Fluorides are a versatile class of compounds, offering 0-dimensional to 3-dimensional structural motifs. The fundamental building blocks may consist of tetrahedral up to nonahedral units , . Among these, the octahedral coordination sphere is one of the most commonly observed structural motifs , . These entities can not only occur isolated, but also connected via common corners, edges, or faces , . This can lead to the formation of quasi-isolated structural units, chains, or networks . Most prominent representatives of the fluorides are the hexafluoridometallates A2MF6 (A=monovalent cation, M=tetravalent cation), A3M′F6 (M′=trivalent cation), and A′MF6 (A′=divalent cation) , in which quasi-isolated octahedral MF6 building blocks dominate the solid state structures. Recently, hexafluoridosilicates like K2SiF6 or Li2SiF6 exhibiting quasi-isolated octahedral units received a lot of attention as host materials for Mn4+. Ever since K2SiF6:Mn4+ was adopted as a red luminescent material in white light emitting diodes, numerous novel hexafluoridometallate phosphors have been discovered , , . However, within the group of tin fluorides not only the hexafluoridostannates are known, but also compositions like ASn2F5 (A=monovalent cation), KSnF3, A′SnF4 (A′=divalent cation), A″SnF7 (A″=trivalent cation), or Na4Sn3F10 have been mentioned in the literature , , , , , , , , , , . Nearly all of the published fluoridostannates contain tin in the oxidation state 2+. Therefore, these structures commonly exhibit a square pyramidal coordination of tin, which leaves space for the lone pair of electrons. The oxidation state of 4+ is only observed within the hexafluoridostannates like Cs2SnF6, KNaSnF6, RbNaSnF6, and CsNaSnF6, and within A″SnF7 (A″=trivalent cation) , , . These materials containing Sn4+ show an octahedral coordination sphere around the tin atom. Interestingly, materials with large cell parameters containing solely isolated octahedral units are only mentioned scarcely. One example is (NH4)3SnF7, which crystallizes in the cubic crystal system with the cell parameter a=1208.6(1) pm and a volume of 1.765 nm3 . Here, we took the chance to characterize K15NaSn5F36, which also exhibits an unusually large unit cell (hexagonal, a=1060.3(2), c=2011.9(4) pm, V=1.9588(7) nm3), whilst containing only simple but uniquely arranged building blocks.
2 Experimental section
K15NaSn5F36 was synthesized in welded copper ampoules. KHF2 and SnO2 (Strem Chemicals, Newburyport, USA, 99.9%) were ground together in an agate mortar with a molar ratio of 3:1 under argon atmosphere (glove box, MBraun Inertgas-System GmbH, Germany). The powder was then transferred into copper ampoules, which were welded in a miniaturized arc-welding installation (further information on the arc-welding installation can be retrieved from ). The closed ampoule was subsequently transferred into a silica glass ampoule, which was filled with 400 mbar argon atmosphere, placed into a tube furnace and heated up to T=300°C with a heating rate of 3 K min−1. The temperature was kept at 300°C for 96 h and subsequently cooled to 250°C at a cooling rate of 0.1 K min−1. This temperature was kept for another 24 h and followed by cooling to 100°C at a cooling rate of 0.1 K min−1. Subsequently, the furnace was turned off and the sample was quenched to room temperature. A colourless crystalline material was recovered, from which a single crystal of K15NaSn5F36 could be isolated. The incorporated sodium in the compound originates from the starting material KHF2 contaminated by sodium. As only a few single crystals could be isolated under oil (Fomblin) no EDX analysis was possible. The presence of sodium is not unexpected, as it has been observed in a number of crystals of samples prepared with the contaminated educt KHF2. As is shown below, the shorter Na–F bonds in comparison to the K–F bonds, as well as the difference in electron density of sodium in contrast to potassium, clearly show the presence of sodium (Na1 site).
First attempts to synthesize bulk samples of K15NaSn5F36 by solid-state synthesis have not been successful as yet.
2.2 X-ray analysis
The analysis of the crystal was carried out at the P24 Beamline of the PETRA III facility at DESY Hamburg (Germany). For the analysis, a single crystal was mounted onto a Huber 4-circle Kappa-diffractometer. The synchrotron radiation was monochromatized by a water-cooled double crystal monochromator [Si(111), Si(311)] to yield a wavelength of λ=0.47686 Å. The collected intensity data were reduced, and a cell refinement was carried out with the help of XDS (Version March 15, 2019, W. Kabsch, MPI Heidelberg). On the basis of the extinction conditions, the space groups P63/m (no. 176), P63 (no. 173), and P6322 (no. 182) were considered for the structure solution (Shelxs-98 ) and refinement. The centrosymmetric space group P63/m (no. 176) was found to be correct.
A refinement in space group P63, which lacks the additional mirror plane of P63/m, is possible but does not lead to better R values. In space group P63 residual electron density is distributed equally for both possible sites of K4 in an approximate ratio of 1:1. As no suitable twinning model was found either, we believe that the crystal structure of K15NaSnF36 is best described in the centrosymmetric space group P63/m.
The parameter refinement (full-matrix least-squares against F2) was carried out with Shelxl-2013 ,  as implemented in the WinGX-2013.3 suite . The anisotropic refinement led to values of 0.0291 and 0.0631 for R1 and wR2 (all data), respectively.
Further information on the crystal structure investigation can be obtained from the joint CCDC/FIZ Karlsruhe deposition service on quoting the deposition number CCDC 1968496 for K15NaSn5F36.
3 Results and discussion
K15NaSn5F36 crystallizes in the hexagonal crystal system with space group P63/m (no. 176). Its lattice parameters are 1060.3(2) and 2011.9(4) pm for a and c, respectively. The unit cell contains 114 atoms. This equals two formula units. Therefore, the Pearson code of K15NaSn5F36 is hP114. The asymmetric unit of K15NaSn5F36 consists of two tin, four potassium, one sodium, and seven fluorine positions. K1, K2, F1, F3, F5, F6, and F7 are located on the general Wyckoff positions 12i. Sn1, F2, and F4 are located at the special Wyckoff positions 6h, Sn2 and K4 (site occupancy factor (SOF)=0.5) at the special Wyckoff positions 4f, K3 at 4e, and Na1 is located at 2b. Therefore, the Wyckoff sequence of K15NaSn5F36 is i7h3f2eb. Information about the crystal structure refinement is listed in Table 1. Further information about atomic coordinates, displacement parameters, as well as selected interatomic distances and angles are shown in Tables 2–5.
Crystal data and structure refinement of K15NaSn5F36.
|Molar mass, g mol−1||1886.94|
|Space group||P63/m (no. 176)|
|Single-crystal diffractometer||Kappa-diffractometer, P24 Beamline, DESY Hamburg|
|Radiation/wavelength λ, Å||Synchrotron/0.47686|
|Formula units per cell, Z||2|
|Calculated density, g cm−3||3.20|
|Absorption coefficient, mm−1||1.2|
|2θ range, °||3.0–39.2|
|Range in hkl||±14, ±14, ±28|
|Total no. of reflections||17486|
|Independent reflections/ref. parameters/Rint||1956/94/0.0328|
|Reflections with I>2 σ(I)||1941|
|Goodness-of-fit on Fi2||1.269|
|Absorption correction||Semi-empirical (from equivalents)|
|Final R1/wR2 [I>2 σ(I)]||0.0289/0.0630|
|Final R1/wR2 (all data)||0.0291/0.0631|
|Largest diff. peak/hole, e Å−3||1.89/−1.34|
Wyckoff positions, atomic coordinates, and equivalent isotropic displacement parameters Ueq (Å2) of K15NaSn5F32.
Anisotropic displacement parameters Uij in Å2 (standard deviations in parentheses).
Interatomic distances in pm (standard deviations in parentheses).
Bond angles in deg (standard deviations in parentheses).
The main structural motifs of K15NaSn5F36 are quasi-isolated [SnF6]2− and [NaF6]5− octahedra (Fig. 1). Within the two different [SnF6]2− octahedra the Sn‒F bond lengths vary from 195.1(2) to 198.7(3) pm (Table 4). This is in accordance with the value given for Cs2SnF6 in the literature (dSn‒F=195.2 pm) . The bond angles confirm a distorted octahedral coordination sphere for both, Sn1 and Sn2 (Table 5). Sodium, in contrast, experiences a nearly perfect octahedral coordination sphere with Na‒F bond lengths of 226.3(2) pm and angles close to 90° (Table 5). The bond length is in accordance with the Na‒F bond length of 231.7 pm in NaF .
These octahedral units are imbedded into a matrix of potassium cations. Within the structure, there are four crystallographically different potassium positions. The corresponding coordination spheres are depicted in Fig. 2. The K‒F bond lengths vary between 253.5(2) and 317.2(3) pm. It is noteworthy that the site occupancy of K4 is only 0.5. This is likely a result of a statistical disorder at this site. Around the K4 sites (Fig. 3, K4_A and K4_B created by symmetry operations), a coordination sphere is built up, which is surrounded solely by [SnF6]2− units (Fig. 3). In comparison, all other potassium cation positions are coordinated by a mixture of [SnF6]2− and [NaF6]5− units, which does not leave enough space for a disorder on these sites. Therefore, just one of the positions K4_A or K4_B is occupied, with a probability of 50% for each option.
K15NaSn5F36 exhibits a layer-like arrangement. Layer one (L1) consists of a mixture of quasi-isolated [SnF6]2− and [NaF6]5− octahedra (Fig. 4, marked blue), whereas layer two (L2) consists solely of quasi-isolated [SnF6]2− octahedra (Fig. 4, marked green). These layers are stacked along the crystallographic c axis (Fig. 4, top). Within every second layer of L1 the orientation of the octahedral units changes, like mirrored on (110). Additionally, every second layer of L2 is turned around the c axis by 180°.
The structural model was confirmed by bond-length/bond-strength (BLBS) ,  and charge-distribution (CHARDI, program Chardi2015) ,  calculations (Table 6). At the site K4, the CHARDI calculations predict a charge of +0.48. This is in accordance with the structural data, as the SOF of K4 is 0.5. In contrast, the site occupancy cannot be taken into account during the BLBS calculations. This leads to a calculated charge of +0.90 for K4.
Values of the charge contributions according to both the bond-valence sums (∑V) and the CHARDI (∑Q) concept.
K15NaSn5F36 was characterized by means of single-crystal structure determination. It is, next to KNaSnF6, the second representative in the class of potassium sodium fluoridostannates . K15NaSn5F36 exhibits its own structure type, showing quasi-isolated [SnF6]2− and [NaF6]5− entities that are imbedded into a matrix of potassium cations, which leads to an unusually large unit cell. One of the potassium positions shows a statistical disorder, because of a large void created by four [SnF6]2− octahedra. The structural model was confirmed by BLBS and CHARDI calculations. Crystals of K15NaSn5F36 therefore have a large unit cell with an unusually high alkali metal to tin ratio of 3:1 (K:Sn). Additionally, this substance, if accessible as bulk material, might qualify as host material for doping with Mn4+ to yield a red phosphor, as it shows the preferable octahedral coordination sphere around the metal cations.
We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III and we would like to thank Carsten Paulmann and Heiko Schulz-Ritter for assistance in using the Kappa-diffractometer. Special thanks go to Prof. Dr. Mathias Wickleder and his research group for the isolation of the single crystal and the collection of the data at DESY. We also want to express our gratitude to Prof. Dr. Klaus Wurst for the help with the single-crystal structure refinement. For financial support of this work, we want to thank OSRAM Opto Semiconductors GmbH.
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