Rerefinement of the crystal structure of BiF5

Redetermination of the crystal structure of BiF5 was undertaken to a much higher precision and quantum chemical calculations for an assignment of the Raman and IR bands.


Chemical context
Bismuth(V) fluoride was first synthesized in the year 1940 (von Wartenberg et al., 1940).Hebecker first determined its crystal structure in 1971 (Hebecker, 1971).During our studies of the chemistry of BiF 5 , we rerefined its crystal structure on basis of single-crystal data and recorded its IR and Raman spectra.

Structural commentary
Bismuth(V) fluoride crystallizes in the tetragonal space group I4/m, Pearson symbol tI12 and Wyckoff sequence 87.hba.BiF 5 adopts the �-UF 5 structure type and exhibits Bi-F bond lengths of 1.941 (4) (4�) and 2.1130 (5) A ˚(2�).The structure consists of chains of trans-corner-sharing [BiF 6 ] octahedra (point group symmetry 4/m..; Fig. 1) running parallel to the c axis.The chains can be described by the Niggli formula van der Waals contacts exist between neighbouring chains with interatomic distances of 2.944 (5) A ˚(F2� � �F2 v ) and 2.952 (5) A ˚(F1� � �F2 v ) [symmetry code: (v) 1 2 À x, 1 2 À y, 1 2 + z).Compared with the distances reported previously (Hebecker, 1971), these are 0.12 A ˚shorter for d(F2� � �F2) and 0.05 A longer for d(F1� � �F2).Regarding the crystal packing, a Bi atom is surrounded by 14 other Bi atoms in the shape of a distorted rhombic dodecahedron, which would correspond to the arrangement of the atoms in the W structure type.However, the rhombic dodecahedron is compressed due to the shorter intrachain and the longer interchain Bi� � �Bi distances.The F1 atoms reside in the octahedral voids of the (idealized) body-centered cubic packing, while the F2 atoms are strongly displaced from these as the octahedra are rotated around the c axis (Fig. 2).

Vibrational spectra
IR and Raman spectra were recorded on a polycrystalline sample of BiF 5 at 293 K. Quantum-chemical calculations at the DFT-PBE0/TZVP level of theory (Dovesi et al., 2018;Zicovich-Wilson et al., 2004;Pascale et al., 2004;Maschio et al., 2013) were performed on basis of the crystal structure of BiF 5 to obtain the theoretical IR and Raman spectra.The recorded and calculated spectra are in good agreement, as shown in Figs. 3 and 4. A comparison of the observed and calculated bands is given in Table 1.

Table 1
Band positions (cm À 1 ) and band assignment of the IR and Raman spectra of solid BiF 5 based on the calculated spectrum.� = stretching vibration, � = bending vibration, s = symmetric, as = asymmetric, -= not observed.

IR
Raman Assignment Recorded Raman spectrum (532 nm laser) of crystalline BiF 5 in black, calculated Raman spectrum of solid BiF 5 at the DFT-PBE0/TZVP level of theory in red.No bands were observed in the region from 1000 to 4000 cm À 1 .

Figure 3
Recorded IR spectrum of crystalline BiF 5 in black and calculated IR absorbance spectrum of solid BiF 5 at the DFT-PBE0/TZVP level of theory in red.No bands were observed in the region from 1000 to 4000 cm À 1 .

Figure 1
The coordination sphere of the Bi atom in the crystal structure of BiF 5 .Displacement ellipsoids are drawn at the 70% probability level.
[Symmetry codes: material, bismuth(V) fluoride was synthesized at 723 K, using a heating rate of 4 K min À 1 from room temperature to 723 K and a holding time of 10 h while diluted fluorine was passed with a flow rate of 5 standard cubic centimeters per minute.

1 1 [
BiF 4/1 F 2/2 ].The F1 atom bridges adjacent Bi atoms in the straight chain.As expected, the axial Bi-F1 bond lengths are longer than the four equatorial Bi-F2 bond lengths to the terminal ligands.Sections of the crystal structure showing the chains are shown in Fig. 2.

Figure 2
Figure 2 Crystal structure of BiF 5 viewed along the b axis (left) and the c axis (right).The [BiF 6 ] octahedra are shown in grey.

Table 2
Experimental details.