Pistorius & Clark (1969
), Meisalo & Kalliomäki (1976
), Adams et al.
), Lee et al.
) and Grzechnik & Friese (2008
) reported previously on the high-pressure behaviour of Tl2
. 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.
) also observed phase transitions near 1.3 and 3.8 GPa using IR and Raman spectroscopic analyses. However, a single-crystal diffraction study demonstrated that thallium carbonate (C
= 4) is structurally stable to at least 3.56 GPa (Grzechnik & Friese, 2008
). The most likely reason for this discrepancy is that the X-ray and spectroscopic data of Meisalo & Kalliomäki (1976
) and Adams et al.
), respectively, were poorly resolved and the pressures were nonhydrostatic. In this study, we continue our work on Tl2
using single-crystal X-ray diffraction in a diamond anvil cell to characterize the postulated (Meisalo & Kalliomäki, 1976
; Adams et al.
) pressure-induced polymorphs above about 4 and 6.7 GPa.
A crystal of dithallium carbonate was compressed slowly 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 (C
= 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 Tl2
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.
) 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)–3.4 (3) Å (Table 1). 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 distances 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.
Selected geometric parameters (Å, °)
When only the Tl—O distances below 3 Å are considered, the crystal structure under 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 fact that the axial compressibility along the a
axis (within the estimated standard deviations) changes little between 3.56 and 5.82 GPa.
Figure 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 black and thick grey lines, respectively. The C—O (more ...)
The other striking aspect of the high-pressure behaviour of Tl2
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 Tl2
cause a decrease of the shortest C
C distance from 3.46 Å under ambient conditions (Marchand et al.
) 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 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.
), in which the C
C distances in high-pressure phases of Li2
are expected to be below 2.5 Å.