluorine atoms is found for 0.54 Յ x Յ 1.7. Both
distinguished. For x Ͻ 0.54 no hydride fluoride exists, i.e. there is no
hydride of the composition KCaH3 and the solid solution starts only
distances resulting from the structure refinements are in the range of
typical K–H, K–F, Ca–H, and Ca–F distances for mainly ionic com-
at x = 0.54. The tetragonal SrTiO3 type structure with partial ordering pounds.
Mn).[11] K2PtCl6-type related structures like MIIFeH6,
Introduction
MIIRuH6, MIICoH5, MIIIrH5, MIINiH4, and MIIPdH4 (MII = Ca,
Sr, Ba, Eu, Yb) are also related to the cubic perovskite type as
vacancy derivatives and are classified as coordination com-
pounds with 18-electron hydridometalate complexes with
covalent and ionic bonding for hydrogen atoms.[12–15]
The cubic perovskite type is one of the basic structure types
in structural chemistry. Its large structural family with many
distortion and substitutional derivatives[1] comprises many in-
organic solids, with numerous examples especially for oxides
and halides. Many representatives show interesting chemical
and physical properties and are thus of importance for various
applications. Examples are ferroelectrics and multiferroics,[2–4]
solar cell materials,[5] nonlinear optical materials,[6] or lumi-
nescent materials,[7,8] just to name a few.
As compared to oxides and halides, perovskite type hydrides
are less numerous and less well investigated, which is due on
one hand to the necessity of neutron diffraction for the location
of hydrogen in many cases and on the other hand to their con-
siderable sensitivity to air and moisture. Nevertheless, they
constitute a substantial body of the known metal hydride crys-
tal structures and thus comprise an important subsection of
this class of compounds. Chemical bonding within perovskite
related metal hydrides varies considerably with ionic com-
pounds such as LiMH3 (inverse perovskite, M = Sr, Ba, Eu)[9]
and defect variants such as EuMg2H6,[10] and metallic hydrides
with perovskite (SrPdH3, EuPdH3) or anti-perovskite type
structure such as MPd3H1–x (x Յ 1; M = Mg, In, Tl, Sn, Pb,
The cubic perovskite type is an attractive host for lumines-
cence materials. These require a large bandgap, i.e. only ionic
hydrides may be considered suitable. The hydride ligand offers
the possibility to greatly influence the 4f7–4f65d transition of
EuII-activated phosphors. Due to its extreme polarizability, ten-
dency for more covalent bond character and its strong ligand
field, the hydride ion enormously affects the energetic position
of d levels. The bary center of the excited state is located at
lower energies in comparison to phosphors containing harder
anions with weaker ligand fields. Furthermore, the crystal field
strength increases the splitting of the 5d state. Both effects
yield in a strong redshift of emission in comparison with analo-
gous fluoride compounds.[8]
Solid solutions between hydrides and fluorides thus offer
excellent opportunities for tuning emission wavelengths of
EuII-doped host materials. Since the exact coordination of eu-
ropium atoms in such hosts is of importance, we have rein-
vestigated the crystal structure of potential host materials with
distorted perovskite type. The well-known hydride-fluoride
analogy[9] states that ionic metal hydrides and fluorides often
exhibit the same crystal structure type or at least similar struc-
tures. This offers the opportunity for substitutional hydride-
fluoride solid solutions, a concept, which has already been ex-
* Prof. Dr. H. Kohlmann
E-Mail: holger.kohlmann@uni-leipzig.de
[a] Institute of Inorganic Chemistry
Universität Leipzig
Johannisallee 29
04103 Leipzig, Germany
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201900309 or from the au- ploited.[16,17] Hydride fluorides of the composition KCaH3–xFx
thor.
(x = 1, 1.5, 2, 2.5) have been reported to crystallize in the
© 2020 The Authors. Published by Wiley-VCH Verlag GmbH &
orthorhombic GdFeO3 type,[18–20] similar to
Co. KGaA. · This is an open access article under the terms of the
NaMgH3–xFx.[17,21,22] In contrast to the latter, the hydride
Creative Commons Attribution-NonCommercial-NoDerivs Li-
cense, which permits use and distribution in any medium, provided
boundary phase KCaH3 does not seem to exist and has been
investigated so far by theoretical calculations only.[23] They
the original work is properly cited, the use is non-commercial and
no modifications or adaptations are made.
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Zeitschrift für anorganische und allgemeine Chemie
predict an orthorhombic GdFeO3 type structure and suggests
that preparation of KCaH3 from KH and CaH2 is not likely to
be successful.[23] By determination of the standard enthalpies
of formation by calorimetry, it was shown that the thermo-
dynamic stability decreases upon increasing hydrogen con-
tent.[20] Some questions about the symmetry of the members
of this solid solution series and the anion distribution in
KCaH3–xFx remain as well as the exact boundary for the
hydrogen richest compounds. Therefore, we have rein-
vestigated the series
KCaH3–xFx and characterized chemical composition and crys-
tal structure.
Results and Discussion
Members of the solid solution series KCaH3–xFx were syn-
thesized via solid state reactions of binary hydrides and fluor-
ides under inert conditions. The colorless powders were ana-
lyzed by X-ray powder diffraction and structures were refined
by the Rietveld method. For all compounds with 0.54 Յ x
Figure 1. Unit cell parameters in KCaH3–xFx (x determined by Riet-
veld refinement on X-ray diffraction patterns, see Table 1).
solid solutions KCaH3–xFx were found with a few percent of mation on the hydride-fluoride order within the anionic posi-
calcium oxide and unreacted starting materials as secondary tions, i.e. preferred crystallographic sites for hydrogen and flu-
phases (for details see Experimental Section and Supporting orine atoms (vide infra). For most compounds, location or re-
Information.).
finement of occupational parameters for hydrogen atoms in
The orthorhombic GdFeO3 type[18,19] was used as starting crystal structures requires neutron diffraction.[24] In case of the
model for Rietveld refinements. The cell parameters a, b, and solid solution series KCaH3–xFx, however, this is not necessary,
c show a continuous decrease with increasing fluorine content because hydrogen positions are well determined by fluorine
x. This observation is expected as ionic hydrides usually have positions. Since calcium with only twenty electrons is the elec-
larger unit cells in comparison to their corresponding iso- tron richest atom and 60% of all atoms in KCaF3 are fluorine,
structural fluorides.[9] The parameters a and c converge for the atomic form factor of fluorine atoms for X-rays contributes
decreasing x giving rise to the assumption that representatives considerably to the X-ray diffraction pattern, i.e. occupation
with high hydrogen content might crystallize in a tetragonal factors can be determined with good accuracy from XRPD pat-
space group. Therefore, several structure types with tetragonal terns and Rietveld analysis in this case. The hydrogen content
space groups were tested as models for the Rietveld refine- of samples was also determined by elemental analyses (CHN,
ments. Within the family of perovskite related structures six Table 1).
common structure types have a tetragonal space group
The comparison of the two analysis methods requires taking
(P4/mmm, P4/mbm, I4/mcm, I4/mmm, P4mm, I4/m[1]). Riet- into account the content of secondary phases. Considering the
veld refinements show satisfactory agreement only for the te- uncertainty of the CHN analysis for small hydrogen contents
tragonal SrTiO3 type with space group I4/mcm. For x up to values from elemental analysis are in good agreement with
1.73 the refinements confirm the tetragonal SrTiO3 structure those from crystal structure refinements (Table 1).
type, while for x higher than 1.73 the refinements show better
R values in the GdFeO3 structure type and a splitting of reflec- distinguished: no compound for x Ͻ 0.54, a solid solution with
tions becomes apparent (Figure 1). tetragonal SrTiO3 type structure for 0.54 Յ x Յ 1.7 and a solid
Three sections within the series KCaH3–xFx may be
Three experiments aimed to synthesize phases with x = 0.3, solution with orthorhombic GdFeO3 type structure for
0.39, and 0.5. Crystal structure refinements resulted in similar 1.8 Յ x Յ 3.
lattice parameters, the formula KCaH0.54F2.46 and considerable
For 1.8 Յ x Յ 3, the solid solution series KCaH3–xFx crys-
amounts of hydride secondary phases in all three cases. The tallizes with the orthorhombic GdFeO3 type structure. In com-
series KCaH3–xFx thus seems to have a phase boundary at x = parison to the cubic perovskite aristotype the Ca2+ surrounding
0.54. Accordingly, structure and properties of the hypothetical corner connected octahedra are distorted and tilted by angles of
compound KCaH3 have been determined so far from theoreti- 156.7° to 163.8°. The K+ surrounding polyhedra are strongly
cal calculations only.[23]
distorted compared to the ideal cuboctahedra, as well (Fig-
The exact hydride and fluoride content was determined by ure 2). The refined atomic position parameters agree with those
two independent methods. In Rietveld refinements, hydrogen from previous investigations.[19] Interatomic angles and poly-
and fluorine atoms were placed on the same crystallographic hedral distortions are similar to related crystal structures of this
sites with the constraint of occ(F) + occ(H) = 1. Refinement type, for example KCaF3.[25] Interatomic distances resulting
of occupation factors does thus not only allow to calculate the from the structure refinements are in the range of typical ionic
substitutional degree x in KCaH3–xFx, but also gives infor- hydrides and fluorides.[16,26]
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Table 1. Comparison of hydrogen content in potassium calcium hydride fluorides from XRPD based Rietveld refinements and elemental analyses;
hydrogen content by XRPD accounts for secondary phases and is thus directly comparable to hydrogen content by elemental analysis; entries
in italics: orthorhombic GdFeO3 type, other entries: tetragonal SrTiO3 type.
Phase
Secondary phases content /wt%
Hydrogen content in sample by
XRPD /wt%
Hydrogen content in sample by ele-
mental analysis /wt%
KCaH0.31F2.69
6% CaO
4% KF
0.22
0.03
KCaH0.82F2.18
KCaH1.27F1.73
6% CaO
9% CaO
2% KH
0.64
1.06
0.37
1.11
KCaH1.28F1.72
7%CaO
1.00
1.09
2% CaF2
16% CaO
11% CaO
2%KH
KCaH1.48F1.52
KCaH2.07F0.93
1.14
1.89
1.07
1.65
KCaH2.16F0.84
KCaH2.18F0.82
6% CaO
3% CaO
3% CaF2
7%CaO
17%CaO
9%CaH2
11%CaO
10%KH
6%CaH2
17% CaO
23% KH
15% CaH2
2.11
2.13
1.53
1.42
KCaH2.29F0.71
KCaH2.46F0.54
2.43
2.43
2.14
2.01
KCaH2.46F0.54
KCaH2.46F0.54
2.51
2.51
2.48
2.38
network was already identified.[18,19] Cuboctahedra and octa-
hedra are distorted and twisted by angles of 20.7° to 34.7°.
These torsion angles are much higher than in tetragonal SrTiO3
with 3.6°.[27] Interatomic K–H, K–F, Ca–H, and Ca–F dis-
tances are comparable to other typical ionic hydrides and fluor-
ides[16,26] (Figure 3).
Figure 2. Crystal structure of KCaH0.82F2.18 in the orthorhombic
GdFeO3 type structure (space group type Pnma); interatomic dis-
tances / Å and angles / °: K–H1/F1: 2.815(5) – 3.409(5), K–H2/F2:
2.647(4) – 3.688(4), Ca–H1/F1: 2.228(1), Ca–H2/F2: 2.236(3)–
2.275(4), Ca–H1/F1–Ca: 162.00(18), H1/F1–Ca–H1/F1: 180, H2/F2–
Ca–H2/F2: 88.68(12), 91.32(12), Ca–H2/F2–Ca: 154.04(17), F1/H1–
F2/H2–F1/H1: 103.77(10), 75.40(8).
Figure 3. Crystal structure of KCaH2.18F0.82 in tetragonal SrTiO3 type
structure (space group type I4/mcm); interatomic distances / Å and
angles / °: K–H1/F1: 3.139(0), K–H2/F2: 2.764(4)– 3.572(5), Ca–H1/
F1: 2.222(0), Ca–H2/F2: 2.293(7), Ca–H1/F1–Ca: 180, H1/F1–Ca–
H1/F1: 180, H2/F2–Ca–H2/F2: 90, Ca–H2/F2–Ca: 150.90(33), F1/
H1–F2/H2–F1/H1: 88.07(17).
Phases with higher hydrogen content crystallize in the te-
tragonal SrTiO3 structure type (space group I4/mcm, no. 140).
This is in contrast to earlier studies, where an orthorhombic
structure had been derived for the whole range of solid solu-
No hydrogen-fluorine ordering, i.e. perfect statistical distri-
tions although a progressive shifting of the anions with in- bution over both crystallographic anion sites, is observed for
creasing hydrogen proportion towards a less tilted octahedral fluorine-rich compounds with GdFeO3 type. This is in agree-
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ment with earlier investigations[18–20] and in analogy to similar
factors. Thermal parameters were constrained to be the same for all
anions (H and F atoms). Occupation factors were constrained to sum
up to fully occupied sites 4c and 8d.
solid solution series like NaMgH3–xFx.[17] For hydrogen-rich
compounds with tetragonal SrTiO3 type structure a partial or-
dering of hydrogen and fluorine atoms is found (0.54 Յ x Յ
1.7). Both anion positions show mixed occupation, but with
some preference of hydrogen atoms for 8h and fluorine atoms
for 4a sites, e.g. occ(H1, 4a) = 0.250(9) and occ(H2, 8h) =
0.616(6) in KCaH1.48F1.52 with occ(H) + occ(F) = 1 for each
position (I4/mcm, SrTiO3 type, see Supporting Information).
Calculating the Goldtschmidt tolerance factor t is a common
method to predict structural deviation from the cubic perovsk-
ite type structure.[28] Based on the crystal radii determined by
Shannon[29] the factor for KCaF3 is t = 0.90 in agreement with
a non-cubic (orthorhombic GdFeO3 type) perovskite type crys-
tal structure. For increasing fluoride substitution by hydride
ions, the tolerance factor t is expected to decrease, since ionic
radius of a hydride radius is usually slightly larger than that of
a fluoride ion. However, due to the extremely large polarizabil-
ity of the hydride anion, its ionic radius is not a constant,[30]
which is why no value for the tolerance factor t is given here
for hydride containing compounds. Temperature and pressure
dependent phase transitions between tetragonal and ortho-
rhombic perovskites are numerous, for example CaTiO3 under-
going a transition from room temperature orthorhombic
Further details of the crystal structures investigations may be obtained
from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leo-
poldshafen, Germany (Fax: +49-7247-808-666; E-Mail: crysdata@fiz-
karlsruhe.de, http://www.fiz-karlsruhe.de/request for deposited data.
html) on quoting the depository numbers CSD-1950921, CSD-
1950922, CSD-1950923, CSD-1950924, CSD-1950925, CSD-
1950926, CSD-1950927, CSD-1950928, CSD-1950929, CSD-
1950930, CSD-1950931.
Elemental Analysis: The hydrogen content of the hydrides was deter-
mined by elemental analysis with a VARIO EL (Elementar Analyse-
nsysteme GmbH, Hanau, Germany) microanalyzer using the carrier
gas-hot extraction method in triplicate repetition.
Supporting Information (see footnote on the first page of this article):
Crystal structures of KCaH3-xFx (0.54 Յ x Յ 3) based on X-ray powder
diffraction (XRPD) and Rietveld refinements
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) (Ko1803/12).
(Pnma) to tetragonal (I4/mcm) structure at 1373 K followed by
[31]
¯
a further transition to cubic (Pm3m) at 1523 K.
An accord-
ing change from tetragonal to orthorhombic perovskite type
structure as a function of composition in a solid solution series
is known for cation substitution. Increasing the barium content
x in Sr1–xBaxZrO3 changes the crystal structure from ortho-
rhombic (Pnma) to orthorhombic (Imma) to tetragonal and fi-
nally to cubic perovskite type.[32] To the best of our knowl-
edge, no switching between tetragonal and orthorhombic per-
ovskite type structures by compositional changes of anions in
solid solution series was reported as yet. The solid solution
Keywords: Metal hydride; Metal fluoride; X-ray diffraction;
Rietveld refinement; Perovskite
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