The caesium phosphates Cs3(H1.5PO4)2(H2O)2, Cs3(H1.5PO4)2, Cs4P2O7(H2O)4, and CsPO3

The caesium phosphates Cs3(H1.5PO4)2(H2O)2 and Cs3(H1.5PO4)2 were obtained from aqueous solutions, and Cs4P2O7(H2O)4 and CsPO3 from solid state reactions, respectively. Cs3(H1.5PO4)2, Cs4P2O7(H2O)4, and CsPO3 were fully structurally characterized for the first time on basis of single-crystal X-ray diffraction data recorded at − 173 °C. Monoclinic Cs3(H1.5PO4)2 (Z = 2, C2/m) represents a new structure type and comprises hydrogen phosphate groups involved in the formation of a strong non-symmetrical hydrogen bond (accompanied by a disordered H atom over a twofold rotation axis) and a very strong symmetric hydrogen bond (with the H atom situated on an inversion centre) with symmetry-related neighbouring anions. Triclinic Cs4P2O7(H2O)4 (Z = 2, P1¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{1}$$\end{document}) crystallizes also in a new structure type and is represented by a diphosphate group with a P–O–P bridging angle of 128.5°. Although H atoms of the water molecules were not modelled, O···O distances point to hydrogen bonds of medium strengths in the crystal structure. CsPO3 is monoclinic (Z = 4, P21/n) and belongs to the family of catena-polyphosphates (MPO3)n with a repetition period of 2. It is isotypic with the room-temperature modification of RbPO3. The crystal structure of Cs3(H1.5PO4)2(H2O)2 was re-evaluated on the basis of single-crystal X-ray diffraction data at − 173 °C, revealing that two adjacent hydrogen phosphate anions are connected by a very strong and non-symmetrical hydrogen bond, in contrast to the previously described symmetrical bonding situation derived from room temperature X-ray diffraction data. In the four title crystal structures, coordination numbers of the caesium cations range from 7 to 12.


Introduction
The recent interest in the family of caesium phosphates is mainly connected with the high proton conductivity of Cs(H 2 PO 4 ) to be utilized as a potential electrolyte for intermediate temperature fuel cells [1][2][3] or for water electrolysis [4]. Another motivation to search for new caesium phosphates is related to acidic salts with formulae M x H y (AO 4 ) z (M = Cs, Rb, K, Na, Li, NH 4 ; A = S, Se, As, P) that likewise exhibit proton conductivity or have ferroelectric properties.

Results and discussion
Results of bond valence sum (BVS) calculations [21] using the parameters provided by Brese and O'Keeffe [22] reveal values for Cs 1 and P atoms in all structures very close to the expected formal total valencies of + I and + V, respectively, with the highest deviation being 0.15 valence units for some P atoms (Table 1).

Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2
Concerning the previous single-crystal X-ray study of Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 at room temperature [5], the results of the current low-temperature study have a higher precision. However, the principal structural arrangement is the same with respect to the two refinements/models. Differences in bond lengths and angles for individual structure units between the two models are negligible and might be caused by different measurement temperatures. Selected bond lengths and angles resulting from the current refinement are collated in Table 1. Since the crystal structure of Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 has been discussed in detail, here only the main features are given. The crystal structure is built up of hydrogen phosphate tetrahedra connected through strong hydrogen bonds involving the hydrogen atoms H1 and H2 (Table 2) into undulating layers parallel (001). Under further contribution of two hydrogen-bonding interactions of medium strengths involving the water molecule (OW), a three-dimensional network is formed (Fig. 1). The two independent caesium cations are located in the voids of this arrangement and are bonded to eight (Cs1) and twelve (Cs2) O atoms (Fig. 2, Table 1).
The chief difference between the two models pertains to the very strong hydrogen bond developed between two phosphate tetrahedra involving O2 and its symmetry-related counterpart (O2···O2(− x + 1, − y, − z)) at a distance of ≈ 2.44 Å. In the previous room-temperature model [5], this hydrogen bond was suggested as being symmetric, with the H atom exactly positioned between the two O2 atoms at an inversion centre of space group Pbca (Wyckoff position 1a). Based on difference Fourier maps obtained from the current data set (Fig. 3), which clearly revealed two symmetryrelated maxima in the vicinity of the inversion centre, we modelled the corresponding H atom (H2) as being statistically disordered, resulting in a non-symmetrical O2-H···O2′ hydrogen bond ( Table 2). The bonding situation regarding such a very strong hydrogen bond between two hydrogen phosphate groups with disordered hydrogen atoms is similar as in other structures comprising tetrahedral oxoanions with OH groups, e.g. in (NH 4 )H 5 (PO 4 ) 2 [23], Tl I H 5 (AsO 4 ) 2 [24], or Na 5 H 3 (SeO 4 ) 4 (H 2 O) 2 [25].
Although the present low-temperature X-ray diffraction data for Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 clearly point to an unsymmetrical hydrogen bond O2-H2···O2(− x + 1, − y, − z) with a disordered H2 atom, it remains unclear whether this model also applies at room temperature, or whether the reported model [5] with a symmetrical hydrogen bond O2-H2-O2′ and with H2 situated at an inversion centre is correct at this temperature. Note that the O2···O2′ distance derived from the room-temperature measurement (2.445(7) Å [5]) is slightly longer than in the current low-temperature measurement (2.4343(19) Å), indicating an expansion of the structure. Therefore, the likelihood of a symmetric hydrogen bond is expected to decrease with higher temperature. In the end, this question (unsymmetrical versus symmetrical hydrogen bond) can be answered without ambiguity only on basis of temperature-dependent neutron diffraction data.

Cs 3 (H 1.5 PO 4 ) 2
Cs 3 (H 1.5 PO 4 ) 2 was reported to exist as a dehydration product of Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 and to be stable between ~ 50 and 275 °C. Except for unit cell parameters and space group assignment on basis of a laboratory X-ray powder study, no further structural details were given for this phase [5]. The unit cell parameters determined from polycrystalline Cs 3 (H 1.5 PO 4 ) 2 at 160 °C (a = 11.1693(4), b = 6.4682(2),  [5] are in good agreement with the values from the current singlecrystal X-ray data at − 173 °C (Table 3). However, the space group derived from the powder study was reported to be C2 (No. 5), whereas the current refinement clearly indicates the higher C2/m space group symmetry. The unique crystal structure of Cs 3 (H 1.5 PO 4 ) 2 at − 173 °C resembles that of the corresponding dihydrate described above and comprises two Cs, one P, three O, and two H atoms in the asymmetric unit. Cs1 is situated on Wyckoff position 2a (site symmetry 2/m), Cs2, P1, O1, O2, O3 are all situated on position 4i (m), H1 (8j; 1) is disordered about a twofold rotation axis, and H3 is situated on Wyckoff position 2b (2/m).
In comparison with hydrated Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 where the very short hydrogen bond is unsymmetrical and associated with disorder of the hydrogen atom, the bonding situation of the hydrogen phosphate tetrahedron is different in Cs 3 (H 1.5 PO 4 ) 2 . As noted above, the hydrogen bond is symmetrical as revealed by difference Fourier maps, which clearly show a maximum at the inversion centre in-between the two symmetry-related oxygen atoms O1 (Fig. 5). However, it has to be noted that the electron density about hydrogen atoms is diffuse. Thus, even for a perfectly determined electron density, an H atom disordered about the 2/m position may result in a single maximum, if the disordered atoms are close to said position. In consequence, as previously, only neutron diffraction studies can unambiguously demonstrate the correctness of this model, since neutrons diffract at the nucleus.  The two caesium cations are situated between the hydrogen-bonded chains (Fig. 6). Each Cs1 symmetrically links four chains; it exhibits a coordination number of 10 in form of a distorted hexadecahedron [26]. Cs1 is bonded to two (non-H atom bearing) O3 atoms of two neighbouring chains with the shortest Cs-O bonds (3.07 Å) observed for this polyhedron, to four (disordered H atom bearing) symmetrically related O1 atoms at longer distances (3.22 Å), and to another four (H atom bearing) O3 atoms at the longest distance (3.41 Å). Each Cs2 likewise links four chains and is surrounded by eight O atoms in the shape of a distorted hexagonal bipyramid. The two shortly bonded O atoms O3 and O2 (d(Cs-O) ≈ 3.05 Å) define the axial O atoms, and the symmetrically related three pairs of O1 and O2 atoms define the six equatorial atoms with bond lengths ranging from 3.16 to 3.30 Å (Table 1).

Cs 4 P 2 O 7 (H 2 O) 4
In the crystal structure of Cs 4 P 2 O 7 (H 2 O) 4 (Fig. 7), isolated diphosphate anions are organised in layers parallel (010), thereby sandwiching adjacent layers composed of caesium cations (Cs3, Cs4) and the four water molecules along the [010] direction. The remaining two caesium cations, Cs1 and Cs2, are situated in-between individual diphosphate groups in the anionic layers.
The diphosphate anion has a staggered conformation with a P-O-P angle of 128.49(10)°. Characteristic for condensed phosphate anions [27], the two P-O bond lengths to the bridging O atom are significantly longer (1.6350(13) and 1.6458(16) Å) than the terminal P-O bond lengths (averaged values 1.518 (P1) and 1.516 (P2) Å) for the two tetrahedra of the anion. The dihedral angle between the atoms of the "backbone" of the anion (O3-P1-O4; O4-P2-O7) and the P···P distance amount to 43.23 (12)  from three neighbouring diphosphate groups and to one water molecule in the adjacent layer, and Cs2 is bonded to eight O atoms from three diphosphate groups and to two water molecules. On the other hand, Cs3 and Cs4 each have six water molecules and three atoms from two and three diphosphate groups, respectively, as bonding partners. The four water molecules are either bonded to four (in case of OW1, OW2, OW3) or to three (OW4) caesium cations. Although H atoms of water molecules could not be located from the current data set, O···O distances between water O atoms and phosphate O atoms in the range 2.682-2.760 Å indicate hydrogen-bonding interactions of medium strengths ( Table 2) that help to consolidate the crystal packing.

CsPO 3
Corbridge reported crystallographic data of CsPO 3 and other alkali long-chain polyphosphates of formula (MPO 3 ) n 2 (M = Na, K, Rb, Cs), showing that the Rb and Cs catena-polyphosphates crystallize isotypically in space group type P2 1 /n [20]. The crystal structure of the corresponding room-temperature modification of RbPO 3 was subsequently determined [29] and later re-examined twice [30,31]. The given lattice parameters from the first study of CsPO 3 at room temperature (a = 12.71, b = 4.32, c = 6.99 Å, β = 83° [20]) are in good agreement with the current lowtemperature data (Table 3, with β > 90° according to convention). Next to CsPO 3 , TlPO 3 is so far the only other known catena-polyphosphate crystallizing isotypically with the RT-form of RbPO 3 . However, crystallographic details of the thallium phase are restricted to lattice parameter and an indexed powder diffractogram [32]. The crystal structure of CsPO 3 (Fig. 8) comprises a polyphosphate chain extending parallel to [010], with a repeating unit of two phosphate tetrahedra. The bond lengths distribution is typical for polyphosphate chains [27], with two short P-O distances (average 1.486 Å) to terminal O atoms (O2, O3) and two considerably longer P-O distances (average 1.616 Å) to bridging O atoms (O1 and O1(− x + 1/2,    Fig. 2. The inset shows the diphosphate group with atom labelling y − 1/2, − z + 1/2)). The Cs cations are situated in-between the chains and exhibit a coordination number of 7 with a monocapped prism as coordination polyhedron; Cs-O distances range from 3.03 to 3.37 Å ( Table 1).
The program compstru [33], available at the Bilbao Crystallographic Server [34], was employed for a quantitative structural comparison of the isotypic CsPO 3 and RbPO 3 structures. For that purpose, the current low-temperature structure data of CsPO 3 and the room-temperature structure data of RbPO 3 were used, neglecting the effect of different measurement temperatures. The comparison revealed a close structural similarity between the structures. The degree of lattice distortion is 0.0193, and the distances between the atomic positions of paired atoms are 0.0211 Å for Cs/ Rb, 0.0658 Å for P1, 0.0836 Å for O1, 0.1244 Å for O2, and 0.1340 Å for O3. The arithmetic mean of all distances between paired atoms is 0.0858 Å, and the measure of similarity is 0.042.

Conclusion
The crystal structures of the four caesium phosphates Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 , Cs 3 (H 1.5 PO 4 ) 2 , Cs 4 P 2 O 7 (H 2 O) 4 , and CsPO 3 were refined from low-temperature X-ray diffraction data at − 173 °C. Although for the two hydrogen phosphates Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 and Cs 3 (H 1.5 PO 4 ) 2 all hydrogen atoms could clearly be located from difference Fourier maps, an uncertainty regarding the hydrogen-bonding situation between symmetry-related hydrogen phosphate tetrahedra remains. Future neutron diffraction studies are definitely required to evidence the correctness of the structure models, here in terms of the corresponding H atom positions. Neutron diffraction data may also help to determine the H atom positions of the water molecules in hydrous Cs 4 P 2 O 7 (H 2 O) 4 , which was not possible on basis of the current X-ray data. However, O···O distances involving the water molecules indicate the presence of two hydrogen-bonding interactions for each of the water molecules.

Preparation
Crystals of Cs 3 (H 1.5 PO 4 ) 2 were isolated from a batch intended to produce Cs 2 (HPO 4 ). To diluted phosphoric acid (≈ 5% wt ), an aqueous solution of Cs 2 CO 3 was added in the molar ratio 1:1. The mixture was carefully evaporated until dryness and kept in an oven at 130 °C for one night. Colourless plate-like crystals were isolated from the hygroscopic product that also contained bulky crystals of Cs 2 (HPO 4 ). After longer contact with ambient humidity at room temperature (about 2 days), crystals of the hydrate phase Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 were subsequently isolated from the original reaction product.  Fig. 1 [5] or Cs 2 (HPO 4 ) 2 (H 2 O) 2 and was reported to be polymorphic [35]. Cs 2 CO 3 and (NH 4 ) 2 HPO 4 were mixed in equimolar amounts, ground and placed in a porcelain crucible that was heated from room temperature to 950 °C within 3 h, kept at that temperature for 4 h and cooled to room temperature by turning off the furnace. Colourless plates of the tetrahydrate were harvested from the bulk product. CsPO 3 was prepared by mixing a diluted solution of phosphoric acid (≈ 5% wt ) and an aqueous solution of Cs 2 CO 3 in the molar ratio of 2:1. The mixture was subsequently warmed until dryness and heated within 2 h to 750 °C, kept at that temperature for 2 h, cooled within 10 h to 300 °C and then quickly removed from the furnace.

Structure determination
All crystals were hygroscopic and thus were embedded in perfluorinated oil for protection. Diffraction experiments on optically preselected crystals followed standard measurement procedures with corresponding software packages for data collection and data reduction [36]. All data sets were corrected for absorption effects by using the semiempirical multi-scan method [37]. The crystal structures were solved by charge flipping [38] and refined with JANA2006 [39].
All H atoms in the structures of Cs 3 (H 1.5 PO 4 ) 2 and Cs 3 (H 1.5 PO 4 ) 2 (H 2 O) 2 were located from difference Fourier maps and were refined with O-H distance constraints of 0.86(2) Å, except for the symmetrical hydrogen bond in Cs 3 (H 1.5 PO 4 ) 2 where the H atom is located on Wyckoff position 2b (2/m). H atoms of the water molecules could not be localized reliably for Cs 4 P 2 O 7 (H 2 O) 4 and therefore are not included in the final crystal structure model. The CsPO 3 crystal under investigation was twinned by mirroring at (100). Reflections of the individuals were separated and processed as HKLF5 data. The refined ratio for the two twin domains was 0.7480(6):0.2520 (6). Coordinates and atom numbering of CsPO 3 were adapted from the isotypic room-temperature modification of RbPO 3 [31].
Details of the data collections and structure refinements are gathered in Table 3. Further details of the crystal structure investigations may be obtained from The Cambridge Crystallographic Data Centre (CCDC) on quoting the depository numbers listed at the end of Table 3. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/struc tures .