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S1063774523600795

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ISSN 1063-7745, Crystallography Reports, 2023, Vol. 68, No. 7, pp. 1055–1059. © Pleiades Publishing, Inc., 2023.
STRUCTURE OF INORGANIC
COMPOUNDS
Neutron Study of Cesium Hydrogen Sulfate–Phosphate Crystals
I. P. Makarovaa,*, N. N. Isakovab, A. I. Kalyukanovb, and V. A. Komornikova
a Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics”
b
of Russian Academy of Sciences, Moscow, 119333 Russia
National Research Centre “Kurchatov Institute,” Moscow, 123182 Russia
*e-mail: makarova@crys.ras.ru
Received June 19, 2023; revised June 19, 2023; accepted June 29, 2023
Abstract―Structural data on the Cs4(HSO4)3(H2PO4) single crystals have been obtained by neutron diffraction methods at the MOND experimental station of the National Research Centre “Kurchatov Institute.”
The structural model of the crystals has been refined, hydrogen atoms have been localized with high accuracy,
and the existence of hydrogen bonds of three types in the structure has been shown.
DOI: 10.1134/S1063774523600795
INTRODUCTION
This study continues the investigations of the
MmHn(AO4)(m + n)/2 ⋅yН2О (М = K, Rb, Cs, or NH4 and
AO4 = SO4, SeO4, HPO4, or HAsO4) superprotonic
crystals, including complex cesium hydrogen sulfate
phosphates, in order to identify regular relations
between their composition, atomic structure, and
physical properties [1–3]. In contrast to other hydrogen-containing compounds, superprotonic crystals
undergo changes in the system of hydrogen bonds with
increasing temperature, which cause changes in the physical properties of the crystals, in particular, the occurrence
of the proton conductivity (~10–3–10–1 Ω–1 cm–1) at relatively low temperatures (150–300°C). Study of the
crystal structure and its modifications under temperature variations is aimed at establishing the structural
conditionality of the changes in physical properties
and the effect on the stabilization of superprotonic
phases of the crystals, which is a necessary condition
for creating new functional materials.
Since the investigated crystals are proton conductors, special attention in studying their structure is
paid to the localization of hydrogen atoms and systems
of hydrogen bonds. In most of hydrogen-containing
crystals, hydrogen atoms occupy entirely one or more
crystallographic sites in the structure, and hydrogen
bonds form an ordered network. The uniqueness of
superprotonic crystals lies in the fact that, with
increasing temperature, a system of dynamically disordered hydrogen bonds is formed in them, which
ensures additional sites for protons; the possibility of
their motion; and, as a result, the occurrence of high
protonic conductivity in the superprotonic phase [4].
It was found that the superprotonic phase can be
formed already at room temperature in the case of sub-
stitution of cations accompanied by an increase in the
symmetry of their coordination environment and,
consequently, the formation of a dynamically disordered system of hydrogen bonds [5].
A group of the MmHn(AO4)(m + n)/2 ⋅yН2О crystal
family promising for both fundamental research and
application includes crystals of CsH2PO4–CsHSO4–
H2O water‒salt system. Studies of this system yielded
a series of cesium hydrogen sulfate phosphate crystals,
including the Cs4(HSO4)3(H2PO4) compound, in
which a superprotonic phase transition was revealed at
a temperature of ~409 K [2, 6]. The choice of the neutron diffraction method for investigations was dictated
by the need to obtain precise structural data on the
Cs4(HSO4)3(H2PO4) crystals taking into account
hydrogen atoms, which is important for characterizing
this compound and revealing general regularities for
the family of superprotonic crystals.
EXPERIMENTAL
The Cs4(HSO4)3(H2PO4) crystals were obtained
using two methods of crystal growth from aqueous solutions: isothermal evaporation from initially unsaturated
solutions and a controlled decrease in the solubility of
saturated aqueous solutions with a seed obtained by isothermal evaporation to grow single crystals of required
size. The synthesis of the Cs4(HSO4)3(H2PO4) single
crystals was discussed in detail in [6, 7].
Optically transparent single-crystal samples without cracks and inclusions for structural investigations
were selected on a Nikon SMZ1270 stereomicroscope
with a magnification of up to ×80. Using the method
of diffraction on a monochromatic neutron beam,
1055
1056
MAKAROVA et al.
Table 1. Main crystallographic characteristics, experimental neutron diffraction data, and results of structural refinement
for the Cs4(HSO4)3(H2PO4) single crystal
X-rays
Neutrons
293
293
Sample size, mm
0.24 (diameter)
1.0 × 0.8 × 2.5
System, sp. gr., Z
Monoclinic, С2/с, 3
Monoclinic , С2/с, 3
a, b, c, Å
19.945(2), 7.8565(5), 8.9945(9)
19.95(3), 7.856(9), 8.98(1)
β, deg
100.12(1)
100.12(2)
3
1387.5(2)
1386(3)
T, K
V, Å
, g/cm3
3.301
3.307
Diffractometer
Xcalibur S
MOND
Radiation, λ, Å
MoKα, 0.7107
1.06
Dx
μ, mm
–1
Adsorption correction; Tmin, Tmax
8.31
0.483, 0.532
Scan mode
ω
φ
θmax, deg
73.13
41.94
Ranges of indices h, k, l
Number of reflections: measured
(N1)/unique with I >3σ(I) (N2), Rint
Refinement method
–52 < h < 53, –20 < k < 20, –23 < l < 22 –24 < h < 24, –5 < k < 5, –10 < l < 10
62 183/2258, 0.031
4046/293, 0.11
Least-squares method on F,
w = 1/(σ2(F) + 0.0001F)2
Least-squares method on F,
w = 1/(σ2(F) + 0.0001F)2
105
106
Number of refined parameters
Extinction correction, coefficient (isotropic, type 1, Lorentzian distribution [12])
R1/wR2, S
Δρmin/Δρmax, e/Å3
Programs
0.13(1)×10
4
0.27(8)×104
0.022/0.021, 1.08
0.127/0.139, 3.50
–0.44/0.34
–0.87/0.94
CrysAlis PRO [13]; Jana 2006 [11];
DIAMOND [14]
Jana 2006 [11]; DIAMOND [14]
a single-domain sample without intergrowths and
twinnings was selected.
The neutron diffraction experiment was carried out
on a single-crystal four-circle diffractometer MOND,
commissioned at the IR-8 reactor of the National
Research Centre “Kurchatov Institute,” using a monochromatic neutron beam with a wavelength of 1.06 Å
from a double focusing monochromator PG[002] [8].
Diffraction data were collected at room temperature
with a MAR345n position-sensitive detector. The
experimental strategy consisted of successive φ-scans
from 0° to 180° with a step of 1° at two angular positions (0° and 45°) of the detector along the 2θ axis.
The space group and unit-cell parameters were determined in the DIRAX program [9] and the collection
of integral intensities and indication of diffraction
reflections were performed in the EVAL14 program [10].
The initial data for the structural analysis were the
coordinates of the Cs4(HSO4)3(H2PO4) basis atoms
from [2], obtained using X-ray diffraction analysis.
The crystallographic calculation was performed in the
Jana2006 crystallographic software package [11].
Table 1 contains the main crystallographic characteristics, experimental data, and the results of structural
refinement of the Cs4(HSO4)3(H2PO4) single crystal
sample. The refined positional and equivalent isotropic displacement parameters of the basic atoms of the
crystal structure are listed in Table 2. For comparison,
the tables present also the X-ray diffraction data.
RESULTS AND DISCUSSION
An analysis of the systematic reflection extinctions
confirmed the sp. gr. С2/с in the Cs4(HSO4)3(H2PO4)
crystals [2]. It can be noted that the unit-cell parameters of the crystals are close to the corresponding X-ray
diffraction data.
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Table 2. Sites, site occupancies (q), coordinates (x/a, y/b, and z/c), and equivalent isotropic displacement parameters
(U, Å2) of basic atoms of the Cs4(HSO4)3(H2PO4) crystal structure
Atom
Wyckoff
position
q
Cs1
4e
1.0
0.5
0.5
0.392(4)
0.25
0.020(12)
Cs2
8f
1.0
0.323369(9)
0.13514(3)
0.38286(5)
0.0485(1)
0.3230(6)
0.143(3)
0.3848(19)
0.046(11)
x/a
y/b
0.40344(3)
z/c
0.25
U
0.03905(7)
S1
8f
1.0
0.15907(3)
0.12937(7)
0.07001(7)
0.0359(2)
0.1615(13)
0.130(8)
0.057(5)
0.07(2)
P
4e
0.75
0.5
0.9086(1)
0.25
0.0276(3)
0.5
0.905(6)
0.25
0.03(2)
0.5
0.9086(1)
0.25
0.0276(3)
0.5
0.905(6)
0.25
0.03(2)
0.4425(1)
0.7995(2)
0.1715(2)
0.0446(6)
0.4435(6)
0.797(3)
0.167(2)
0.041(9)
0.5250(1)
0.0214(2)
0.1348(2)
0.0366(5)
0.5249(5)
0.019(3)
0.140(1)
0.030(8)
0.1665(1)
0.7693(2)
0.7169(2)
0.0455(6)
0.1668(6)
0.766(3)
0.721(2)
0.044(9)
0.1472(2)
0.7482(3)
0.4467(3)
0.0669(9)
0.1482(10)
0.742(4)
0.443(3)
0.079(15)
0.1026(1)
0.9859(3)
0.5589(2)
0.0591(8)
0.1032(6)
0.984(3)
0.559(2)
0.047(9)
0.2224(1)
0.9616(3)
0.5774(3)
0.0751(9)
0.2214(8)
0.962(3)
0.571(2)
0.063(11)
S2
O1
O2
O3
O4
O5
O6
4e
8f
8f
8f
8f
8f
8f
0.25
1.0
1.0
1.0
1.0
1.0
1.0
H1
8f
1.0
0.384(3)
0.245(7)
0.744(7)
0.18(2)
0.394(1)
0.246(5)
0.741(3)
0.07(2)
H2
4b
0.75
0
0.5
0
0.09(2)
0
0.5
0
0.04(2)
0.182(4)
0.711(10)
0.472(9)
0.07(3)
0.227(3)
0.704(11)
0.467(10)
0.15(5)
H3
8f
0.5
X-ray and neutron diffraction data are given in the first and second rows, respectively. Based on the X-ray data, hydrogen atoms were
refined in the isotropic approximation of thermal parameters.
Figure 1 shows the atomic structure of the
Cs4(HSO4)3(H2PO4) crystal. The independent region
of the unit cell of the Cs4(HSO4)3(H2PO4) crystals
contains two sites of Cs1 (4e) and Cs2 (8f) atoms and
two symmetrically nonequivalent tetrahedral groups
AO4, in which the A(4e) and A(8f) sites can be occupied by S or P atoms (Table 2). The refinement of the
structural model showed that the A(8f) site is fully
occupied by S1 atoms, which is evidenced by the occupancy qS1 = 1.0 within standard deviations. The A(4e)
site is occupied by statistical P and S2 atoms with
occupancies of qP = 0.75 and qS1 = 0.25; i.e., in the
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crystal unit cell, the A(4e) site is occupied statistically
by three P atoms and one S atom. These results confirm the conclusions drawn based on the X-ray diffraction data [2].
It can be noted that, in coordination polyhedra of
cesium atoms in the Cs4(HSO4)3(H2PO4) structure,
the interatomic distances obtained from the X-ray [2]
and neutron diffraction data are consistent within
standard deviations: the average Cs1–O distances are
3.241(2) and 3.25(3) Å and the average Cs2–O distances are 3.273(2) and 3.27(4) Å, respectively. The
correlation of the interatomic distances (taking into
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MAKAROVA et al.
Cs
SO4
(P,S)O4
H
c
a
Fig. 1. Atomic structure of the Cs4(HSO4)3(H2PO4) crystal. SO4 and (P,S)O4 tetrahedra with hydrogen bonds are shown.
account the participation of O atoms as donors or
acceptors in hydrogen bonds) was also observed in tetrahedral groups: the average S1–O distances are
1.466(2) and 1.49(5) Å and the average (P,S2)–O distances are 1.509(2) and 1.49(3) Å.
The neutron diffraction study allowed us to refine
the positional and anisotropic displacement parameters of hydrogen atoms and significantly increase the
accuracy of determining the hydrogen-bond geometry
in the Cs4(HSO4)3(H2PO4) crystals. Three hydrogen
atoms, H1, H2, and H3, are localized in the crystal
structure.
It was revealed that the H1 atom is involved in the
formation of strong O3–H1–O1 hydrogen bonds with
the following parameters: 2.61(2) Å for O3–O1,
1.32(3) Å for O3–H1, 1.32(3) Å for H1–O1, and
163(3)° for ∠O3–H1–O1 between the PO4 and SO4
tetrahedra. The H2 atom is involved in the formation
of O2–H2–O2' hydrogen bonds (2.55(2) Å for O2–
O2', 1.27(1) Å for O2–H2, and 180° for ∠O2–H2–
O2'), which connect chains of PO4 tetrahedra. The H2
site has an occupancy of qH2 = 0.75 within standard
deviations, which corresponds to the established statistical substitution of SO4 for one of the four PO4 tetrahedra in the unit cell; i.e., there is no O2–H2–O2'
hydrogen bond in the presence of S atoms. The H3
atom forms a weak O4–H3⋅⋅⋅O6' hydrogen bond
(3.08(3) Å for O4–O6', 1.58(6) Å for O4–H3, 1.73(8) Å
for H3–O6', and 137(4)° for ∠O4–H3–O6'), which
connects the SO4 tetrahedra and occupies a disordered
site with an occupancy of qH3 = 1/2. The distance
between the H3 and H3' sites is 1.2(1) Å. According to
the structural data obtained, Cs4(HSO4)3(H2PO4)
crystals contain hydrogen bonds with incompletely
occupied Н sites already at room temperature.
ACKNOWLEDGMENTS
The neutron experiments were performed on the equipment of the NRC IR-8 Unique Scientific Facility.
FUNDING
This study was carried out within the State assignment of
the Ministry of Science and Higher Education of the Russian Federation for the Federal Scientific Research Сenter
“Crystallography and Photonics” of the Russian Academy
of Sciences.
CONFLICT OF INTEREST
The authors declare that they have no conf licts of
interest.
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Translated by E. Bondareva
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