Tl₂CO₃ at 3.56 GPa

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.

S1. Comment

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

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).

S2. Experimental

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.

S3. Refinement

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 0.5U_iso 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

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

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

Crystal data

Tl2CO3	F(000) = 768
Mr = 468.8	Dx = 8.126 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 971 reflections
a = 12.006 (6) Å	θ = 4.8–28.6°
b = 5.2173 (7) Å	µ = 83.87 mm^−1
c = 7.292 (1) Å	T = 300 K
β = 123.01 (3)°	Irregular shape, colourless
V = 383.0 (2) Å^3	0.10 × 0.09 × 0.06 mm
Z = 4

Data collection

Stoe IPDS 2T diffractometer	176 independent reflections
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus	110 reflections with I > 3σ(I)
Plane graphite monochromator	Rint = 0.087
Detector resolution: 6.67 pixels mm^-1	θmax = 28.4°, θmin = 4.8°
rotation method scans	h = −9→10
Absorption correction: numerical (X-RED; Stoe & Cie, 1998)	k = −6→6
Tmin = 0.004, Tmax = 0.013	l = −9→9
475 measured reflections

Refinement

Refinement on F	4 restraints
R[F^2 > 2σ(F^2)] = 0.047	Weighting scheme based on measured s.u.'s w = 1/[σ^2(F) + 0.0001F^2]
wR(F^2) = 0.054	(Δ/σ)max = 0.022
S = 2.18	Δρmax = 1.65 e Å^−3
176 reflections	Δρmin = −2.49 e Å^−3
21 parameters

Special details

Refinement. Restrained refinement for the carbonate group.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Tl1	0.0730 (8)	0	0.3094 (8)	0.042 (9)
Tl2	0.1387 (9)	0.5	0.7498 (9)	0.056 (9)
C1	0.148 (14)	0.5	0.188 (13)	0.022 (4)*
O1	0.177 (13)	0.5	0.379 (12)	0.045 (7)*
O2	0.158 (8)	0.293 (5)	0.103 (8)	0.045 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl1	0.059 (15)	0.0357 (14)	0.039 (5)	0	0.032 (8)	0
Tl2	0.092 (16)	0.0438 (17)	0.050 (5)	0	0.051 (9)	0

Geometric parameters (Å, º)

Tl1—Tl1^i	3.849 (8)	Tl2—C1^xii	3.13 (13)
Tl1—Tl1^ii	4.014 (13)	Tl2—C1^ii	3.71 (17)
Tl1—Tl2^iii	3.871 (7)	Tl2—C1^v	3.51 (11)
Tl1—Tl2	3.871 (7)	Tl2—C1^xiii	3.51 (11)
Tl1—Tl2^iv	3.501 (10)	Tl2—O1	2.98 (13)
Tl1—Tl2^ii	3.501 (10)	Tl2—O1^ii	3.37 (16)
Tl1—Tl2^v	3.714 (15)	Tl2—O1^v	3.85 (12)
Tl1—C1^iii	3.05 (9)	Tl2—O1^xiii	3.85 (12)
Tl1—C1	3.05 (9)	Tl2—O2^xii	2.68 (7)
Tl1—C1^vi	4.06 (6)	Tl2—O2^xiii	2.57 (8)
Tl1—C1^i	4.06 (6)	Tl2—O2^viii	2.57 (8)
Tl1—C1^v	3.36 (8)	Tl2—O2^xiv	2.68 (7)
Tl1—O1^iii	2.82 (6)	C1—C1^i	3.09 (16)
Tl1—O1	2.82 (6)	C1—O1	1.24 (14)
Tl1—O1^v	2.59 (11)	C1—O1^i	3.84 (12)
Tl1—O2	2.71 (9)	C1—O1^v	3.73 (7)
Tl1—O2^i	3.16 (5)	C1—O1^xiii	3.73 (7)
Tl1—O2^v	3.85 (5)	C1—O2	1.28 (9)
Tl1—O2^vii	3.16 (5)	C1—O2^i	3.27 (15)
Tl1—O2^viii	3.85 (5)	C1—O2^xv	3.27 (15)
Tl1—O2^ix	2.71 (9)	C1—O2^xvi	1.28 (9)
Tl2—Tl2^ii	3.338 (8)	O1—O2	2.18 (12)
Tl2—Tl2^x	4.079 (6)	O1—O2^xvi	2.18 (12)
Tl2—Tl2^xi	4.079 (6)	O2—O2^xvi	2.16 (4)
O1^iii—Tl1—O1	136 (4)	O2^v—Tl1—O2^ix	96.8 (16)
O1^iii—Tl1—O1^v	70 (2)	O2^vii—Tl1—O2^viii	164.2 (15)
O1^iii—Tl1—O2	111 (4)	O2^vii—Tl1—O2^ix	66 (2)
O1^iii—Tl1—O2^i	133.9 (16)	O2^viii—Tl1—O2^ix	116 (2)
O1^iii—Tl1—O2^v	61.3 (15)	O1—Tl2—O1^ii	117 (3)
O1^iii—Tl1—O2^vii	78.3 (19)	O1—Tl2—O1^v	52.1 (17)
O1^iii—Tl1—O2^viii	92.2 (15)	O1—Tl2—O1^xiii	52.1 (17)
O1^iii—Tl1—O2^ix	47 (3)	O1—Tl2—O2^xii	153.6 (18)
O1—Tl1—O1^v	70 (2)	O1—Tl2—O2^xiii	78 (3)
O1—Tl1—O2	47 (3)	O1—Tl2—O2^viii	78 (3)
O1—Tl1—O2^i	78.3 (19)	O1—Tl2—O2^xiv	153.6 (18)
O1—Tl1—O2^v	92.2 (15)	O1^ii—Tl2—O1^v	129.7 (18)
O1—Tl1—O2^vii	133.9 (16)	O1^ii—Tl2—O1^xiii	129.7 (18)
O1—Tl1—O2^viii	61.3 (15)	O1^ii—Tl2—O2^xii	76 (2)
O1—Tl1—O2^ix	111 (4)	O1^ii—Tl2—O2^xiii	141.7 (17)
O1^v—Tl1—O2	83 (3)	O1^ii—Tl2—O2^viii	141.7 (17)
O1^v—Tl1—O2^i	146 (2)	O1^ii—Tl2—O2^xiv	76 (2)
O1^v—Tl1—O2^v	33 (3)	O1^v—Tl2—O1^xiii	85 (2)
O1^v—Tl1—O2^vii	146 (2)	O1^v—Tl2—O2^xii	102 (2)
O1^v—Tl1—O2^viii	33 (3)	O1^v—Tl2—O2^xiii	88 (2)
O1^v—Tl1—O2^ix	83 (3)	O1^v—Tl2—O2^viii	32.6 (17)
O2—Tl1—O2^i	66 (2)	O1^v—Tl2—O2^xiv	138.2 (17)
O2—Tl1—O2^v	116 (2)	O1^xiii—Tl2—O2^xii	138.2 (17)
O2—Tl1—O2^vii	98.4 (18)	O1^xiii—Tl2—O2^xiii	32.6 (17)
O2—Tl1—O2^viii	96.8 (16)	O1^xiii—Tl2—O2^viii	88 (2)
O2—Tl1—O2^ix	68.9 (18)	O1^xiii—Tl2—O2^xiv	102 (2)
O2^i—Tl1—O2^v	164.2 (15)	O2^xii—Tl2—O2^xiii	106 (2)
O2^i—Tl1—O2^vii	57.8 (9)	O2^xii—Tl2—O2^viii	78 (3)
O2^i—Tl1—O2^viii	133.7 (9)	O2^xii—Tl2—O2^xiv	47.4 (12)
O2^i—Tl1—O2^ix	98.4 (18)	O2^xiii—Tl2—O2^viii	73.2 (19)
O2^v—Tl1—O2^vii	133.7 (9)	O2^xiii—Tl2—O2^xiv	78 (3)
O2^v—Tl1—O2^viii	32.5 (6)	O2^viii—Tl2—O2^xiv	106 (2)

Symmetry codes: (i) −x, y, −z; (ii) −x, y, −z+1; (iii) x, y−1, z; (iv) −x, y−1, −z+1; (v) −x+1/2, y−1/2, −z+1; (vi) −x, y−1, −z; (vii) −x, −y, −z; (viii) −x+1/2, −y+1/2, −z+1; (ix) x, −y, z; (x) −x+1/2, y−1/2, −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.