Single-crystal structures of A ₂SiF₆ (A = Tl, Rb, Cs), a better structure model for Tl₃[SiF₆]F, and its novel tetragonal polymorph

2.1 Single-crystal structures of A ₂SiF₆ (A = Tl, Rb, Cs)

Single-crystals of Tl₂SiF₆, Rb₂SiF₆, and Cs₂SiF₆ were synthesized by dissolving the powdery compounds in 40% hydrofluoric acid and evaporating the respective solution at room temperature. The compounds crystallize in the cubic space group Fm 3 ‾ m (No. 225, cF36) with Z = 4 and a = 8.4749(10) Å, V = 608.7(2) Å³ for the thallium, a = 8.3918(10) Å, V = 591.0(2) Å³ for the rubidium and a = 8.8638(11) Å, V = 696.4(3) Å³ for the cesium compound. The atom positions and equivalent isotropic displacement parameters are shown in Table 1. For selected crystallographic data and details of the crystal structure determinations see Tables S1–S3 (Supplementary Material available online), and for anisotropic displacement parameters Table S4.

The previously reported lattice parameters were determined at room temperature, so they are slightly larger than those determined by us at T = 100 or 200 K, respectively. The higher measurement temperature used by us for the cesium compound also explains the larger anisotropic displacement parameter of the fluorine atom in comparison with the other compounds. A detailed description of the crystal structure (Figure 1) is omitted as the K₂PtCl₆ structure type is well known and the structural motifs have previously been described well [1, 2, 7].

Figure 1:

The crystal structure of cubic Tl₂SiF₆ (Rb₂SiF₆ and Cs₂SiF₆ are isotypic). Anisotropic displacement ellipsoids are depicted at the 70% probability level at T = 100 K. Grey polyhedra represent [SiF₆]²⁻ anions, the F atoms in these polyhedra are shown as yellow spheres with arbitrary radii.

Table 2 compares the measurement temperatures, the radii of the A ^I cations, the lattice parameters a, the x coordinates of the F atoms, and the Si–F bond lengths of the compounds A ₂ SiF₆ presented here with literature data. Previously, the Si–F bond lengths were reported as 1.71 Å at room temperature [1]. We observe 1.693(3), 1.686(6), and 1.679(4) Å for the Rb, Tl, and Cs compound, respectively, all matching within tripled standard uncertainties.

2.2 Syntheses and single-crystal structures of two polymorphs of Tl₃[SiF₆]F

By reaction of TlF with SiO₂ under hydrofluorothermal conditions single-crystals were formed and identified as Tl₃[SiF₆]F by powder X-ray diffraction at T = 298 K and by single-crystal structure determination at T = 100 K. The Rietveld refinement is shown in Figure 2, its refinement details and those of the single-crystal structure determination are listed in Tables S5 (single-crystal data) and S6 (Rietveld data) in the Supplementary Material. The atomic positions and equivalent isotropic displacement parameters as refined from the single-crystal data are given in Table 4, for anisotropic displacement parameters see Table S7 in the Supplementary Material.

Figure 2:

Recorded (black) and calculated (red) powder X-ray diffraction pattern of hexagonal Tl₃[SiF₆]F after Rietveld refinement. The calculated reflection positions are indicated by the vertical bars below the patterns. The curve at the bottom represents the difference between the observed and the calculated intensities. The grey-shaded area was excluded from the refinement, it contains reflections belong to an unknown by-product. R _p = 7.18, wR _p = 10.68 (not background-corrected R values), S = 1.76.

We describe the crystal structure of Tl₃[SiF₆]F in the hexagonal space group P6₃ mc (No. 186, hP22) with a = 7.8248(4), c = 6.8768(4) Å, V = 364.64(4) Å³, and Z = 2, T = 100 K, instead of the previously used trigonal space group P 3 ‾ 1c (No. 163, hP22, a = 7.908(2), c = 6.927(1)  Å, V = 375.15 Å³, Z = 2, temperature not given, likely T = 298 K) [6]. Figure 3 shows a comparison of the previous structure model with the one presented here. In our structure model no disorder is present and two crystallographically independent fluorine atoms form an almost regular coordination octahedron around the Si atom. Therefore, the Si–F bond lengths (1.688(6) and 1.695(6) Å) are much more uniform and also lie in between the previously given values of 1.57(3) and 1.78(2) Å; see also  Table 3 [6]. This clearly shows the superiority of our structure model. The third fluorine atom, here called the “free” fluorine atom, is located on the corners and in the middle of the edges of the unit cell (3m.), as described previously [6].

Figure 3:

Comparison of the crystal structure of Tl₃[SiF₆]F previously published in space group P 3 ‾ 1c (left) [6] with the present description in P6₃ mc (right). The atoms are shown with arbitrary radii. [SiF₆]²⁻ anions are shown as yellow and grey octahedra, indicating the disorder.

The crystal structure is related to the ZrBeSi structure type (P6₃/mmc, No. 194, hP6) which derives from the AlB₂ structure type (P6/mmm, No. 191, hP3) [9, 10]. The “free” F⁻ anions of Tl₃[SiF₆]F (P6₃ mc, No. 186, hP22) correspond to the positions of the Zr atoms, the center of gravity of the [SiF₆]²⁻ anions to the Be atoms, and the center of gravity between three Tl⁺ cations at ⅓, ⅔, z ≈ ¼ to the Si atoms of ZrBeSi. Figure 4 and the Bärnighausen-tree in Scheme 1 illustrate the structural relation between the crystal structures of AlB₂, ZrBeSi, and Tl₃[SiF₆]F.

Figure 4:

The structural relation between the crystal structures of ZrBeSi and of Tl₃[SiF₆]F. Related atom/atom group positions are shown in the same color. Zr, F in yellow, Be [SiF₆]²⁻ in grey, the white dummy atom in the “Tl₃ center” corresponds directly to the Si atom in green.

Scheme 1:

A Bärnighausen-tree showing the symmetry relation of the crystal structures of AlB₂, ZrBeSi, and Tl₃[SiF₆]F. The colors represent the atoms/atom groups in Figure 4, “Tl₃” means the position of the white dummy atom within the center of the “Tl₃” unit.

Describing the crystal structure of Tl₃[SiF₆]F in space group P6₃/mmc leads to disordered [SiF₆]²⁻ anions. With the translationengleiche transition of index 2 to space group P6₃ mc, the mirror planes perpendicular to the c axis at z = ¼ and z = ¾ are lost and the [SiF₆]²⁻ anions can become ordered. The translationengleiche transition of index 2 from space group P6₃/mmc to the previously chosen space group P 3 ‾ 1c cannot resolve the disorder of the [SiF₆]²⁻ anion as the position of the Si atom has 3.2 symmetry. The authors of the previous structure report should have better used the statistical description in space group P6₃/mmc instead of the one in space group P 3 ‾ 1c. In conclusion, also from a group-theoretical point of view, space group P6₃ mc leads to a better structure model.

So far, no other A ₃ MF₇ compound seems to be known that forms this structure type. Only (NH₄)₃[ReCl₃O₃]Cl is isotypic if the hydrogen atoms are neglected [11].

Unfortunately, all attempts to obtain additional single-crystals of this compound resulted only in a powdery product whose powder pattern (Figure 5) corresponded closely to that of the tetragonal modification of Rb₃SiF₇ [12].

Figure 5:

Observed (black) and calculated (red) powder X-ray diffraction pattern of tetragonal Tl₃[SiF₆]F after Rietveld refinement. The calculated reflection positions are indicated by the vertical bars below the patterns. The upper trace represents TlF and the lower one Tl₃[SiF₆]F. The curve at the bottom shows the difference between the observed and the calculated intensities. R _p = 5.18, wR _p = 8.00 (not background-corrected R values), S = 2.09.

As this result was unexpected, we additionally confirmed the presence of thallium within the compound by its green emission in flame spectroscopy [13]. The tetragonal modification of Tl₃[SiF₆]F crystallizes in space group P4/mbm (No. 127, tP22) with a = 8.0313(8), c = 5.8932(6) Å, V = 380.13(7) Å³ and Z = 2 at T = 298 K. Details of the Rietveld refinement are listed in Table S8 (Supplementary Material), atom positions and isotropic displacement parameters are shown in Table 4. Several powder X-ray patterns of the compound were recorded on flat sample holders and in glass capillaries, however the preferred orientation of the particles led to a pronounced texture effect that was difficult to model in the Rietveld refinement. Therefore, only the atomic positions were refined and fixed isotropic displacement parameters had to be used.

The tetragonal modification of Tl₃[SiF₆]F crystallizes in the known (NH₄)₃[SiF₆]F structure type [12, 14] and we therefore refrain from a detailed description here (Figure 6).

Figure 6:

The crystal structure of tetragonal Tl₃[SiF₆]F. The atoms are shown with arbitrary radii. Grey polyhedra are [SiF₆]²⁻ anions.

The Si–F bond lengths of the two Tl₃[SiF₆]F modifications presented here and of Tl₂SiF₆ are similar within the tripled standard uncertainties (Table 5). The Si–F bond lengths of the tetragonal modification of Tl₃[SiF₆]F correspond to those of the isotypic Rb and Cs compounds [5].

2.3 Infrared spectroscopy

The similarity of the IR spectra of Tl₂SiF₆ and Tl₃[SiF₆]F mentioned in the literature was also confirmed in this work [12]. The spectra (Figure 7) only show the Si–F vibrations of the [SiF₆]²⁻ octahedra at ν = 690 cm⁻¹ and δ = 470 cm⁻¹, that are very similar in both compounds (see Table 6).

Figure 7:

Experimentally observed IR spectra of Tl₂SiF₆ (black), hexagonal Tl₃[SiF₆]F (red) and tetragonal Tl₃[SiF₆]F (blue).

The infrared spectra also show O–H and H–F valence vibrations at 3309  and 1425 cm⁻¹, respectively [15–17] which can be explained by adhering hydrofluoric acid and moisture on the samples due to the applied synthetic method.

4.1 Preparation of A ₂SiF₆ (A = Tl, Rb, Cs)

A mixture of the starting materials AF (A = Tl, Rb, Cs) and SiO₂ were placed into a Teflon-lined steel autoclave with a total volume of 20 and 3.00 mL of water was added together with 0.126 g NH₄HF₂ to create a hydrofluoric medium (pH ∼ 4). The mixture was heated for seven days at T = 220 ° C and then cooled down to room temperature overnight. After decantation of the hydrofluoric acid, the remaining colorless powder was washed with acetone and petroleum ether. In each case colorless crystals were obtained by dissolving the compounds in 40% hydrofluoric acid and evaporation of the solution at room temperature.

4.2 Preparation of Tl₃[SiF₆]F

200 mg SiO₂ (Alfa Aesar Johnson Matthey GmbH, 99.5%), an excess of TlF (Strem Chemicals, 99%) or Tl₂CO₃ (Merck, 98%), and 3.00 mL of water were placed into a Teflon-lined steel autoclave with a total volume of 20 mL. To create a hydrofluoric medium (pH ∼ 4), 0.126 g NH₄HF₂ was added. The mixture was heated for seven days at 220 ° C and then cooled down to room temperature overnight. After decantation of the hydrofluoric acid, the remaining product was washed with acetone and with petroleum ether. This synthesis led once to the formation of single-crystals of the hexagonal modification of Tl₃[SiF₆]F, in all other attempts we could only obtain powders of the tetragonal one.

4.3 Powder X-ray diffraction

The powder X-ray diffraction patterns were recorded at ambient temperature with an STOE Stadi MP powder diffractometer in transmission geometry. Powders were fixed between two pieces of Scotch tape (3MScotch^® Magic™, flat sample). The diffractometer was operated with CuKα ₁ radiation (λ = 1.5406 Å, Ge(111) monochromator) and equipped with a Mythen1K detector. The sample preparations were carried out on air. The evaluation of the powder X-ray patterns was carried out with the WinXPOW 3.07 software package [18]. The Rietveld refinement was performed with Jana2006 [19].

4.4 Single-crystal X-ray diffraction

Suitable single-crystals were selected under perfluorinated oil and mounted using a MiTeGen loop. Intensity data of suitable crystals were recorded with an IPDS 2 diffractometer (STOE & Cie) where the crystals were kept under a stream of nitrogen. The diffractometer was operated with MoKα radiation (λ = 0.71073 Å, graphite monochromator) and equipped with an image plate detector. Evaluation, integration and reduction of the diffraction data were carried out using the STOE X-Area software suite [20]. The numerical absorption corrections were applied with the modules X-Shape and X-Red32 of the X-Area software suite. The structures were solved with dual-space methods (Shelxt-2015 [21]) and refined against F ² (Shelxl-2018/3 [22]). Representations of the crystal structures were created with the Diamond software [23].

CCDC 2063139–2063144 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

4.5 IR spectroscopy

Infrared spectra were measured on a Bruker Alpha Platinum FT-IR spectrometer using the ATR Diamond module with a resolution of 4 cm⁻¹. The spectrometer was located inside a glovebox under argon (5.0, Praxair) atmosphere. For data collection, the Opus 7.2 software was used [24].