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: [email protected] 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. CRYSTALLOGRAPHY REPORTS Vol. 68 No. 7 2023 NEUTRON STUDY OF CESIUM HYDROGEN 1057 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 CRYSTALLOGRAPHY REPORTS Vol. 68 No. 7 2023 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 1058 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. REFERENCES 1. I. P. Makarova, Phys. Solid State 57 (3), 442 (2015). https://doi.org/10.1134/s1063783415030117 2. I. Makarova, V. Grebenev, E. Dmitricheva, et al., Acta Crystallogr. B 72, 133 (2016). https://doi.org/10.1107/S2052520615023069 3. I. Makarova, E. Selezneva, V. Grebenev, et al., Ferroelectrics 500, 54 (2016). https://doi.org/10.1080/00150193.2016.1215204 CRYSTALLOGRAPHY REPORTS Vol. 68 No. 7 2023 NEUTRON STUDY OF CESIUM HYDROGEN 4. I. P. Makarova, L. A. Shuvalov, and V. I. Simonov, Ferroelectrics 79, 111 (1988). https://doi.org/10.1080/00150198808229410 5. E. Selezneva, I. Makarova, R. Gainutdinov, et al., Acta Crystallogr. 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