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The crystal structure of thallium carbonate, Tl2CO3 (C2/m, Z = 4), is stable at least up to 3.56 GPa, as demonstrated by hydro­static single-crystal X-ray diffraction measurements in a diamond anvil cell at room temperature. Our results contradict earlier observations from the literature, which found a structural phase transition for this compound at about 2 GPa. Under atmospheric conditions, all atoms except for one O atom reside on the mirror plane in the high-pressure structure. The compression mainly affects the part of the structure where the nonbonded electron lone pairs on the Tl+ cations are located.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270108020398/sq3152sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270108020398/sq3152Isup2.hkl
Contains datablock I

Comment top

At atmospheric pressure, thallium carbonate, Tl2CO3, crystallizes in space group C2/m (Z = 4) (Marchand et al., 1975). The planar carbonate groups are parallel to [100]. Two non-equivalent Tl+ cations are in asymmetric coordination environments, attributable to their electron lone pairs which are arranged in tunnels parallel to [010]. The coordination sphere of Tl1 includes five O atoms at distances in the range 2.67–2.82 Å and two at distances of 3.36 Å each. The coordination around the Tl2 cation includes four O atoms at distances in the range 2.67–2.69 Å, and two O atoms at distances of 3.24 and 3.60 Å. Considering only the Tl—O distances below [less than?] 3 Å, the crystal structure could be viewed as a stack of corrugated layers of cations and carbonate groups along the a axis (see Fig. 1). The nonbonded electron lone pairs are located in between the layers.

The high-pressure behaviour of Tl2CO3 has already been studied by various experimental techniques by Pistorius & Clark (1969), Meisalo & Kalliomäki (1976), Adams et al. (1983) and Lee et al. (1993). Based on optical observations of the sample in a diamond anvil cell, a sequence of phase transitions to closely related crystal structures at 2 , 4.2 and 6.7 GPa was postulated by Meisalo & Kalliomäki (1976). They described the X-ray powder patterns for all three polymorphs existing below 6.7 GPa as very similar. The powder pattern of the polymorph at pressures higher than 6.7 GPa was said to be `distinctly different'. Adams et al. (1983) also detected phase transitions near 1.3 and 3.8 GPa using infrared and Raman spectroscopies. They argued that the new polymorph occurring between 1.3 and 3.8 GPa has either a C- or an I-centred orthorhombic lattice.

Crystal structures and high-pressure behaviours of M2CO3 carbonates have been shown to depend on the M+ cation (M = Li, Na, K, Rb, Cs), with the carbonate groups being rigid at extreme conditions (Grzechnik et al., 2003; Cancarevic et al., 2006). These compounds are of interest due to their various phase transitions (including ferroic ones) and their modulated structures (Dušek et al., 2003). The structure of Tl2CO3 at atmospheric pressure is distinct because of the stereochemical influence of the nonbonded electron lone pair on thallium. The presence of the lone pairs could be considered a feature of covalent bonding (Marchand et al., 1975; Grzechnik, 2007). The structural characterization of a new polymorph, presumably forming due to a pressure-induced phase transition at about 2 GPa (Meisalo & Kalliomäki, 1976; Adams et al., 1983), thus offers an opportunity to elucidate the high-pressure behaviour of the lone pair and its participation in the increase in symmetry in Tl2CO3. Hence, we have performed a single-crystal X-ray diffraction study of thallium carbonate to determine its structure between 2 and 4.2 GPa.

The indexing of the single-crystal X-ray diffraction data and analysis of the reconstructed reciprocal space indicated that Tl2CO3 (C2/m, Z = 4) does not undergo any phase transition at about 2 GPa. Its crystal structure is stable upon compression at hydrostatic conditions at least up to 3.56 GPa at room temperature. Our observation clearly contradicts the previous reports by Meisalo & Kalliomäki (1976) and Adams et al. (1983). It is quite likely that these earlier studies suffered from insufficient resolution or non-hydrostatic conditions.

The lattice parameters at 3.56 GPa can be compared with those at ambient pressure (Marchand et al., 1975). This comparison shows that the monoclinic β angle in Tl2CO3 increases upon compression and that the a lattice parameter is the most compressible one. The large compressibility of a can be attributed to the fact that the changes in the interlayer Tl—O and Tl—Tl distances are the largest. The compression thus mainly takes place in the region of the structure where the Tl+ lone pairs are located.

The shortest C—C distance in Tl2CO3 at ambient pressure is 3.46 Å (Marchand et al., 1975), comparable with values observed in other M2CO3 carbonates: 3.16 Å in Li2CO3 (Effenberg & Zemann, 1979) or up to 4 Å for Cs2CO3 (Ehrhardt et al., 1980). In the high-pressure form of Li2CO3 at 10 GPa this distance decreases to 2.57 Å (Grzechnik et al., 2003; Cancarevic et al., 2006). In the case of Tl2CO3, the contraction of the interlayer spaces also results in a decrease in the shortest C—C distance to 3.09 Å at 3.65 GPa.

Related literature top

For related literature, see: Adams et al. (1983); Ahsbahs (1995, 2004); Angel et al. (2007); Cancarevic et al. (2006); Dušek et al. (2003); Effenberg & Zemann (1979); Ehrhardt et al. (1980); Grzechnik et al. (2003, 2007); Mao et al. (1986); Petricek et al. (2006).

Experimental top

The crystals were synthesized according to the method described by Marchand et al. (1975). High-pressure data were collected at 0.69, 2.37 and 3.56 GPa in an Ahsbahs-type diamond anvil cell (Ahsbahs, 1995, 2004) at room temperature using a Stoe IPDS 2T diffractometer with Mo Kα radiation. A 0.25 mm hole was drilled into a stainless steel gasket preindented to a thickness of about 0.12 mm. The intensities were indexed, integrated and corrected for absorption using Stoe software (Stoe & Cie, 1998). The shape of the crystal was approximated by 20 faces using the program X-SHAPE (Stoe & Cie, 1998). Shaded areas of the images by the diamond anvil cell were masked prior to integration. Because of their hemispherical shape, no absorption correction was necessary for the diamond anvils. The ruby luminescence method (Mao et al., 1986) was used for pressure calibration, and 2-propanol, which is hydrostatic to 4.20 GPa (Angel et al., 2007) and does not react with Tl2CO3, was used as a pressure medium.

Refinement top

Data at 3.56 GPa were refined with the program JANA2006 (Petricek et al., 2006). The two Tl atoms were refined anisotropically. Isotropic displacement parameters of the two O atoms were restrained to be equal and the isotropic displacement parameter of C was set to 0.5Uiso of the O atoms. Due to the fact that the C-atom position could not be refined reliably, we had to introduce distance restraints for the carbonate group. These were C—O1 = 1.24 Å and C—O2 = 1.28 Å in accordance with the ambient pressure data published by Marchand et al. (1975). This restriction is justified as the carbonate groups in other M2CO3 compounds are rigid at high pressures (Grzechnik et al., 2003; Cancarevic et al., 2006).

Computing details top

Data collection: X-AREA (Stoe & Cie, 1998); cell refinement: X-AREA (Stoe & Cie, 1998); data reduction: JANA2006 (Petricek et al., 2006); program(s) used to solve structure: Coordinates from model; program(s) used to refine structure: JANA2006 (Petricek et al., 2006); software used to prepare material for publication: JANA2006 (Petricek et al., 2006).

Figures top
[Figure 1] Fig. 1. The crystal structure of Tl2CO3 at ambient pressure. The Tl—O bond distances are shown up to 3.5 Å (those less than and greater than 3 Å are drawn as thick and thin lines, respectively).
Thallium carbonate top
Crystal data top
Tl2CO3F(000) = 768
Mr = 468.8Dx = 8.126 Mg m3
Monoclinic, C2/mMo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yCell parameters from 971 reflections
a = 12.006 (6) Åθ = 4.8–28.6°
b = 5.2173 (7) ŵ = 83.87 mm1
c = 7.292 (1) ÅT = 300 K
β = 123.01 (3)°Irregular shape, colourless
V = 383.0 (2) Å30.10 × 0.09 × 0.06 mm
Z = 4
Data collection top
Stoe IPDS 2T
diffractometer
176 independent reflections
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus110 reflections with I > 3σ(I)
Plane graphite monochromatorRint = 0.087
Detector resolution: 6.67 pixels mm-1θmax = 28.4°, θmin = 4.8°
rotation method scansh = 910
Absorption correction: numerical
(X-RED; Stoe & Cie, 1998)
k = 66
Tmin = 0.004, Tmax = 0.013l = 99
475 measured reflections
Refinement top
Refinement on F4 restraints
R[F2 > 2σ(F2)] = 0.047Weighting scheme based on measured s.u.'s w = 1/[σ2(F) + 0.0001F2]
wR(F2) = 0.054(Δ/σ)max = 0.022
S = 2.18Δρmax = 1.65 e Å3
176 reflectionsΔρmin = 2.49 e Å3
21 parameters
Crystal data top
Tl2CO3V = 383.0 (2) Å3
Mr = 468.8Z = 4
Monoclinic, C2/mMo Kα radiation
a = 12.006 (6) ŵ = 83.87 mm1
b = 5.2173 (7) ÅT = 300 K
c = 7.292 (1) Å0.10 × 0.09 × 0.06 mm
β = 123.01 (3)°
Data collection top
Stoe IPDS 2T
diffractometer
176 independent reflections
Absorption correction: numerical
(X-RED; Stoe & Cie, 1998)
110 reflections with I > 3σ(I)
Tmin = 0.004, Tmax = 0.013Rint = 0.087
475 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.04721 parameters
wR(F2) = 0.0544 restraints
S = 2.18Δρmax = 1.65 e Å3
176 reflectionsΔρmin = 2.49 e Å3
Special details top

Refinement. Restrained refinement for the carbonate group.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Tl10.0730 (8)00.3094 (8)0.042 (9)
Tl20.1387 (9)0.50.7498 (9)0.056 (9)
C10.148 (14)0.50.188 (13)0.022 (4)*
O10.177 (13)0.50.379 (12)0.045 (7)*
O20.158 (8)0.293 (5)0.103 (8)0.045 (7)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Tl10.059 (15)0.0357 (14)0.039 (5)00.032 (8)0
Tl20.092 (16)0.0438 (17)0.050 (5)00.051 (9)0
Geometric parameters (Å, º) top
Tl1—Tl1i3.849 (8)Tl2—C1xii3.13 (13)
Tl1—Tl1ii4.014 (13)Tl2—C1ii3.71 (17)
Tl1—Tl2iii3.871 (7)Tl2—C1v3.51 (11)
Tl1—Tl23.871 (7)Tl2—C1xiii3.51 (11)
Tl1—Tl2iv3.501 (10)Tl2—O12.98 (13)
Tl1—Tl2ii3.501 (10)Tl2—O1ii3.37 (16)
Tl1—Tl2v3.714 (15)Tl2—O1v3.85 (12)
Tl1—C1iii3.05 (9)Tl2—O1xiii3.85 (12)
Tl1—C13.05 (9)Tl2—O2xii2.68 (7)
Tl1—C1vi4.06 (6)Tl2—O2xiii2.57 (8)
Tl1—C1i4.06 (6)Tl2—O2viii2.57 (8)
Tl1—C1v3.36 (8)Tl2—O2xiv2.68 (7)
Tl1—O1iii2.82 (6)C1—C1i3.09 (16)
Tl1—O12.82 (6)C1—O11.24 (14)
Tl1—O1v2.59 (11)C1—O1i3.84 (12)
Tl1—O22.71 (9)C1—O1v3.73 (7)
Tl1—O2i3.16 (5)C1—O1xiii3.73 (7)
Tl1—O2v3.85 (5)C1—O21.28 (9)
Tl1—O2vii3.16 (5)C1—O2i3.27 (15)
Tl1—O2viii3.85 (5)C1—O2xv3.27 (15)
Tl1—O2ix2.71 (9)C1—O2xvi1.28 (9)
Tl2—Tl2ii3.338 (8)O1—O22.18 (12)
Tl2—Tl2x4.079 (6)O1—O2xvi2.18 (12)
Tl2—Tl2xi4.079 (6)O2—O2xvi2.16 (4)
O1iii—Tl1—O1136 (4)O2v—Tl1—O2ix96.8 (16)
O1iii—Tl1—O1v70 (2)O2vii—Tl1—O2viii164.2 (15)
O1iii—Tl1—O2111 (4)O2vii—Tl1—O2ix66 (2)
O1iii—Tl1—O2i133.9 (16)O2viii—Tl1—O2ix116 (2)
O1iii—Tl1—O2v61.3 (15)O1—Tl2—O1ii117 (3)
O1iii—Tl1—O2vii78.3 (19)O1—Tl2—O1v52.1 (17)
O1iii—Tl1—O2viii92.2 (15)O1—Tl2—O1xiii52.1 (17)
O1iii—Tl1—O2ix47 (3)O1—Tl2—O2xii153.6 (18)
O1—Tl1—O1v70 (2)O1—Tl2—O2xiii78 (3)
O1—Tl1—O247 (3)O1—Tl2—O2viii78 (3)
O1—Tl1—O2i78.3 (19)O1—Tl2—O2xiv153.6 (18)
O1—Tl1—O2v92.2 (15)O1ii—Tl2—O1v129.7 (18)
O1—Tl1—O2vii133.9 (16)O1ii—Tl2—O1xiii129.7 (18)
O1—Tl1—O2viii61.3 (15)O1ii—Tl2—O2xii76 (2)
O1—Tl1—O2ix111 (4)O1ii—Tl2—O2xiii141.7 (17)
O1v—Tl1—O283 (3)O1ii—Tl2—O2viii141.7 (17)
O1v—Tl1—O2i146 (2)O1ii—Tl2—O2xiv76 (2)
O1v—Tl1—O2v33 (3)O1v—Tl2—O1xiii85 (2)
O1v—Tl1—O2vii146 (2)O1v—Tl2—O2xii102 (2)
O1v—Tl1—O2viii33 (3)O1v—Tl2—O2xiii88 (2)
O1v—Tl1—O2ix83 (3)O1v—Tl2—O2viii32.6 (17)
O2—Tl1—O2i66 (2)O1v—Tl2—O2xiv138.2 (17)
O2—Tl1—O2v116 (2)O1xiii—Tl2—O2xii138.2 (17)
O2—Tl1—O2vii98.4 (18)O1xiii—Tl2—O2xiii32.6 (17)
O2—Tl1—O2viii96.8 (16)O1xiii—Tl2—O2viii88 (2)
O2—Tl1—O2ix68.9 (18)O1xiii—Tl2—O2xiv102 (2)
O2i—Tl1—O2v164.2 (15)O2xii—Tl2—O2xiii106 (2)
O2i—Tl1—O2vii57.8 (9)O2xii—Tl2—O2viii78 (3)
O2i—Tl1—O2viii133.7 (9)O2xii—Tl2—O2xiv47.4 (12)
O2i—Tl1—O2ix98.4 (18)O2xiii—Tl2—O2viii73.2 (19)
O2v—Tl1—O2vii133.7 (9)O2xiii—Tl2—O2xiv78 (3)
O2v—Tl1—O2viii32.5 (6)O2viii—Tl2—O2xiv106 (2)
Symmetry codes: (i) x, y, z; (ii) x, y, z+1; (iii) x, y1, z; (iv) x, y1, z+1; (v) x+1/2, y1/2, z+1; (vi) x, y1, z; (vii) x, y, z; (viii) x+1/2, y+1/2, z+1; (ix) x, y, z; (x) x+1/2, y1/2, z+2; (xi) x+1/2, y+1/2, z+2; (xii) x, y, z+1; (xiii) x+1/2, y+1/2, z+1; (xiv) x, y+1, z+1; (xv) x, y+1, z; (xvi) x, y+1, z.

Experimental details

Crystal data
Chemical formulaTl2CO3
Mr468.8
Crystal system, space groupMonoclinic, C2/m
Temperature (K)300
a, b, c (Å)12.006 (6), 5.2173 (7), 7.292 (1)
β (°) 123.01 (3)
V3)383.0 (2)
Z4
Radiation typeMo Kα
µ (mm1)83.87
Crystal size (mm)0.10 × 0.09 × 0.06
Data collection
DiffractometerStoe IPDS 2T
diffractometer
Absorption correctionNumerical
(X-RED; Stoe & Cie, 1998)
Tmin, Tmax0.004, 0.013
No. of measured, independent and
observed [I > 3σ(I)] reflections
475, 176, 110
Rint0.087
(sin θ/λ)max1)0.670
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.047, 0.054, 2.18
No. of reflections176
No. of parameters21
No. of restraints4
Δρmax, Δρmin (e Å3)1.65, 2.49

Computer programs: X-AREA (Stoe & Cie, 1998), JANA2006 (Petricek et al., 2006), Coordinates from model.

Selected bond lengths (Å) top
Tl1—O12.82 (6)Tl2—O12.98 (13)
Tl1—O1i2.59 (11)Tl2—O1v3.37 (16)
Tl1—O22.71 (9)Tl2—O2vi2.68 (7)
Tl1—O2ii3.16 (5)Tl2—O2vii2.57 (8)
Tl1—O2iii3.16 (5)Tl2—O2viii2.57 (8)
Tl1—O2iv2.71 (9)Tl2—O2ix2.68 (7)
Symmetry codes: (i) x+1/2, y1/2, z+1; (ii) x, y, z; (iii) x, y, z; (iv) x, y, z; (v) x, y, z+1; (vi) x, y, z+1; (vii) x+1/2, y+1/2, z+1; (viii) x+1/2, y+1/2, z+1; (ix) x, y+1, z+1.
 

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