Crystal structure and stability of Tl₂CO₃ at high pressures

The crystal structure of dithallium carbonate, Tl₂CO₃ (C2/m, Z = 4), was investigated at pressures of up to 7.4 GPa using single-crystal X-ray diffraction in a diamond anvil cell. It is stable to at least 5.82 GPa. All atoms except for one of the O atoms lie on crystallographic mirror planes. At higher pressures, the material undergoes a phase transition that destroys the single crystal.

S1. Comment

Pistorius & Clark (1969), Meisalo & Kalliomäki (1976), Adams et al. (1983), Lee et al. (1993) and Grzechnik & Friese (2008) reported previously on the high-pressure behaviour of Tl₂CO₃. A sequence of phase transitions at 2, 4.2 and 6.7 GPa was discovered by Meisalo & Kalliomäki (1976), in which the phases below 6.7 GPa were supposed to be structurally very similar. Adams et al. (1983) also observed phase transitions near 1.3 and 3.8 GPa using IR and Raman spectroscopies. However, a single-crystal diffraction study demonstrated that thallium carbonate (C2/m, Z = 4) is structurally stable at least to 3.56 GPa (Grzechnik & Friese, 2008). The most likely reason for this discrepancy is that the X-ray and spectroscopic data by Meisalo & Kalliomäki (1976) and Adams et al. (1983), respectively, were poorly resolved and the pressures were non-hydrostatic. In this study, we continue our work on Tl₂CO₃ using single-crystal X-ray diffraction in a diamond anvil cell to characterize the postulated (Meisalo & Kalliomäki, 1976; Adams et al., 1983) pressure-induced polymorphs above about 4 and 6.7 GPa.

A crystal of dithallium carbonate was slowly compressed to a pressure of 5.82 GPa. Indexing of the diffraction data, analysis of the reconstructed reciprocal space, and structure solution and refinement clearly showed that the material (C2/m, Z = 4) does not transform to a new polymorph at about 4 GPa. However, on further compression to a pressure of 7.4 GPa, no single-crystal reflections were detected. Instead, weak and very smeared incomplete Debye–Scherrer rings were visible in the diffraction diagrams on the image plate. These observations indicate that Tl₂CO₃ does undergo a pressure-induced phase transition at about 6.7 GPa (Meisalo & Kalliomäki, 1976) that also destroys the single crystal. It is thus the only transformation of those reported in the literature that we observe in our high-pressure single-crystal X-ray diffraction data under hydrostatic conditions.

As observed at ambient pressure (Marchand et al., 1975) and at 3.56 GPa (Grzechnik & Friese, 2008), all atoms except for one of the O atoms lie on crystallographic mirror planes at 5.82 GPa. Two non-equivalent Tl⁺ cations are in asymmetric coordination environments attributable to their electron lone pairs (E). The Tl1 cation is coordinated to seven O atoms at distances in the range 2.50 (11)–2.9 (2) Å. The coordination around the Tl2 cation includes five O atoms at distances in the range 2.42 (19)–3.2 (3) Å. The compression mainly affects the part of the structure where the Tl⁺ lone pairs are placed. A comparison with the structural data at lower pressures shows that it is the longest Tl—O distances that diminish the most, while the short ones are relatively incompressible or even increase slightly. The fact that the spread of the Tl—O distances becomes smaller on compression to 5.82 GPa indicates that the coordination environments around the Tl atoms tend to become more uniform, due to the diminished (but nevertheless still existent) stereoactivity of the electron lone pairs.

When only the Tl—O distances below 3 Å are considered, the crystal structure at ambient conditions can be viewed as a stack of corrugated layers of cations and carbonate groups along the a axis (Fig. 1). The suppression of the E pairs results in the structure losing its layered character at 5.82 GPa. This is also reflected in the fact that the Tl···Tl distances between adjacent layers are considerably shortened with increasing pressure. Thus, the interlayer Tl1···Tl2 and Tl2···Tl2 distances are 3.588 (1) and 3.693 (1) Å, respectively, at ambient pressure, 3.50 (1) and 3.338 (8) Å, respectively, at 3.56 GPa, and 3.42 (2) and 3.26 (2) Å, respectively, at 5.82 GPa, indicating increasing Tl···Tl interactions between the layers on compression. These interactions might be responsible for the axial compressibility along the a axis that (within the estimated standard deviations) changes little between 3.56 and 5.82 GPa.

The other striking aspect of the high-pressure behaviour of Tl₂CO₃ is the rotation of the carbonate groups to accommodate the electron lone pairs (Fig. 1). The pressure-induced lattice contraction and changes in the orientation of the carbonate group in Tl₂CO₃ cause a decrease of the shortest C···C distance from 3.46 Å under ambient conditions (Marchand et al., 1975) to 2.4 (3) Å at 5.82 GPa. At the intermediate pressure of 3.56 GPa, this distance is 3.09 (16) Å (Grzechnik & Friese, 2008). Since experimental data for the shortest C—C distances in X₂CO₃ (X = Li, Na, K, Rb, Cs or Tl) at higher pressures are not available, the result of this study can only be compared with the theoretical work by Cancarevic et al. (2006), in which the C···C distances in high-pressure phases of Li₂CO₃ are expected to be below 2.5 Å.

S2. Experimental

The crystals were synthesized according to the method described by Marchand et al. (1975). High-pressure data were collected at 5.82 and 7.40 GPa in a Boehler–Almax-type diamond anvil cell (Boehler, 2006) at room temperature using a Stoe IPDS 2T diffractometer with Mo Kα radiation. A 0.250 mm hole was drilled into a stainless steel gasket preindented to a thickness of about 0.08 mm. The intensities were indexed, integrated and corrected for absorption using the Stoe software (Stoe & Cie, 2006). Areas of the images shaded by the diamond anvil cell were masked prior to integration. Corrections for the effect of absorption by the diamond anvil and the crystal were made using the program ABSORB (Angel, 2004). The ruby luminescence method (Mao et al., 1986) was used for pressure calibration and a 1:1 mixture of pentane and isopentane was used as pressure medium. The hydrostatic pressure limit is 7.4 GPa (Piermarini et al., 1973), higher than that of propan-2-ol, which is 4.2 GPa (Angel et al., 2007), used in the previous work on this compound (Grzechnik & Friese, 2008).

S3. Refinement

Data at 5.82 GPa were refined with the program JANA2006 (Petříček et al., 2006). All atoms were refined isotropically. 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(O). Due to the fact that the C-atom position could not be refined reliably, we had to introduce distance and angle restraints for the carbonate group, as in our previous study of Tl₂CO₃ (Grzechnik & Friese, 2008).

Computing details

Data collection: X-AREA (Stoe & Cie, 2006); cell refinement: X-AREA (Stoe & Cie, 2006); data reduction: JANA2006 (Petříček et al., 2006); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: JANA2006 (Petříček et al., 2006); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: JANA2006 (Petříček et al., 2006).

Figures

Fig. 1. Crystal structures of Tl2CO3 at different pressures. Tl—O bond distances below 3.5 Å are shown. Distances longer than and shorter than 3 Å are drawn as thin and thick grey lines, respectively. The C—O distances are drawn as thick black lines.

Dithallium carbonate

Crystal data

Tl2CO3	F(000) = 768
Mr = 468.8	Dx = 8.244 Mg m^−3
Monoclinic, C2/m	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2y	Cell parameters from 931 reflections
a = 12.02 (2) Å	θ = 4.4–28.1°
b = 5.150 (2) Å	µ = 85.08 mm^−1
c = 7.198 (4) Å	T = 300 K
β = 122.08 (10)°	Irregular shape, colourless
V = 377.6 (8) Å^3	0.10 × 0.07 × 0.05 mm
Z = 4

Data collection

Stoe IPDS 2T diffractometer	81 reflections with I > 3σ(I)
Radiation source: sealed X-ray tube, 12 x 0.4 mm long-fine focus	Rint = 0.114
Plane graphite monochromator	θmax = 28.1°, θmin = 4.4°
Detector resolution: 6.67 pixels mm^-1	h = −9→9
rotation method scans	k = −6→6
931 measured reflections	l = −8→9
142 independent reflections

Refinement

Refinement on F	1 constraint
R[F^2 > 2σ(F^2)] = 0.091	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
wR(F^2) = 0.088	(Δ/σ)max = 0.041
S = 2.99	Δρmax = 4.31 e Å^−3
142 reflections	Δρmin = −4.44 e Å^−3
15 parameters	Extinction correction: B-C type 1 Gaussian isotropic (Becker & Coppens, 1974)
6 restraints	Extinction coefficient: 0

Special details

Refinement. All atoms isotropic. C—O distances and O—C—O angles in Carbonate group restricted. Uiso(O1) restricted to be equal to Uiso(O2). Uiso(C) restricted to 0.5*Uiso(O)

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

	x	y	z	Uiso*/Ueq
Tl1	0.065 (2)	0	0.306 (2)	0.033 (2)*
Tl2	0.3650 (18)	0	0.2592 (19)	0.028 (2)*
O2	0.10 (2)	0.280 (8)	0.002 (19)	0.046 (17)*
O1	0.16 (3)	0.5	0.32 (2)	0.046 (17)*
C1	0.12 (2)	0.5	0.105 (19)	0.023 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
?	?	?	?	?	?	?

Geometric parameters (Å, º)

Tl1—Tl2^i	3.42 (2)	Tl2—O2^ix	2.42 (19)
Tl1—Tl2^ii	3.42 (2)	Tl2—O2^x	2.42 (19)
Tl1—O2	2.82 (18)	Tl2—O2^v	3.07 (18)
Tl1—O2^iii	2.50 (11)	Tl2—O1^xi	3.4 (3)
Tl1—O2^iv	2.50 (11)	Tl2—O1^vii	3.2 (3)
Tl1—O2^v	2.82 (18)	O2—O2^iii	2.4 (4)
Tl1—O1^vi	2.81 (14)	O2—O2^xii	2.26 (6)
Tl1—O1	2.81 (14)	O2—O1	2.3 (2)
Tl1—O1^vii	2.9 (2)	C1—O2	1.31 (11)
Tl1—C1^vi	3.17 (14)	C1—O2^xii	1.31 (11)
Tl1—C1	3.17 (14)	C1—O1	1.4 (2)
Tl2—Tl2^viii	3.26 (2)	C1—C1^iii	2.4 (3)
Tl2—O2	3.07 (18)
O2—Tl1—O2^iii	52 (7)	O1—Tl1—O1^vii	74 (4)
O2—Tl1—O2^iv	89 (5)	O2—Tl2—O2^ix	108 (5)
O2—Tl1—O2^v	61 (3)	O2—Tl2—O2^x	83 (6)
O2—Tl1—O1^vi	106 (7)	O2—Tl2—O2^v	56 (3)
O2—Tl1—O1	48 (5)	O2—Tl2—O1^xi	141 (4)
O2—Tl1—O1^vii	99 (6)	O2—Tl2—O1^vii	88 (6)
O2^iii—Tl1—O2^iv	71 (3)	O2^ix—Tl2—O2^x	56 (4)
O2^iii—Tl1—O2^v	89 (5)	O2^ix—Tl2—O2^v	83 (6)
O2^iii—Tl1—O1^vi	133 (4)	O2^ix—Tl2—O1^xi	59 (5)
O2^iii—Tl1—O1	68 (5)	O2^ix—Tl2—O1^vii	152 (2)
O2^iii—Tl1—O1^vii	142 (4)	O2^x—Tl2—O2^v	108 (5)
O2^iv—Tl1—O2^v	52 (7)	O2^x—Tl2—O1^xi	59 (5)
O2^iv—Tl1—O1^vi	68 (5)	O2^x—Tl2—O1^vii	152 (2)
O2^iv—Tl1—O1	133 (4)	O2^v—Tl2—O1^xi	141 (4)
O2^iv—Tl1—O1^vii	142 (4)	O2^v—Tl2—O1^vii	88 (6)
O2^v—Tl1—O1^vi	48 (5)	O1^xi—Tl2—O1^vii	121 (6)
O2^v—Tl1—O1	106 (7)	O2—C1—O2^xii	120 (14)
O2^v—Tl1—O1^vii	99 (6)	O2—C1—O1	120 (7)
O1^vi—Tl1—O1	133 (9)	O2^xii—C1—O1	120 (7)
O1^vi—Tl1—O1^vii	74 (4)

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