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The structure of trithallium hydrogen bis­(sulfate), Tl3H(SO4)2, in the super-ionic phase has been analyzed by Rietveld analysis of the X-ray powder diffraction pattern. Atomic parameters based on the isotypic Rb3H(SeO4)2 crystal in space group R\overline 3m in the super-ionic phase were used as the starting model, because it has been shown from the comparison of thermal and electric properties in Tl3H(SO4)2 and M3H(SO4)2 type crystals (M = Rb, Cs or NH4) that the room-temperature Tl3H(SO4)2 phase is isostructural with the high-temperature R\overline 3m-symmetry M3H(SO4)2 crystals. The structure was determined in the trigonal space group R\overline 3m and the Rietveld refinement shows that an hydrogen-bond O—­H...O separation is slightly shortened compared with O—H...O separations in isotypic M3H(SeO4)2 crystals. In addition, it was found that the distortion of the SO4 tetrahedra in Tl3H(SO4)2 is less than that in isotypic crystals.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270102008569/iz1021sup1.cif
Contains datablocks global, I

rtv

Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108270102008569/iz1021Isup2.rtv
Contains datablock I

Comment top

The Tl3H(SO4)2 crystal is one of a family of M3H(XO4)2 compounds (M = Rb, Cs or NH4, and X = S or Se) known as zero-dimensional hydrogen-bonded systems. The M3H(XO4)2 crystals exhibit very interesting characteristics. Firstly, the hydrogen bonds are isolated. Secondly, deuterated crystals show a drastic isotope effect at the low-temperature phase transition. Thirdly, they undergo a super-ionic phase transition from the low-temperature ferroelastic phase of the monoclinic system to the high-temperature paraelastic phase of the trigonal system; for example, Rb3H(SeO4)2 and Cs3H(SeO4)2 undergo super-ionic phase transitions at 449 K and 456 K, respectively.

Recently, it was found that Tl3H(SO4)2 undergoes a super-ionic phase transition at 239 K and displays super-ionic conductivity even at room temperature (Matsuo et al., 2001), in spite of the fact that other M3H(XO4)2-type crystals are good insulators at room temperature. This interesting feature of the Tl3H(SO4)2 crystal would result from the change to the crystal structure due to placing Tl in the M site. Therefore, the determination of the crystal structure of the Tl3H(SO4)2 crystal in the super-ionic phase is very important for understanding the origin of the super-ionic phase transition in M3H(XO4)2-type crystals. However, no atomic coordinates of Tl3H(SO4)2 in the super-ionic phase were known, although the unit-cell parameters at room temperature had been previously determined by Peter & Jolibois (1973) from X-ray powder diffraction measurements. It is very difficult to prepare good quality single crystals of this compound. Therefore, we have measured the X-ray powder diffraction pattern and determined the crystal structure of Tl3H(SO4)2 in the super-ionic phase using Rietveld analysis.

It is known that M3H(XO4)2-type crystals belong to the trigonal system, space group R3 m, in the highest temperature phase below the melting point (Baranov et al., 1987; Merinov et al., 1990; Lukaszewicz et al., 1993). Chen et al. (2000) recently deduced that a new (NH4)3H(SO4)2 phase above 433 K also has R3 m symmetry. Moreover, (NH4)3H(SeO4)2, which is one of the M3H(XO4)2-type crystals, is known to undergo four phase transitions at TI—II = 335 K, TII-III = 308 K, TIII-IV = 275 K and TIV—V = 181 K (Gesi, 1977; Osaka et al., 1979). Phase I, space group R3 m, and phase II, space group R3, are super-ionic conductors (Pawłowski et al., 1990; Lukaszewicz et al., 1993). The phase transition at TI—II is characterized by a small change in the temperature gradients of the dielectric constant and of electrical conductivity (Pawłowski et al., 1990). The Tl3H(SO4)2 crystal also undergoes four phase transitions above 77 K, at TI—II = 267 K, TII-III = 239 K, TIII-IV = 196 K and TIV—V = 156 K (Matsuo et al., 2001).

In phases I and II, Tl3H(SO4)2 is a super-ionic conductor similar to (NH4)3H(SeO4)2. Furthermore, in the phase transition at TI—II = 269 K, the slopes of dielectric constant and electrical conductivity show the same temperature dependence as (NH4)3H(SeO4)2 (Matsuo et al., 2001). These facts indicate that the room temperature Tl3H(SO4)2 phase is isostructural with the high-temperature R3 m symmetry M3H(XO4)2 crystals. Therefore, a starting model based on the isotypic Rb3H(SeO4)2 crystal in the space group R3 m of the super-ionic phase was used in the determination of the structure of the super-ionic phase of Tl3H(SO4)2.

In the M3H(XO4)2-type crystal, it is also known that the O2 atom which forms the hydrogen bond statistically occupies three equivalent positions with probability 1/3 in the super-ionic phase (Baranov et al., 1987; Merinov et al., 1990). We used a similarly disordered model here. Selected geometric parameters are given in Table 1. The crystal structure is shown in Fig. 1 and the fitted diffraction profile for Tl3H(SO4)2 is shown in Fig. 2.

The structure of Tl3H(SO4)2 consists of isolated SO4 tetrahedra with Tl atoms distributed between them. The unit-cell volume of Tl3H(SO4)2 is 674.65 (6) Å3. This is very small in comparison with the unit-cell volumes of 733.5 Å3 for Rb3H(SeO4)2 (Baranov et al., 1987) and 839.0 Å3 for Cs3H(SeO4)2 (Merinov et al., 1990) in the super-ionic phase. In the subsequent discussion, the crystal data for Rb3H(SeO4)2 and Cs3H(SeO4)2 are taken from Baranov et al. (1987) and Merinov et al. (1990), respectively.

The O2—O2 bond lengths, which are hydrogen-bond lengths, are 2.626 (6) Å in Tl3H(SO4)2. This is slightly shorter than in the Rb3H(SeO4)2 [2.67 (2) Å] and Cs3H(SeO4)2 [2.71 Å] crystals. The S—O1 and S—O2 bond lengths in Tl3H(SO4)2 are 1.644 (6) and 1.641 (8) Å, respectively; thus, the S—O2 distance is very similar to the S—O1 distance. In contrast, in isotypic M3H(SeO4)2 crystals, the Se—O2 distances [1.682 (9) Å for Rb3H(SeO4)2 and 1.70 (2) Å for Cs3H(SeO4)2] are considerably longer than the Se—O1 distances [1.617 (3) Å for (Rb3H(SeO4)2) and 1.662 (8) Å for Cs3H(SeO4)2]. Thus, in Tl3H(SO4)2, the distortion of the SO4 tetrahedra is less than that of the isotypic M3H(SeO4)2 crystals.

It is known that the super-ionic phase transition is accompanied by a ferroelastic phase transition from the low-temperature ferroelastic phase to the high-temperature paraelastic phase. That is, the distortion in the crystal is closely related to the super-ionic phase transition. Therefore, the changes of the crystal structure accompanied by filling the M sites with Tl, in conjunction with the lesser distortion of the SO4 tetrahedra in comparison with the distortion of the tetrahedra in the isotypic crystals, would be the primary causes of the super-ionic phase transition at lower temperatures.

In this work, the coordinates of the H atom could not be determined, because H atoms migrate rapidly in the crystal in the super-ionic phase (Matsuo, 2001).

Experimental top

Crystals of Tl3H(SO4)2 were obtained from aqueous solutions of Tl2SO4 and H2SO4 in a molar ratio of 3:2. These crystals were grown by slow evaporation from a saturated solution at 313 K after several recrystallizations for purification.

Refinement top

The diffraction data were analyzed by the Rietveld method, with the atomic parameters of the isotypic Rb3H(SeO4)2 in its super-ionic phase as a starting model (Baranov et al., 1987). The profile shape was represented by a pseudo-Voigt function. In addition to the profile, lattice and structure parameters, the zero-point shift, ten background parameters, and the scale factor were determined with corrections for preferred orientation along (001) (the crystal shapes are very thin plates). Isotropic thermal vibrations were assumed. The interatomic distances and bond angles were calculated with ORFFE (Busing et al., 1964).

Computing details top

Data collection: RINT server software (Rigaku Corporation, 19??); cell refinement: RIETAN2000 (Izumi & Ikeda, 2000); data reduction: RINT server software; program(s) used to solve structure: RIETAN2000; program(s) used to refine structure: RIETAN2000; molecular graphics: ORTEP-3 for Windows (Farrugia, 2001); software used to prepare material for publication: RIETAN2000.

Figures top
[Figure 1] Fig. 1. A view of the unit cell of the crystal structure of Tl3H(SO4)2. The O2 atom occupies three equivalent positions with probability 1/3.
[Figure 2] Fig. 2. The fitted diffraction profile for Tl3H(SO4)2, showing calculated (line), observed (+) and difference (lower) profiles.
Trithallium hydrogen bis(sulfate) top
Crystal data top
Tl3H(SO4)2Dx = 5.946 (1) Mg m3
Mr = 806.28Cu Kα radiation, λ = 1.54184 Å
Trigonal, R3mT = 293 K
Hall symbol: -R 3 2Particle morphology: plate-like
a = 5.9376 (4) Åwhite
c = 22.0966 (9) Åflat sheet, 15 × 20 mm
V = 674.65 (6) Å3Specimen preparation: Prepared at 313 K
Z = 3
Data collection top
Rigaku RINT-1400
diffractometer
Data collection mode: reflection
Radiation source: rotating-anode X-ray tubeScan method: step
Specimen mounting: packed powder pellet2θmin = 5.000°, 2θmax = 110.00°, 2θstep = 0.006°
Refinement top
Refinement on InetProfile function: pseudo-Voigt
Least-squares matrix: full with fixed elements per cycle38 parameters
Rp = 0.065H-atom parameters not refined
Rwp = 0.094(Δ/σ)max = 0.01
Rexp = 0.076Background function: square polynomial for each range
χ2 = 1.513Preferred orientation correction: March-Dollase function, axis (001) (Dollase, 1986)
17501 data points
Crystal data top
Tl3H(SO4)2V = 674.65 (6) Å3
Mr = 806.28Z = 3
Trigonal, R3mCu Kα radiation, λ = 1.54184 Å
a = 5.9376 (4) ÅT = 293 K
c = 22.0966 (9) Åflat sheet, 15 × 20 mm
Data collection top
Rigaku RINT-1400
diffractometer
Scan method: step
Specimen mounting: packed powder pellet2θmin = 5.000°, 2θmax = 110.00°, 2θstep = 0.006°
Data collection mode: reflection
Refinement top
Rp = 0.06517501 data points
Rwp = 0.09438 parameters
Rexp = 0.076H-atom parameters not refined
χ2 = 1.513
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Tl10.00.00.00.038 (2)*
Tl20.00.00.20754 (3)0.081 (2)*
S10.00.00.4063 (2)0.103 (6)*
O10.1489 (6)0.1489 (6)0.4334 (2)0.0929 (6)*
O20.0390 (2)0.0390 (2)0.3343 (4)0.0542 (8)*0.3333333
Geometric parameters (Å, º) top
Tl1—S1i3.788 (2)S1—O11.644 (6)
Tl2—S1ii3.621 (2)S1—O21.641 (8)
Tl1—O1iii2.913 (5)O1—O1iv2.65 (1)
Tl2—O1ii3.028 (3)O1—O2v2.586 (7)
Tl2—O22.830 (8)O2—O2vi2.626 (6)
O1—S1—O1iv107.52 (6)O1—S1—O2v103.85 (6)
Symmetry codes: (i) x+1/3, y+2/3, z1/3; (ii) y+1/3, x+2/3, z+2/3; (iii) y+1/3, xy+2/3, z1/3; (iv) x+y, x, z; (v) y, xy, z; (vi) y+1/3, x1/3, z+2/3.

Experimental details

Crystal data
Chemical formulaTl3H(SO4)2
Mr806.28
Crystal system, space groupTrigonal, R3m
Temperature (K)293
a, c (Å)5.9376 (4), 22.0966 (9)
V3)674.65 (6)
Z3
Radiation typeCu Kα, λ = 1.54184 Å
Specimen shape, size (mm)Flat sheet, 15 × 20
Data collection
DiffractometerRigaku RINT-1400
diffractometer
Specimen mountingPacked powder pellet
Data collection modeReflection
Scan methodStep
2θ values (°)2θmin = 5.000 2θmax = 110.00 2θstep = 0.006
Refinement
R factors and goodness of fitRp = 0.065, Rwp = 0.094, Rexp = 0.076, χ2 = 1.513
No. of data points17501
No. of parameters38
No. of restraints?
H-atom treatmentH-atom parameters not refined

Computer programs: RINT server software (Rigaku Corporation, 19??), RIETAN2000 (Izumi & Ikeda, 2000), RINT server software, RIETAN2000, ORTEP-3 for Windows (Farrugia, 2001).

Selected geometric parameters (Å, º) top
S1—O11.644 (6)O1—O2ii2.586 (7)
S1—O21.641 (8)O2—O2iii2.626 (6)
O1—O1i2.65 (1)
O1—S1—O1i107.52 (6)O1—S1—O2ii103.85 (6)
Symmetry codes: (i) x+y, x, z; (ii) y, xy, z; (iii) y+1/3, x1/3, z+2/3.
 

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