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Tetragonal caesium hydro­xide monohydrate, CsOH·H2O, a clathrate hydrate, is a polymorph of three known hexagonal or pseudo-hexagonal modifications. It was obtained as a by-product in a high-pressure experiment. Whether it is a high-pressure polymorph, however, remains to be verified. The Cs atoms are situated in cavities of the form of a bicapped pentagonal prism, within an infinite three-dimensional hydrogen-bonded oxy­gen framework that is locally identical to layers found in the hexagonal modifications. The Cs atom and one of the two H atoms are at sites with {\overline 4}m2 symmetry, the O atom has mm site symmetry and the second H atom has 2/m symmetry.

Supporting information


Crystallographic Information File (CIF)
Contains datablocks global, I


Rietveld powder data file (CIF format)
Contains datablock I

Comment top

Caesium hydroxide monohydrate, CsOH·H2O (or CsH3O2), is known to crystallize in three different hexagonal or pseudo-hexagonal modifications (Jacobs et al., 1982). Below 233 K the symmetry is monoclinic, between 233 and 340 K it is trigonal, and above 340 K the structure becomes truly hexagonal, at least so far as the positions of the Cs and O atoms are concerned (space group P6/mmm, a = 4.574 and c = 4.440 Å, Cs at 1a, O at 2 d). The O atoms form (001) layers with hexagonal symmetry, each O atom being connected to three neighbouring O atoms via hydrogen bonds. It has been shown by incoherent neutron scattering (Stahn et al., 1983) that at 402 K the H atoms are dynamically disordered in a double-well potential between adjacent O atoms, indicating that at this temperature it is impossible to distinguish between OH- ions and H2O molecules, and the structure can thus be rationalized in terms of (001) two-dimensional H3O2- ions separated by (001) layers of Cs+ ions.

The Rietveld refinement of the synchrotron data (Fig. 1.) measured on the present tetragonal polymorph shows that the same triangular coordination of O atoms as in the hexagonal polymorph can be found locally in the tetragonal phase, as bands running along the a axis at z ~1/4 and 3/4, and along the b axis at z ~0 and 1/2 (Fig. 2.). These bands form an infinite three-dimensional oxygen framework that is locally identical to the layers found in the hexagonal modification.

No H atoms were identified during the refinement, so the concept of bond valences (Brown & Wu, 1976) was used to locate the hydrogen bonds. The data of Brese & O'Keeffe (1991) were used to calculate the bond valences for all non-H atoms. The bond valences sum to 0.94 for Cs and to 0.47 for O, indicating that each O atom participates in three hydrogen bonds. Neutron diffraction on a deuterated sample would be necessary to localize the H atoms precisely and to determine whether, at room temperature, they are fully ordered (which would lead to a decrease of the symmetry), or disordered as in the hexagonal polymorph.

The Cs atoms are situated in the cavities of the oxygen framework, taking the form of a bicapped pentagonal prism. This form can be derived from a hexagonal prism (the Cs coordination observed in the hexagonal modification) by a 90° rotation of a half of the hexagonal prism.

The tetragonal polymorph of CsOH·H2O is a clathrate hydrate. The calculated density of the tetragonal modification of CsOH·H2O at ambient pressure and 293 K is 3.79 Mg m-3, which is higher than the density of the hexagonal polymorph, which was found to be 3.51 Mg m-3 at the same temperature and pressure.

Experimental top

The tetragonal modification of CsOH·H2O was initially obtained as a by-product during the high-pressure synthesis of caesium-based ternary metal hydrides. In the first step, binary caesium hydride powder was synthesized by direct hydrogenation of metallic caesium ingot (STREM, 99.9%) at 700 K and 80 bar (1 bar = 10 5 Pa) hydrogen pressure for 15 d in an autoclave. In the second step, caesium-based metal hydrides were synthesized by heating mixtures of CsH and the corresponding metal to 800 K in a multi-anvil press at 30 kbar pressure in air. The reaction products contained CsOH and tetragonal CsOH·H2O as impurities. The same experiments were performed without the metal powders, and the reaction product contained CsH, CsOH and tetragonal CsOH·H2O. The product was white, pyrophoric and extremely sensitive to air and moisture.

Computing details top

Data collection: Please give details; cell refinement: FULLPROF98 (Rodríguez-Carvajal, 1998); data reduction: FULLPROF98; program(s) used to solve structure: FOX (Favre-Nicolin & Černý, 2002); program(s) used to refine structure: FULLPROF98; molecular graphics: ATOMS (Dowty, 1993); software used to prepare material for publication: WinPLOTR (Roisnel & Rodríguez-Carvajal, 1998) and ATOMS.

Figures top
[Figure 1] Fig. 1. Observed (circles) and calculated (solid line) intensities for CsOH·H2O. The three rows of tick-marks indicate, from top to bottom, the positions of the Bragg peaks of the main (CsOH·H2O) and impurity (CsH, CsOH) phases. The difference pattern appears below.
[Figure 2] Fig. 2. The structure of CsOH·H2O viewed approximately along the a axis. The positions of the H atoms (small spheres) were predicted from bond-valence analysis. Cs atoms are denoted by large spheres and O atoms by mid-sized spheres.
Caesium hydroxide monohydrate top
Crystal data top
CsOH·H2ODx = 3.785 (2) Mg m3
Mr = 167.93Synchrotron radiation, λ = 0.48562 Å
Tetragonal, I41/amdµ = 24.21 mm1
Hall symbol: -I 4bd 2T = 293 K
a = 4.38088 (4) ÅParticle morphology: plate-like
c = 15.46525 (17) Åwhite
V = 296.81 (1) Å3cylinder, 50 × 0.4 mm
Z = 4Specimen preparation: Prepared at 800 K and 3000000 kPa
F(000) = 332
Data collection top
Two-axis goniometer
Data collection mode: transmission
Radiation source: synchrotron, Swiss-Norwegian Beam Line BM1BScan method: step
Channel-cut Si 111 monochromator2θmin = 6.012°, 2θmax = 36.488°, 2θstep = 0.002°
Specimen mounting: glass capillary
Refinement top
Refinement on Inet14 parameters
Least-squares matrix: full with fixed elements per cycle0 constraints
Rp = 0.125H-atom parameters not refined
Rwp = 0.133Calculated w = 1/Yi
Rexp = 0.059(Δ/σ)max = 0.01
χ2 = 5.198Background function: linear interpolation between 16 estimated points
15238 data pointsPreferred orientation correction: no
Profile function: pseudo-Voigt
Crystal data top
CsOH·H2OZ = 4
Mr = 167.93Synchrotron radiation, λ = 0.48562 Å
Tetragonal, I41/amdµ = 24.21 mm1
a = 4.38088 (4) ÅT = 293 K
c = 15.46525 (17) Åcylinder, 50 × 0.4 mm
V = 296.81 (1) Å3
Data collection top
Two-axis goniometer
Scan method: step
Specimen mounting: glass capillary2θmin = 6.012°, 2θmax = 36.488°, 2θstep = 0.002°
Data collection mode: transmission
Refinement top
Rp = 0.12515238 data points
Rwp = 0.13314 parameters
Rexp = 0.059H-atom parameters not refined
χ2 = 5.198
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
Cs00.750.1250.0328 (5)*
O00.250.4560 (5)0.004 (2)*
Geometric parameters (Å, º) top
Cs—O3.341 (3)Cs—Oii3.410 (6)
Cs—O3.341 (3)Cs—O3.410 (6)
Cs—O3.341 (3)Cs—O3.410 (6)
Cs—O3.341 (3)Cs—O3.410 (6)
Cs—O3.341 (3)O—Oii2.505 (11)
Cs—Oi3.341 (3)O—O2.579 (6)
Cs—O3.341 (3)O—O2.579 (6)
Cs—O3.341 (3)
O—Cs—O81.92 (6)O—Cs—O135.96 (17)
O—Cs—O81.92 (6)O—Cs—Oii135.10 (10)
O—Cs—O135.96 (18)O—Cs—O82.31 (13)
O—Cs—O98.08 (6)O—Cs—O44.90 (10)
O—Cs—Oi98.08 (6)O—Cs—O97.69 (13)
O—Cs—O44.04 (17)O—Cs—Oii82.31 (13)
O—Cs—O180.0000O—Cs—O135.10 (10)
O—Cs—Oii97.69 (13)O—Cs—O97.69 (13)
O—Cs—O44.90 (10)O—Cs—O44.90 (10)
O—Cs—O82.31 (13)Oii—Cs—O79.93 (12)
O—Cs—O135.10 (10)Oii—Cs—O125.97 (7)
O—Cs—O135.96 (18)Oii—Cs—O125.97 (7)
O—Cs—O81.92 (6)O—Cs—O125.97 (7)
O—Cs—O180.0000O—Cs—O125.97 (7)
O—Cs—Oi44.04 (17)O—Cs—O79.93 (12)
O—Cs—O98.08 (6)Oii—O—O121.9 (2)
O—Cs—O98.08 (6)Oii—O—O121.9 (2)
O—Cs—Oii44.90 (10)Oii—O—Cs67.98 (12)
O—Cs—O97.69 (13)Oii—O—Cs67.98 (12)
O—Cs—O82.31 (13)Oii—O—Csi67.98 (12)
O—Cs—O135.10 (10)Oii—O—Cs67.98 (12)
O—Cs—O81.92 (6)Oii—O—Cs140.03 (8)
O—Cs—O44.04 (17)Oii—O—Cs140.03 (8)
O—Cs—Oi180.0000O—O—O116.3 (4)
O—Cs—O98.08 (6)O—O—Cs139.00 (6)
O—Cs—O98.08 (6)O—O—Cs68.96 (10)
O—Cs—Oii97.69 (13)O—O—Csi139.00 (6)
O—Cs—O44.90 (10)O—O—Cs68.96 (10)
O—Cs—O135.10 (10)O—O—Cs66.14 (17)
O—Cs—O82.31 (13)O—O—Cs66.14 (17)
O—Cs—O98.08 (6)O—O—Cs68.96 (10)
O—Cs—Oi98.08 (6)O—O—Cs139.00 (6)
O—Cs—O180.0000O—O—Csi68.96 (10)
O—Cs—O44.04 (17)O—O—Cs139.00 (6)
O—Cs—Oii44.90 (10)O—O—Cs66.14 (17)
O—Cs—O97.69 (13)O—O—Cs66.14 (17)
O—Cs—O135.10 (10)Cs—O—Cs136.0 (2)
O—Cs—O82.31 (13)Cs—O—Csi81.92 (9)
O—Cs—Oi135.96 (17)Cs—O—Cs81.92 (9)
O—Cs—O81.92 (6)Cs—O—Cs82.31 (5)
O—Cs—O81.92 (6)Cs—O—Cs135.10 (12)
O—Cs—Oii135.10 (10)Cs—O—Csi81.92 (9)
O—Cs—O82.31 (13)Cs—O—Cs81.92 (9)
O—Cs—O97.69 (13)Cs—O—Cs135.10 (12)
O—Cs—O44.90 (10)Cs—O—Cs82.31 (5)
Oi—Cs—O81.92 (6)Csi—O—Cs136.0 (2)
Oi—Cs—O81.92 (6)Csi—O—Cs135.10 (12)
Oi—Cs—Oii82.31 (13)Csi—O—Cs82.31 (5)
Oi—Cs—O135.10 (10)Cs—O—Cs82.31 (5)
Oi—Cs—O44.90 (10)Cs—O—Cs135.10 (13)
Oi—Cs—O97.69 (13)Cs—O—Cs79.93 (17)
Symmetry codes: (i) x+1/2, y, z+1/2; (ii) y+1/4, x+1/4, z+3/4.

Experimental details

Crystal data
Chemical formulaCsOH·H2O
Crystal system, space groupTetragonal, I41/amd
Temperature (K)293
a, c (Å)4.38088 (4), 15.46525 (17)
V3)296.81 (1)
Radiation typeSynchrotron, λ = 0.48562 Å
µ (mm1)24.21
Specimen shape, size (mm)Cylinder, 50 × 0.4
Data collection
DiffractometerTwo-axis goniometer
Specimen mountingGlass capillary
Data collection modeTransmission
Scan methodStep
2θ values (°)2θmin = 6.012 2θmax = 36.488 2θstep = 0.002
R factors and goodness of fitRp = 0.125, Rwp = 0.133, Rexp = 0.059, χ2 = 5.198
No. of data points15238
No. of parameters14
No. of restraints?
H-atom treatmentH-atom parameters not refined

Computer programs: Please give details, FULLPROF98 (Rodríguez-Carvajal, 1998), FULLPROF98, FOX (Favre-Nicolin & Černý, 2002), ATOMS (Dowty, 1993), WinPLOTR (Roisnel & Rodríguez-Carvajal, 1998) and ATOMS.

Selected bond lengths (Å) top
Cs—Oi3.341 (3)O—Oii2.505 (11)
Cs—Oii3.410 (6)O—O2.579 (6)
Symmetry codes: (i) x+1/2, y, z+1/2; (ii) y+1/4, x+1/4, z+3/4.

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