Binary Lead Fluoride Pb3 F8.

The binary lead fluoride Pb3 F8 was synthesized by the reaction of anhydrous HF with Pb3 O4 or by the reaction of BrF3 with PbF2 . The compound was characterized by single-crystal and powder X-ray diffraction, IR, Raman, and solid-state MAS 19 F NMR spectroscopy, as well as thermogravimetric analysis, XP and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Solid-state quantum-chemical calculations are provided for the vibrational analyses and band assignments. The electronic band structure offers an inside view of the mixed valence compound.

Introduction

The binary lead fluorides PbF2 and PbF4 are well established compounds.1, 2 Their first lab synthesis dates back to the first half of the 19th century and the determination of their crystal structures to 1944 and 1962, respectively.1, 2, 3, 4 In addition, PbF3, which is better described as Pb2F6 containing PbII and PbIV atoms, was reported.5 While for Sn and Ge also the mixed valence compounds M 3F8 (M=Sn, Ge), and even Ge5F12 and Ge7F16 are known, only the three binary lead fluorides mentioned above are unambiguously known.6, 7, 8, 9 Therefore, the existence of a mixed valence compound of the composition Pb3F8 appears to be likely. For lead, mixed valence compounds are nothing special and the well‐characterized compound Pb3O4 (latin: Minium), which was used as a pigment in ancient Rome and in anti‐corrosion coatings, or which is even today in usage for charlatanism, comes to the mind.10, 11, 12, 13 A compound of the average chemical composition {Pb3F8} was mentioned only twice in the literature. Nothing besides this average composition has been reported. Pb3F8 was first mentioned in 1972 by Banner and co‐workers as a result of the reaction of Pb3O4 with gaseous HF on a thermogravimetric scale.14 In their search for Pb2F6, Charpin and co‐workers described reactions leading to Pb3F8 as a product or side product. Again, no details on Pb3F8 were given, even not how the compound was identified as Pb3F8.15 Herein, we present the synthesis and characterization of the binary lead(II/IV) fluoride Pb3F8.

Results and Discussion

The formation of the title compound can be envisaged by the following stoichiometric Equation (1).

Pb3O4 is reacted with an excess of anhydrous HF (aHF) at room temperature, so that the equilibrium of the reaction is shifted to the product side. After a few minutes of reaction time the deep‐orange color of Pb3O4 is already gone and the reaction is complete within one hour at room temperature. After the removal of the volatiles (HF and H2O), the product is obtained as a slightly beige powder (Figure S1, Supporting Information) that is easily ground. The dry powder of Pb3F8 is stable for several hours in air. The compound prepared in this way always contains small amounts of PbF2 (typically 5–8 %), as evidenced by Rietveld analysis (Table S1, Figure S2, Supporting Information) on powder X‐ray diffraction patterns. The obtained lattice parameters are a=8.8434(1), b=7.5427(1), c=10.2339(1) Å, β=98.810(1)°, V=672.3(3) Å3 at T=298 K. They agree well with those obtained from single‐crystal X‐ray diffraction, see below. To suppress the back reaction by hydrolysis, a large excess of circa 100 equivalents of aHF is needed. If the reaction mixture is allowed to stand for three days at room temperature, or, if an excess of aHF is used that is too small, a product is obtained that always contains larger amounts of PbF2 than mentioned above. To obtain phase pure Pb3F8 we attempted to oxidize PbF2 using an excess of BrF3 under warming up to 130 °C. However, after evaporation of the residual BrF3, the remaining colorless powder consists of Pb3F8 and small amounts of Pb2F6 (circa 3 %). Thermogravimetric investigations (Figures S3, S4, Supporting Information, and for further details see the Supporting Information) indicate that the thermal decomposition of Pb3F8 is complex. The decomposition under loss of fluorine gas starts roughly around 80 °C. After thermal decomposition, pure PbF2 is obtained as evidenced by powder XRD (Figure S5, Supporting Information). The overall mass loss during this procedure has been determined twice, once to 5.2 and once to 4.7 %. Both values are in reasonably good agreement with the theoretically expected mass loss of 4.9 %. Thus, Pb3F8 decomposes thermally to three equivalents of PbF2 and one equivalent of F2. Further details will be reported elsewhere. Helium pycnometric density determination (see the Supporting Information) yields a density of circa 7.68 g cm−3 for the used sample of Pb3F8. Due to the presence of circa 15 % PbF2 (ρ=8.44 g cm−3) in the sample used for density determination, a value of 7.74 g cm−3 is to be expected from the measurements. Thus, the experimentally determined density is in very good agreement with the measurement and with the crystallographic density of Pb3F8 of circa 7.61 g cm−3.

Single‐crystal X‐ray diffraction shows Pb3F8 to crystallize in the monoclinic space group I2/a (No. 15, mS44, 15ef  5) with the lattice parameters a=8.7800(18), b=7.4927(15), c=10.196(5) Å; β=98.78(3)°; V=662.9(4) Å3; Z=4, at T=100 K, while at room temperature lattice parameters of a=8.8400(5), b=7.5398(5), c=10.2297(7) Å, β=98.82(2)°, V=673.77(8) Å3 are obtained. The latter agree well with the values determined from powder X‐ray diffraction at room temperature. No phase change was observed upon cooling from room temperature to 100 K and Table S2, Supporting Information, holds details of the single crystal structure determination. Surprisingly, Pb3F8 is not isotypic to the compounds M 3F8 (M=Ge, Sn) but, to the best of our knowledge, represents a novel structure type.6, 7 As the crystal structure of Pb3F8 is complicated we will start with the local structure description before we describe it globally. There are two types of Pb atoms, Pb(1) and Pb(2). The Pb(1) atoms (Wyckoff position 4e) are coordinated by F atoms (8f) in the shape of an irregular octahedron, while the coordination polyhedron around the Pb(2) atom (8f) reminds of a pentagonal pyramid (Figure 1).

Figure 1

The coordination spheres of the two lead atoms of Pb3F8. The Pb(1) atom is coordinated octahedron‐like, the Pb(2) atom like a pentagonal pyramid. Pb atoms are shown in grey, F atoms in yellow. Displacement ellipsoids at 70 % probability at 100 K.

All F atoms around the Pb(1) atom are μ2‐bridging to Pb(2) atoms. The Pb(1)–F atomic distances are observed in the range from 2.048(3) to 2.063(3) Å. They agree well with reported ones for hexafluoridoplumbates(IV) in compounds such as M IIPbF6 (M II=Mg (1.99 Å), Ni (1.99 Å), Zn (1.97 Å), Sr (2.042–2.060 Å), Ba (2.04 Å), Pb (1.991–2.011 Å)) or M I 2PbF6 (M I=Ag (2.021–2.100 Å), Li (1.997 Å)), which however all contain spatially separated [PbF6]2− octahedra.16, 17, 18, 19, 20 Therefore, we assign oxidation state +IV to these octahedron‐like coordinated Pb(1) atoms. As stated above, the Pb(2) atoms are coordinated by six fluorine atoms in a shape similar to a pentagonal pyramid (Figure 1) and the Pb(2)–F distances span a rather broad range from 2.330(3) to 2.651(3) Å. As they are clearly longer than the Pb(1)–F distances, we assign oxidation state +II to the Pb(2) atoms. Charge distribution (CHARDI) calculations21 (Table S3, Supporting Information) agree with the description of Pb3F8 as a mixed valence compound as charges of +4.12 and +1.94 are calculated for the Pb(1) and Pb(2) atoms, respectively. Thus, the assignment of the oxidation states is supported.

One Pb(2)–F(4) distance within the pentagonal pyramid is shortest with 2.330(3) Å, and represents the “tip” of the pyramid pointing to the bottom in Figure 1. The other two Pb(2)–F(4) distances are longer and equal within the standard uncertainty (2.446(3) and 2.449(3) Å). The other Pb(2)–F distances are significantly longer and range from 2.505(3) to 2.651(3) Å. As can be seen in Figure 1, the Pb(2) atom is not located in the center of the coordination polyhedron but resides close to the pentagonal face. Such a coordination polyhedron is reminiscent of the text‐book anion [XeOF5]−,22, 23, 24 and the peculiar location and coordination sphere of the Pb(2) atom is attributed to an accumulation of electron density in real space as shown in the quantum chemical calculations below. Due to the chemical hardness of the fluoride anion and its extremely low polarizability, its electron density leads to repulsion and deformation of the electron density at the Pb atom. Some call this effect the “sterically active lone‐pair” and its influence on local as well as crystal structure is known for example from α‐ and β‐PbO, or from the black and pigeon blood red modifications of SnO.25, 26, 27 However, above the “lone‐pair” of the PbII atom there are three additional F atoms with Pb–F distances of 2.851(4), 2.874(3), and 3.051(3) Å. According to the distance histogram one could count those three F atoms to the coordination sphere of Pb(2) leading to coordination number 6+3. The coordination polyhedron around Pb(2) is then irregular with ten triangles and one tetragon as the faces. Also, the calculated effective coordination number (ECoN) of 6.9 hints to a small contribution of the three next‐nearest fluorine atoms to its coordination sphere, whereas the calculated ECoN for Pb(1) agrees well with C. N.=6 as assigned by our structure analysis.

We will now come to the global structure description by explaining how the coordination polyhedra are interconnected. The F(4) atoms are μ3‐bridging between Pb(2) atoms and that leads to the formation of a 1D infinite zigzag ladder shown in Figure 2 a. The two longer Pb(2)–F(4) distances form the stringers of the ladder, while the short Pb(2)–F(4) distances represent the rungs of the ladder (Figure 2 a). Thus, the “lone‐pairs” on the Pb(2) atoms point to the left and right in Figure 2 a.

Figure 2

a) Ladder‐like connection of the Pb(2) atoms (grey) via μ3‐bridging F(4) atoms (yellow). b) Connection of the Pb(2) containing ladder to the Pb(1) containing octahedra. Displacement ellipsoids shown with 70 % probability at 100 K.

The topside and underside of the infinite ladder are coordinated by [Pb(1)F6]2− octahedra as shown in Figure 2 b. The ladders are sandwiched between the octahedra and vice versa, leading to a 2D infinite layer of ladders interconnected by octahedra. A section is shown in Figure 3 a.

Figure 3

a) A part of the 2D infinite layer formed by the sandwiching of [Pb(1)F6]2 octahedra by Pb(2) containing ladders. Displacement ellipsoids at 70 % probability at 100 K. b) A section of the crystal structure of Pb3F8. Atoms are shown isotropic with arbitrary radii. Pb atoms grey, F atoms yellow. The 2D infinite layers run parallel to the ab plane. The height along the b axis is shown with the approximate y coordinate of the gravimetric center of the building units.

Thus, the “sterically active lone‐pairs” of the Pb(2) atoms point out of the topside and underside of these layers (Figure 3 b) separating them from each other. Figure 3 b shows a section of the crystal structure of Pb3F8 with the 2D infinite layers parallel to the ab plane stacked along the c axis. The Niggli formula indicates the coordination number and environment of the Pb atoms nicely. For the PbII atom [PbF3/2F3/3] and for the PbIV atom [PbF6/2] is obtained. Thus, Pb3F8 can be described by the Niggli formula [PbF3/2F3/3]2[PbF6/2]. The Pb atoms are hexagonally packed and each is anticuboctahedrally surrounded by twelve Pb atoms. Thus, the arrangement of the Pb atoms of Pb3F8 corresponds to the simple Mg structure type. However, the F atoms neither fill the octahedral nor the tetrahedral voids of the sphere packing.

Raman spectroscopic investigations have been carried out on Pb3F8 and on PbF2 for comparison. For experimental details see the Supporting Information. The experimentally obtained spectra were then compared with ones obtained from DFT‐PBE0/TZVP calculations based on the crystal structures of Pb3F8 and PbF2. The most striking difference between the Raman spectrum of Pb3F8 and the spectrum of PbF2 (see Figures S6 and S7, Supporting Information) is the strong vibrational band at 531 cm−1 that is only present in the Raman spectrum of Pb3F8. This band is well reproduced by our theoretical findings and can be attributed to a symmetric stretching of the PbIV−F bonds, which explains the absence of this band in PbF2. Pb3F8 is also clearly identified by the lattice vibrational bands around 100 cm−1 as this frequency region corresponds to a minimum in Raman intensity in the spectrum of PbF2. The two peaks at around 250 cm−1 and the two peaks at around 200 cm−1 belong to a symmetric stretching of the PbII−F bonds and bending modes of the PbIV−F bonds, respectively. In summary, the Raman spectrum supports our classification of Pb3F8 as a mixed valence compound. Full band assignments are available from Tables S4 to S6, Supporting Information.

An IR spectroscopic investigation of Pb3F8 powder in the range from 4000 to 450 cm−1 (Figure S8) shows only a single broader band at 466 cm−1, which is comprised of intense PbIV–F stretch and weaker PbII–F scissoring and rocking modes. For Li2PbF6, which contains [PbF6]2− octahedra, a band at 475 cm−1 has been observed.16 This agrees well considering the different connectivity of the [PbF6]2− octahedra in the two compounds. The experimentally determined band position of Pb3F8 agrees well with the quantum chemically calculated bands at 493, 470, and 456 cm−1. The complete assignment of IR bands is given in Table S5, Supporting Information. The obtained Pb3F8 is essentially free of impurities such as H2O, OH−, or HF, as no bands in the range from 4000 to circa 450 cm−1 are present.

Solid‐state 19F MAS NMR experiments (Figure 4 and Table 1) of Pb3F8 were performed to further corroborate the crystal structure model. The 19F DEPTH MAS NMR spectrum shows four resonances, one occurring at δ=−18.2 ppm and a group of three overlapping signals at δ=−40, −48.5, and −56 ppm. All four resonances have peak areas including spinning sidebands of 1:0.84:1.03:0.90. The spectrum also contains a fifth peak at δ=−24.2 ppm with a lower intensity which is likely to originate from the PbF2 impurity.28 These observations are expected for F atoms which do not have fast ion‐dynamics on the NMR timescale, as four symmetry‐inequivalent F atoms (F(1) to F(4)) with the same site multiplicity are present in the crystal structure.

Figure 4

19F DEPTH MAS NMR spectrum (experimental: solid line, simulated: dashed line) of Pb3F8 at 20 kHz spinning frequency. The spinning side bands are marked with asterisks. The simulation includes a version of the DEPTH[32, 33] sequence with four π‐pulses: π/2‐π‐π‐π‐π‐τdeadtime‐FID. The DEPTH experiment results in MAS NMR spectra free of probe head background. The simulation includes the effect of the deadtime delay and excitation profile of the DEPTH sequences which causes the baseline rolling. Zeroth and first order phase correction are included as variable parameters in the least‐square fit.

Table 1

Estimates for the 19F solid‐state NMR chemical shift parameters for Pb3F8 obtained by a least‐square fit of the experimentally obtained spectrum (Figure 4) with SIMPSON version 3.1.229 simulations of the used version of the DEPTH30, 31 experiment.

Site	δ iso [ppm]	δ aniso [ppm]	η	δ 11 [ppm]	δ 22 [ppm]	δ 33 [ppm]
F(4)	−18.2	66.1	0.60	47.9	−31.5	−71.2
F(1)–F(3)	−40.0	−111.8	0.61	50.5	−18.5	−151.7
F(1)–F(3)	−48.5	−121.0	0.65	51.5	−27.4	−169.5
F(1)–F(3)	−56.0	−119.2	0.47	31.6	−24.5	−175.2

A tentative peak assignment of the 19F resonances follows the idea that neighboring cations contribute to the 19F chemical shift according to their coordination number and distance to F atoms in ionic fluorides.32 Consequently, F atoms with a similar bonding situation should feature similar isotropic and anisotropic chemical shift values. In the present case (Table 1) the group of three resonances has an anisotropic chemical shift which is larger by about a factor of two compared to the peak which appears at the highest ppm values. In the structure three F atoms are coordinated to two Pb atoms, one F atom is coordinated to three. Therefore, the resonance at −18.2 ppm is assigned to the three‐fold coordinated fluorine site (F(4)) and the three signals at −40, −48.5, and −56 ppm are assigned to the fluorine atoms F(1), F(2), and F(3) coordinated by the two lead Pb(1) and Pb(2) atoms.

We have performed X‐ray photoelectron spectroscopy (XPS) as well as near‐edge X‐ray absorption fine structure (NEXAFS) measurements to get information about the electronic structure of Pb3F8. The survey XP spectrum of Pb3F8 on carbon tape is presented in Figure 5 a. The spectrum only shows contributions from Pb and F atoms, besides minor C 1s and O 1s peaks from the carbon tape.

Figure 5

(a) Survey XP spectrum of Pb3F8 on carbon tape, taken with monochromatic Al K radiation. (b) Valence band spectrum of Pb3F8 measured with Al Kα radiation in comparison to DFT calculations (PBE0/NCPW). A Shirley background was subtracted from the experimental data to compare it to the theoretical results. The contribution of the Pb 6s orbitals to the total calculated DOS is highlighted. Further details concerning the data treatment are given in the Supporting Information. (c) Pb M5‐edge NEXAFS spectra of Pb3F8, PbF2 and Pb3O4 measured by the X‐ray fluorescence yield. Inset: Zoom‐in of the pre‐edge feature.

Another sample that was studied with hard X‐ray photoelectron spectroscopy (HAXPES, Figure S9 in the Supporting Information) shows the same features and even less contributions from the carbon tape. During the XPS and HAXPES measurements, the sample exhibits substantial photoemission‐induced charging, which results in peak shifts and broadening. For this reason, a refined analysis of the XPS peak shapes with a discrimination between PbII and PbIV contributions is not possible. Instead, we performed NEXAFS spectroscopy measurements on the Pb M5‐edge to gain further insight into the electronic structure of Pb3F8 (Figure 5 c). As a reference, we also studied PbF2 and Pb3O4. Between 2490 and 2495 eV, a pre‐edge feature is observed, which is followed by the M5‐edge for all three species. PbF2 shows a sharp peak at 2490 eV with a minimum at 2495 eV. In contrast, there is only a broad feature between 2490 and 2495 eV for Pb3O4. The Pb3F8 spectrum resembles a mixture of both reference samples. A peak at 2490 eV is observed, whereas there is no minimum at 2495 eV like for PbF2. Instead, there is a broad feature similar to the case of Pb3O4. This is in line with the presence of both PbII and PbIV species in the Pb3F8 sample and with a small contamination of PbF2, as stated above. The differences in the M5‐edge itself are more complicated as there are nearly no similarities between the three compounds. In the range from 2500 to 2510 eV, PbF2 and Pb3F8 show similar spectral features, but above that range PbF2 exhibits a local minimum, whereas Pb3F8 shows a peak. A similar peak is observed in the Pb3O4 spectrum but shifted by nearly 10 eV to higher energies.

We have calculated the electronic structure of Pb3F8 by DFT methods using the hybrid functional PBE0 and fully relativistic pseudopotentials.33, 34 To estimate the accuracy of our calculations we compared the experimentally determined valence band XP spectrum with the calculated partial density of states (pDOS) that is corrected by background and cross‐section effects (see the Supporting Information). The results are shown in Figure 5 b. The valence band width as well as its three‐peaked shape are well reproduced by the DFT calculations.

In the following, we investigate the electronic structure of Pb3F8 in more detail by calculating its band structure and charge distribution. The band structure as well as the total DOS are given in Figure 6.

Figure 6

Left: Electronic band structure of Pb3F8. Right: Total Density of States (DOS) and the projected DOS of the 6s orbitals of PbII and PbIV. The position of the band gap (4.5 eV) is highlighted (DFT‐PBE0/NCPP with SOC).

The band structure calculations show Pb3F8 to be an insulator with a band gap of approximately 4.5 eV in line with its off‐white color. The DOS of the valence band is dominated by F 2p states that range from −1 eV to 2 eV. At about 4 eV the top of the valence band consists of four bands with only a small amount of dispersion that can be attributed to the filled PbII 6s bands. The conduction band is located at about 9 eV and consists of two bands. Both show nearly exclusive PbIV 6s character as illustrated by the pDOS in the right of Figure 6. A small amount of the PbIV 6s states is located at the bottom of the valence band at about −4 eV due to some covalent PbIV−F bond character. For the same reason PbII 6s states are present at about −2 eV. The band structure of the mixed valence compound Pb3O4 shows similar characteristics.11 We thus conclude that like Pb3O4 also Pb3F8 is a mixed valence compound with the lead atoms in the oxidation states +II and +IV.

The crystal structure of Pb3F8 indicates that the PbII atoms feature “sterically active lone‐pairs”. We calculated electron‐density difference maps of Pb3F8 which display the difference of the electron density of the compound compared to a superposition of the electron density of free atoms, yielding information where electron density is accumulated or depleted. The electron‐density difference map of Pb3F8 is shown in Figure 7. It is drawn in a view perpendicular to the ladder‐like connection of the Pb(2) atoms and the F(4) atoms, compare Figure 2 a.

Figure 7

Electron‐density difference map (DFT‐PBE0/NCPP+SOC) of Pb3F8 along the ladder‐like connection (sketched) of the Pb(2) atoms (grey color) via μ 3‐bridging F(4) atoms (yellow color). An increase in electron density is shown in blue color and solid black lines, while a decrease in electron density is shown in brown color and dashed black lines.

The map displays a strong polarization of the electron density around the PbII atoms. The electron density along the Pb–F bonds is minimized (brownish colors) in line with the expected high amount of ionic bonding character. Moreover, there is an accumulation of electron density (in blue colors) besides the PbII atoms, pointing to the left and right side of the depicted ladder. Therefore, the electron density at the PbII atoms is “pushed” away from the fluorine atoms inside the ladder. This effect is often referred to “sterically active lone‐pairs” of the PbII atoms. The electron density around the fluorine atoms (in yellow color) is strongly and nearly spherically increased as is expected for F− anions due to the high electronegativity of the F atom.

Conclusions

The binary lead(II/IV) fluoride Pb3F8 was synthesized from Pb3O4 in anhydrous HF at room temperature. The bulk phase appears off‐white while single crystals are colorless. It is thermally stable up to circa 80 °C and then decomposes to PbF2 under loss of F2. The compound crystallizes in the monoclinic space group I2/a (No. 15) with the lattice parameters a=8.7800(18), b=7.4927(15), c=10.196(5) Å; β=98.78(3)°; V=662.9(4) Å3; Z=4 at T=100 K, as evidenced by single‐crystal X‐ray analysis. The description of Pb3F8 as a mixed valence PbII/PbIV compound is evidenced by the thermal decomposition products, the crystal structure, the 19F solid‐state NMR, valence and core level photoelectron, as well as near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopic investigations and further supported by IR and Raman spectra. Additionally, quantum chemical calculations were carried out to elucidate the electronic structure of Pb3F8. The calculated band gap is in line with the color of the compound. An accumulation of electron density next to the PbII atoms that some call “sterically active lone‐pairs” seems to be responsible for the formation of the peculiar layer structure of Pb3F8.

Conflict of interest

The authors declare no conflict of interest.

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Supplementary

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