K₂SnOF₄ and K₂WO₃F₂ – different but similar

2.1 Synthesis

K₂SnOF₄ was synthesized from SnO₂ and KHF₂via a high-pressure/high-temperature route. The educts were weighed in with an atomic ratio of 1 : 2.5 for Sn : K and firmly ground together. This step was carried out within a glove box (MBraun Inertgas-Systeme GmbH, Garching, Germany) providing an inert gas atmosphere to prevent the hydration of KHF₂ upon exposure to moisture. The mixture was transferred into a platinum capsule which was subsequently inserted into a boron nitride crucible (Henze Boron Nitride Products AG, Lauben, Germany). This crucible was finally placed into an 18/11 assembly. Details on the assembly are described in the literature [21], [22], [23]. The assembly was then inserted into the middle of eight tungsten carbide cubes (Hawedia, Marklkofen, Germany) which transfer the pressure from six steel wedges to the 18/11 assembly and to the sample. A 1000 t multi-anvil press (Max Voggenreiter GmbH, Mainleus, Germany) ensures the compression of the sample via a Walker-type module, which includes the steel wedges and the tungsten carbide cubes.

The sample was compressed within 65 min to 2.5 GPa and held at that pressure during the following heating program. The sample was then heated to 350 °C within 10 min, held at that temperature for further 60 min, and cooled down to room temperature within 40 min. After the end of the heating program, the sample was decompressed to room pressure within 200 min. A colorless, crystalline sample was recovered. It is mandatory to open the sample under inert gas atmosphere, as the substance is hygroscopic.

In a second step, doping of K₂SnOF₄ with Mn⁴⁺ was achieved by a ball-milling process. For this step, 70 mg (0.24 mmol) K₂SnOF₄, 2.6 mg (0.01 mmol) K₂MnF₆, and 200 mg ZrO₂ milling balls (Ø = 1 mm) were weighed in under argon atmosphere and transferred into a zirconia vessel. The milling process of 6 × 10 min at 300 rpm (15 min breaks between the steps) was carried out in a planetary mill (Pulverisette 7, FRITSCH, Idar-Oberstein, Germany).

K₂WO₃F₂ was also synthesized via a high-pressure/high-temperature route. The starting materials KF and WO₃ were ground together under inert gas atmosphere and subsequently transferred into a platinum capsule. The capsule was placed into a boron nitride crucible, which was subsequently inserted into an 18/11 assembly. The assembly was placed into the multi-anvil set-up as described above.

Compression of the sample to 5.5 GPa took 145 min. This pressure was kept during the following heating program. The sample was heated to 900 °C within 10 min, kept at that temperature for 60 min and slowly cooled to 350 °C within 30 min. Afterwards the sample was decompressed to ambient pressure within 430 min. A colorless and crystalline product was recovered.

2.2 X-ray analysis

A STOE Stadi P powder diffractometer was used to collect the diffraction data of the powder sample in the 2θ range of 2.0–40.4° and with a step size of 0.015°. The instrument is equipped with a Mythen 2 DCS4 detector, a Ge(111) primary beam monochromator, and a molybdenum radiation source MoKα₁ (λ = 70.93 pm). The data was recorded at room temperature.

For the structure analysis, single crystals were isolated under a polarization microscope and mounted onto a sample holder. For data collection, a Bruker D8 Quest diffractometer was used (BRUKER, Billerica, USA). The crystal of K₂SnOF₄ was cooled to 173(2) K and the crystal of K₂WO₃F₂ was cooled to 183(2) K during the measurements. The diffractometer was equipped with an Incoatec microfocus X-ray tube (Incoatec, Geesthacht, Germany) using MoKα radiation (λ = 71.07 pm). Collection of the intensity data was accomplished by a Photon 100 detector. A multi-scan absorption correction was applied, using Sadabs 2014/5. For K₂SnOF₄, the space groups Pna2₁ (no. 33) and Pnma (no. 62) were considered for the structure solution with Shelxtl-xt-2014/4 on the basis of the extinction conditions. The space group Pnma (no. 62) was found to be correct during the refinement. The WinGX-2013.3 [24] suite was used to carry out the parameter refinement (full-matrix least-squares against F²) with Shelxl-2013 [25], [26].

For K₂WO₃F₂, the space groups Pna2₁ (no. 33) and Pnma (no. 62) were considered for the structure solution (Shelxtl-xt-2014/4), and Pnma (no. 62) was found to be correct during the refinement. The WinGX-2013.3 [24] suite was used to carry out the parameter refinement (full-matrix least-squares against F²) with Shelxl-2013 [25], [26].

Further information on the crystal structure investigations can be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the deposition number CCDC 2019752 for K₂SnOF₄, and CCDC 2019753 for K₂WO₃F₂.

2.3 EDX-spectroscopy

The chemical composition was analyzed by energy dispersive X-ray spectroscopy (EDX) using a SUPRA^TM35 scanning electron microscope (SEM, CARL ZEISS, Oberkochen, Germany, field emission) equipped with a Si/Li EDX detector (OXFORD INSTRUMENTS, Abingdon, Great Britain, model 7426).

2.4 Vibrational spectroscopy

A Bruker Alpha-P spectrometer (BRUKER, Billerica, USA), equipped with a DTGS detector and a 2 × 2 mm diamond ATR crystal, was used for the characterization of K₂SnOF₄ and K₂WO₃F₂ by FT-IR-ATR (Attenuated Total Reflection) spectroscopy. The handling of the data was carried out using the Opus 7.2 software.

2.5 Luminescence spectroscopy

K₂SnOF₄:Mn⁴⁺ was excited by a blue light laser diode (THORLABS, Newton, USA) with a wavelength of λ = 448 nm. The emission spectrum was recorded using a CCD-Detector (AVA AvaSpec 2048, AVANTES, Apeldoorn, The Netherlands). To ensure proper measurements, the set-up was calibrated for spectral radiance prior to the experiments using a tungsten-halogen calibration lamp. The software AVA AvaSoft full version 7 was used for data handling.

3.1 Crystal structure of K₂SnOF₄

K₂SnOF₄ crystallizes in the orthorhombic crystal system with space group Pnma (no. 62). It is isostructural to (NH₄)₂FeF₅ and its lattice parameters are 612.4(1), 738.4(1), and 1082.9(1) pm for a, b, and c, respectively [27]. The volume of the unit cell of K₂SnOF₄ amounts to 0.4896(1) nm³ with 32 atoms comprised by four formula units. The asymmetric unit is formed by one tin, two potassium, one oxygen, and two fluorine atoms. Tin is located at the special Wyckoff site 4a, both potassium as well as the oxygen positions are located at 4c, and the two fluorine positions are located at general Wyckoff sites 8d. Therefore, the Wyckoff sequence of K₂SnOF₄ is d²c³a and the Pearson code is oP32.

Information on the crystal structure refinement can be found in Table 1. The atomic coordinates and displacement parameters, as well as selected distances and angles can be found in Tables S1–S4 (Supporting Information available online).

The main structural motif of K₂SnOF₄ is an [SnO₂F₄]⁴⁻ octahedron (Figure 1, left). Within this octahedron, the Sn–F bond lengths are 201.0(1) and 202.1(1) pm for Sn1–F1 and Sn1–F2, respectively, whereas the Sn–O bond length is 196.7(1) pm (Table S3; Supporting Information). These bond lengths are in accordance to the ones reported in the literature for K₅Sn₂OF₁₁ (d_Sn–F = 196.4(6)–200.3(4) pm; d_Sn–O = 194.7(3)–195.3(2) pm) [19]. The bond angles confirm an octahedral coordination sphere (Table S4; Supporting Information).

Figure 1:

[SnO₂F₄]⁴⁻ octahedron (left) and infinite chain of octahedral units (right).

The octahedra are interconnected via common oxygen atoms (O1) and build infinite chains along [010] (Figure 1, right). The Sn1–O1–Sn1 tilting angles are approx. 140°. These chains are quasi isolated from each other through potassium cations. Within the structure, there are two crystallographically different potassium atoms. Both are nine-fold coordinated by eight fluorine atoms and one oxygen atom. The K–F bond lengths vary between 268.1(1) and 271.4(1) pm, the K–O bonds are 276.7(2) and 288.3(2) pm for K1–O1 and K1–O2, respectively. The coordination spheres of both potassium cations are depicted in Figure 2. A unit cell of K₂SnOF₄ is shown in Figure 3.

Figure 2:

Coordination spheres of K1 (left) and K2 (right), with bond lengths given in pm.

Figure 3:

Unit cell projection of K₂SnOF₄ along [100] (top) and along [010] (bottom).

Bond-length/bond-strength (BLBS) and charge-distribution calculations (CHARDI, program Chardi2015) confirmed the structural model (Table S5; Supporting Information) [28], [29], [30], [31]. EDX measurements proved the cation composition of K₂SnOF₄ to be 2:1.2(2) for the ratio potassium to tin. An SEM image of a polycrystalline sample of K₂SnOF₄ is shown in Figure S1 (Supporting Information).

K₂SnOF₄ was synthesized as the main phase but not phase pure. Analysis via the Rietveld technique determined the powder composition to be 70(2) wt% K₂SnOF₄ with SnO₂ (4(1) wt%), KHF₂ (26(2) wt%), and an unknown substance as side-phases (Figure 4).

Figure 4:

Plot of the powder analysis via the Rietveld method. The experimental powder pattern is plotted with black crosses, the calculated curve as the red solid line and the difference curve as the blue solid line. Reflection positions of K₂SnOF₄, SnO₂ and KHF₂ are shown as black tick marks in the top, middle and bottom rows, respectively. Reflections belonging to an unknown side phase are marked with purple asterisks.

3.2 Crystal structure of K₂WO₃F₂

K₂WO₃F₂ crystallizes in the orthorhombic crystal system with space group Pnma (no. 62). Its crystal structure is isostructural to the one of K₂SnOF₄ described above. The lattice parameters of K₂WO₃F₂ are 607.71(3), 735.19(3), and 1077.38(5) pm for a, b, and c, respectively. Its cell volume is 0.4814(1) nm³. Within the K₂WO₃F₂ structure there are seven crystallographically different positions. These are two tungsten positions W1 and W2 at Wyckoff site 8d with an occupancy of 0.25 each. Additionally, there are two fully occupied potassium positions K1 and K2 at the Wyckoff site 4c. A fully occupied oxygen position is located at Wyckoff site 4c, and two positions exhibiting a mixed occupancy of oxygen and fluorine (1:1) are located at Wyckoff site 8d. Therefore, the Wyckoff sequence of K₂WO₃F₂ is d⁴c³. The unit cell consists of four formula units, which amounts to 32 atoms. This leads to the Pearson code oP32.

Information on the crystal structure refinement can be found in Table 1. Atomic coordinates, anisotropic displacement parameters, as well as important distances and angles are listed in Tables S6–S9 (Supporting Information).

The main motif of the crystal structure is an octahedral [WO₄F₂]⁴⁻ unit (Figure 5, left). Within this unit, tungsten is shifted from the central position in the direction of the equatorial oxygen atoms. Since these equatorial positions exhibit a statistical disorder of oxygen and fluorine (1:1), the deflection of tungsten in the direction of the oxygen atoms from the Wyckoff site 4a to the site 8d introduces four possible locations for tungsten within this unit (Figure 5, right top). This leads to the assumption that the cis-conformation of fluorine is observed more often than the trans-conformation (Figure 5, right bottom), which would not deflect tungsten from the center of the octahedral unit. A cis-conformation is also observed for the two compounds Cs₂WO₃F₂ and Ba₂WO₃F₄ [2], [8], [10]. The W–O bond lengths are shorter in comparison to the W–F bond lengths due to the deflection of tungsten. This is consistent with literature data, describing shorter bond lengths for W–O than for W–F, e.g., within the compound Ag(WOF₅)₂ (d_W–O = 172.2 to 177.2 pm; d_W–F = 182.6 to 207.5 pm), which is likely a result of the more covalent bonding character of tungsten and oxygen in comparison to tungsten and fluorine [9].

Figure 5:

Octahedral [WO₄F₂]⁴⁻ unit (left), possible cis-conformation (right, top) and possible trans-conformations (right, bottom). The bond lengths are given in pm.

The octahedral units are connected via common oxygen atoms and form chains along [010] (Figure 6, left). Within the chain, the octahedra are tilted towards each other with a tilting angle (W–O–W) of approx. 149° (Figure 6, right). These chains are quasi-isolated by potassium cations, which occupy two crystallographically independent positions (Figure 6, left). The coordination spheres of the two potassium cations are shown in Figure 7. Both are coordinated nine-fold by eight mixed O/F positions and one oxygen position. The K–O bond lengths are 289.6(1) and 294.6(1) pm, whereas the K–O/F lengths vary from 271.7(1) to 312.0(1) pm. The mixed occupancy of the fluorine and oxygen positions as well as the deflection of tungsten hints at the possibility that this phase is a high-temperature modification. At low temperatures, an ordering of the structures might be possible, although no signs of ordering were observed down to T = 183(2) K.

Figure 6:

Central projection of the unit cell of K₂WO₃F₂ along [100] (left-top) and along [010] (left-bottom). A chain of [WO₄F₂]⁴⁻ units is shown on the right.

Figure 7:

Coordination sphere of K1 (left) and K2 (right).

EDX analysis confirmed the atomic ratio of K:W to 1.9(1):1 for K₂WO₃F₂. An SEM image of a polycrystalline sample is shown in Figure S2 (Supporting Information). Phase analysis via the Rietveld technique led to K₂WO₃F₂ as main product with small amounts of an unknown side phase present (Figure 8).

Figure 8:

Phase analysis via the Rietveld technique. The black curve shows the measured pattern, the red curve the calculated one, and the blue line marks the difference curve. The black tick marks correspond to the hkl positions of K₂WO₃F₂. Purple asterisks mark reflections belonging to an unknown side phase.

3.3 Vibrational spectroscopy

3.3.1 FT-IR spectrum of K₂SnOF₄

IR bands for the compound K₂SnOF₄ were reported in 1963 by Kolditz und Preiss [20]. They observed a strong absorption band at 442 cm⁻¹, as well as two very strong bands at 490 and 532 cm⁻¹. These three absorption bands are also found for the sample prepared during this work (448, 487, and 520 cm⁻¹). Kolditz und Preiss also observed bands at 755 and 820 cm⁻¹. In contrast to their work, these bands are absent for our sample (Figure 9), but a medium strong absorption band can be noticed at 783 cm⁻¹. Another quite broad, but weak absorption band, located at 1590 cm⁻¹, as well as two weak but sharp absorption bands (1181 and 1122 cm⁻¹) likely belong to KHF₂ (unreacted starting material) [32]. Lastly, some very weak but sharp absorption bands are located at 618 and 644 cm⁻¹. These bands might be due to the unknown side phase. Above 1600 cm⁻¹, no absorption bands are noticeable, suggesting that no O–H bonds were overlooked during the structure determination by X-ray diffraction.

Figure 9:

FT-IR spectrum of K₂SnOF₄ in the range of 400–4000 cm⁻¹.

3.3.2 FT-IR spectrum of K₂WO₃F₂

The FT-IR spectrum of K₂WO₃F₂ in the range of 400 to 4000 cm⁻¹ is shown in Figure 10. No bands above 1500 cm⁻¹ are noticeable; therefore, the structure does not contain any water. In analogy to Cs₂WO₃F₂ and (NH₄)₂MoO₃F₂, the bands at around 920 cm⁻¹, 850 cm⁻¹, and 800 cm⁻¹ can be attributed to W–O bonds [8], [33]. The bands resulting of the W–F bonds are expected below 500 cm⁻¹ [8], [33]. Similar data have been reported for Ba₂WO₃F₄, confirming similar structural motifs in Ba₂WO₃F₄, Cs₂WO₃F₂, and K₂WO₃F₂ [10].

Figure 10:

FT-IR spectrum of K₂WO₃F₂ in the range of 400–4000 cm⁻¹.