Binary Lead Fluoride Pb3F8

Abstract The binary lead fluoride Pb3F8 was synthesized by the reaction of anhydrous HF with Pb3O4 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 19F 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 PbF 2 and PbF 4 are well established compounds. [1,2] Their first lab synthesis dates back to the first half of the 19th centurya nd the determinationo ft heir crystal structures to 1944 and 1962, respectively. [1][2][3][4] In addition, PbF 3 , which is betterd escribed as Pb 2 F 6 containing Pb II and Pb IV atoms, was reported. [5] While forS na nd Ge also the mixed valence compounds M 3 F 8 (M = Sn, Ge), and even Ge 5 F 12 and Ge 7 F 16 are known, only the three binaryl ead fluorides mentioned above are unambiguously known. [6][7][8][9] Therefore, the existence of am ixed valence compound of the composition Pb 3 F 8 appearst ob el ikely.F or lead, mixed valence compounds are nothing special and the well-characterized compound Pb 3 O 4 (latin:M inium), whichw as used as ap igment in ancient Rome andi na nti-corrosion coatings, or which is even today in usage for charlatanism, comest ot he mind. [10][11][12][13] Ac ompound of the average chemical composition {Pb 3 F 8 }w as mentioned only twice in the literature. Nothing besidest his average composition has been reported. Pb 3 F 8 wasf irst mentioned in 1972 by Banner and co-workers as ar esult of the reaction of Pb 3 O 4 with gaseous HF on at hermogravimetric scale. [14] In their search for Pb 2 F 6 ,C harpina nd co-workersd escribed reactions leadingt oP b 3 F 8 as ap roduct or sidep roduct. Again, no details on Pb 3 F 8 were given, even not how the compound was identified as Pb 3 F 8 . [15] Herein, we present the synthesis and characterizationo ft he binary lead(II/IV) fluoride Pb 3 F 8 .

Results and Discussion
The formation of the title compound can be envisaged by the following stoichiometric Equation (1).
Pb 3 O 4 is reacted with an excess of anhydrous HF (aHF) at room temperature, so that the equilibrium of the reactioni s shiftedt ot he product side. After af ew minutes of reaction time the deep-orange color of Pb 3 O 4 is already gone and the reactioni sc omplete within one hour at room temperature. After the removal of the volatiles (HF and H 2 O), the product is obtained as as lightly beige powder ( Figure S1, Supporting Information)t hat is easily ground.T he dry powder of Pb 3 F 8 is stable for several hours in air.T he compound prepared in this way alwaysc ontains small amounts of PbF 2 (typically 5-8 %), as evidenced by Rietveld analysis( Ta ble 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) , b = 98.810(1)8, V = 672.3(3) 3 at T = 298 K. They agree well with those obtained from single-crystal X-ray diffraction, see below.T os uppress the back reactionb yh ydrolysis, 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,i fa ne xcess of aHF is used that is too small, a product is obtained that alwaysc ontainsl arger amounts of PbF 2 than mentioned above.T oobtain phase pure Pb 3 F 8 we attempted to oxidize PbF 2 using an excesso fB rF 3 under warming up to 130 8C. However,a fter evaporation of the residual BrF 3 ,t he remaining colorless powder consists of Pb 3 F 8 and small amounts of Pb 2 F 6 (circa 3%). Thermogravimetrici nvestigations( Figures S3, S4, Supporting Information, andf or further details see the Supporting Information) indicate that the thermal decomposition of Pb 3 F 8 is complex. The decomposition under loss of fluorine gas starts roughlya round 80 8C. After thermald ecomposition, pure PbF 2 is obtained as evidenced by powderX RD (Figure S5, Supporting Information). The overall mass loss duringt his procedureh as been determinedt wice, 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,P b 3 F 8 decomposest hermally to three equivalents of PbF 2 and one equivalent of F 2 .F urther details will be reported elsewhere. Helium pycnometric density determination (see the Supporting Information) yields ad ensity of circa 7.68 gcm À3 for the used sample of Pb 3 F 8 .D ue to the presence of circa 15 %P bF 2 (1 = 8.44 gcm À3 )i nt he sample used for density determination,avalue of 7.74 gcm À3 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 Pb 3 F 8 of circa 7.61 gcm À3 .
Single-crystal X-ray diffraction shows Pb 3 F 8 to crystallize in the monoclinic space group I2/a (No. 15, mS44,  3 are obtained. The latter agree wellw ith the values determined from powder X-ray diffractiona tr oom temperature. No phase change was observed upon cooling from room temperature to 100 Ka nd Ta ble S2, Supporting Information, holds details of the single crystal structure determination.S urprisingly,P b 3 F 8 is not isotypic to the compounds M 3 F 8 (M = Ge, Sn) but, to the best of our knowledge,r epresents an ovel structure type. [6,7] As the crystal structure of Pb 3 F 8 is complicated we will start with the local structure description before we describe it globally.T here are two typeso fP ba toms, Pb(1) and Pb (2). The Pb(1) atoms (Wyckoff position 4e)a re coordinated by Fa toms (8f)i nt he shapeo fa ni rregular octahedron, while the coordination polyhedrona round the Pb(2) atom (8f)r eminds of a pentagonal pyramid ( Figure 1 , which howevera ll contain spatially separated [PbF 6 ] 2À 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 as hape similar to ap entagonal pyramid ( Figure 1) and the Pb(2)-F distances span ar ather broad range from 2.330(3) to 2.651(3) .A st hey are clearly longer than the Pb(1)-F distances, we assign oxidation state + II to the Pb(2) atoms. Charge distribution (CHARDI) calculations [21] (Table S3, Supporting Information) agree with the description of Pb 3 F 8 as am ixed 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 shortestw ith 2.330(3) ,a nd represents the "tip" of the pyramid pointingt ot he bottom in Figure 1. The othert wo Pb(2)-F(4) distances are longer and equalw ithin the standard uncertainty (2.446(3) and 2.449 (3) ). The other Pb(2)-Fdistances are significantly longera nd range from 2.505(3) to 2.651(3) .A s can be seen in Figure1,t he Pb(2) atom is not located in the center of the coordination polyhedronb ut resides close to the pentagonal face. Such ac oordination polyhedron is reminiscent of the text-book anion[ XeOF 5 ] À , [22][23][24] and the peculiar location and coordination sphere of the Pb(2) atom is attributed to an accumulationo fe lectron density in real space as shown in the quantum chemical calculations below.D ue to the chemical hardnesso ft he fluoride anion andi ts 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 examplef rom a-a nd b-PbO, or from the black and pigeon blood red modifications of SnO. [25][26][27] However,a bove the "lone-pair" of the Pb II atom there are three additional Fa toms with Pb-F distances of 2.851(4), 2.874(3), and 3.051 (3) .A ccording to the distance histogramo ne could count those three Fa toms 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 ando ne 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 ECoNf or Pb(1) agrees well with C. N. = 6a sa ssigned 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 m 3 -bridging between Pb(2) atoms and that leads to the formation of a1 Di nfinite zigzagl adder showni n Figure 2a.T he two longer Pb(2)-F(4) distances form the stringers of the ladder,whilethe short Pb(2)-F(4) distances represent The topsidea nd underside of the infinitel adder are coordinated by [Pb(1)F 6 ] 2À octahedra as shown in Figure2b. The ladders are sandwiched between the octahedra and vice versa, leadingt oa2D infinite layer of ladders interconnected by octahedra.As ection is showni nF igure 3a.
Thus, the "sterically active lone-pairs" of the Pb (2)  Raman spectroscopic investigations have been carried out on Pb 3 F 8 and on PbF 2 for comparison. For experimental details see the Supporting Information. The experimentally obtained spectra were then compared with ones obtainedf rom DFT-PBE0/TZVP calculations based on the crystal structures of Pb 3 F 8 and PbF 2 .T he most striking difference between the Raman spectrum of Pb 3 F 8 and the spectrum of PbF 2 (see Figures S6 and S7, Supporting Information) is the strong vibrationalb and at 531 cm À1 that is only presenti nt he Raman spectrum of Pb 3 F 8 .T his band is well reproduced by our theoretical findings and can be attributed to as ymmetric stretching of the Pb IV ÀF bonds, which explainst he absence of this band in PbF 2 .P b 3 F 8 is also clearly identified by the lattice vibrational bands around 100 cm À1 as this frequency region corresponds to am inimum in Raman intensity in the spectrum of PbF 2 .T he two peaks at around2 50 cm À1 andt he two peaks at around2 00 cm À1 belongt oasymmetric stretchingo ft he Pb II ÀFb onds and bending modeso ft he Pb IV ÀFb onds, respectively.I ns ummary, the Ramans pectrum supports our classification of Pb 3 F 8 as a mixed valence compound. Full band assignmentsa re available from Ta bles S4 to S6, Supporting Information.
An IR spectroscopic investigation of Pb 3 F 8 powder in the range from 4000 to 450 cm À1 ( Figure S8) shows only as ingle broader band at 466 cm À1 ,w hich is comprised of intenseP b IV -Fs tretcha nd weaker Pb II -F scissoringa nd rocking modes. For Li 2 PbF 6 ,w hich contains [PbF 6 ] 2À octahedra, ab and at 475 cm À1 has been observed. [16] Thisa greesw ell considering the different connectivity of the [PbF 6 ] 2À octahedra in the two compounds. The experimentally determined band position of Pb 3 F 8 agreesw ell with the quantum chemically calculated bands at 493, 470, and456 cm À1 .The complete assignment of IR bandsis giveni nT able S5,S upportingI nformation.T he obtained Pb 3 F 8 is essentially free of impurities such as H 2 O, OH À ,o rH F, as no bands in the range from 4000 to circa 450 cm À1 are present.
Solid-state 19 FMAS NMR experiments (Figure 4a nd Ta ble 1) of Pb 3 F 8 were performed to further corroborate the crystal structure model.T he 19 FDEPTH MAS NMR spectrum shows four resonances,o ne occurring at d = À18.2 ppm andagroup of three overlapping signals at d = À40, À48.5, and À56 ppm. All four resonances have peak areas including spinning sidebands of 1:0.84:1.03:0.90. The spectrum also contains af ifth peak at d = À24.2 ppm with al ower intensity whichi sl ikely to originate fromt he PbF 2 impurity. [28] These observations are expected for Fa toms which do not have fast ion-dynamics on  At entative peak assignment of the 19 Fr esonancesf ollows the idea that neighboring cations contribute to the 19 Fc hemical shift according to their coordination number and distance to Fa toms in ionic fluorides. [32] Consequently,Fatoms with a similar bonding situation should feature similari sotropic and anisotropic chemical shiftv alues. In the present case ( Table 1) the group of three resonances has an anisotropic chemical shift whichi sl arger by about af actor of two compared to the peak which appearsatthe highest ppm values. In the structure three Fa toms are coordinated to two Pb atoms, one Fa tom is coordinated to three. Therefore, the resonance at À18.2 ppm is assigned to the three-fold coordinated fluorine site (F(4)) and the three signals at À40, À48.5, and À56 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 photoelectrons pectroscopy (XPS) as wella sn ear-edge X-ray absorption fine structure (NEXAFS) measurementst og et information about the electronic structure of Pb 3 F 8 .T he survey XP spectrum of Pb 3 F 8 on carbon tape is presented in Figure 5a.T he spectrum only shows contributions from Pb and Fa toms, besides minor C1 sa nd O1 sp eaks from the carbon tape.
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. Duringt he XPS and HAXPESm easurements, the sample exhibits substantial photoemission-induced charging, which results in peak shifts and broadening. For this reason, ar efineda nalysis of the XPS peak shapes with ad iscrimination between Pb II and Pb IV contributions is not possible. Instead, we performed NEXAFS spectroscopy measurements on the Pb M 5 -edge to gain further insight into the electronic structure of Pb 3 F 8 (Figure 5c). As ar eference, we also studied PbF 2 and Pb 3 O 4 .Between 2490 and 2495 eV,apre-edge feature is observed, which is followedbyt he M 5 -edge for all three species. PbF 2 shows as harp peak at 2490 eV with am inimum at 2495 eV.I nc ontrast, there is only ab road feature between 2490 and 2495 eV for Pb 3 O 4 .T he Pb 3 F 8 spectrum resemblesa mixture of both reference samples. Ap eak at 2490 eV is observed, whereas there is no minimum at 2495 eV like for PbF 2 .  [32,33] sequencew ith four p-pulses: p/2-p-p-p-p-t deadtime -FID. The DEPTHe xperiment results in MAS NMR spectrafree of probeh ead background.The simulation includes the effectoft he deadtime delayand excitation profile of the DEPTH sequenceswhich 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 19 Fs olid-state NMR chemical shift parameters for Pb 3 F 8 obtained by al east-square fit of the experimentallyo btained spectrum (Figure 4) with SIMPSON version 3.1.2 [29] simulations of the used version of the DEPTH [30,31]    Instead, there is ab road feature similar to the case of Pb 3 O 4 . This is in line with the presence of both Pb II and Pb IV species in the Pb 3 F 8 sample and with as mallc ontamination of PbF 2 ,a s stated above.T he differences in the M 5 -edge itself are more complicated as there are nearly no similarities between the three compounds. In the range from 2500 to 2510 eV,P bF 2 and Pb 3 F 8 show similar spectral features, but above thatr ange PbF 2 exhibits al ocal minimum, whereas Pb 3 F 8 shows ap eak. A similar peak is observed in the Pb 3 O 4 spectrum but shifted by nearly 10 eV to higher energies. We have calculated the electronic structure of Pb 3 F 8 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 correctedb yb ackground and cross-section effects (see the Supporting Information). The results are shown in Figure 5b.T he 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 Pb 3 F 8 in more detail by calculating its band structure and charge distribution.T he band structure as well as the total DOS are given in Figure 6.
The band structure calculations show Pb 3 F 8 to be an insulator with ab and gap of approximately 4.5 eV in line with its offwhite color. The DOS of the valence band is dominated by F 2p states that range from À1eVt o2eV.A ta bout4eV the top of the valenceb and consistso ff our bands with only as mall amount of dispersion that can be attributed to the filled Pb II 6s bands. The conductionb and is located at about 9eVand consists of two bands. Both show nearly exclusive Pb IV 6s character as illustrated by the pDOS in the right of Figure6.As mall amount of the Pb IV 6s states is located at the bottom of the valence band at about À4eVd ue to some covalent Pb IV ÀFb ond character.F or the same reasonP b II 6s states are present at about À2eV. The band structure of the mixed valence compound Pb 3 O 4 shows similarc haracteristics. [11] We thus conclude that like Pb 3 O 4 also Pb 3 F 8 is am ixed valencec ompound with the lead atoms in the oxidationstates + II and + IV.
The crystal structure of Pb 3 F 8 indicates that the Pb II atoms feature "sterically active lone-pairs". We calculated electrondensity differencem aps of Pb 3 F 8 which display the difference of the electron density of the compound compared to as uperpositiono ft he electron density of free atoms, yielding information where electron density is accumulatedo rd epleted. The electron-density differencem ap of Pb 3 F 8 is shown in Figure 7. It is drawn in av iew perpendicular to the ladder-like connection of the Pb(2) atomsa nd the F(4) atoms, compare Figure 2a.
The map displays as trongp olarization of the electron density around the Pb II atoms. The electron density along the Pb-F bondsi sm inimized (brownish colors) in line with the expected high amount of ionic bonding character. Moreover,t here is an accumulation of electron density (in blue colors) besides the Pb II atoms, pointing to the left and right side of the depicted ladder. Therefore, the electron density at the Pb II atoms is "pushed" away from the fluorine atoms inside the ladder. This effect is often referred to "stericallya ctive lone-pairs" of the Pb II atoms. The electron density around the fluorine atoms (in yellow color) is strongly and nearly spherically increased as is expected for F À anionsdue to the high electronegativity of the Fatom.

Conclusions
The binaryl ead(II/IV) fluorideP b 3 F 8 was synthesized from Pb 3 O 4 in anhydrous HF at room temperature. The bulk phase appearso ff-white while single crystalsa re colorless. It is thermally stable up to circa 80 8Ca nd then decomposest oP bF 2 under loss of F 2 .T he compound crystallizes in the monoclinic space group I2/a (No. 15) with the lattice parameters a = 8.7800 (18)  662.9(4) 3 ; Z = 4a tT = 100 K, as evidencedb ys ingle-crystal Xray analysis. The descriptiono fP b 3 F 8 as am ixed valence Pb II / Pb IV compound is evidencedb yt he thermal decomposition products,t he crystal structure, the 19 Fs olid-state NMR, valence and core level photoelectron, as well as near-edge X-ray absorptionf ine structure (NEXAFS) spectroscopici nvestigations and further supported by IR and Raman spectra.A dditionally, quantum chemical calculations were carried out to elucidate the electronic structure of Pb 3 F 8 .T he calculated band gap is in line with the color of the compound.A na ccumulation of electron density next to the Pb II atoms that some call "sterically active lone-pairs" seems to be responsible for the formation of the peculiar layer structure of Pb 3 F 8 .