Tungsten fluorides: Syntheses and electrochemical characterization in the FLINAK molten salt eutectic

Article abstract of DOI:10.1021/ic000491o

The following tungsten fluorides have been synthesized by simple addition reactions or by reduction with tungsten metal at elevated temperature: KWFT₇, K₂WF₈, MWF₆ (M = K, Na, Rb, Cs), K₂WF₇, M₃WF₈ (M = K, Na, Rb), and K₃WF₆. The compounds were characterized by their Raman spectra and by cyclic voltammetry in the molten FLINAK eutectic melt (46.5, 11.5, and 42.0 mol % of LiF, NaF, and KF, respectively) at 475-800 °C. X-ray crystal structures are reported for two new compounds K₂WF₇ and K₃WF₆. The crystals of K₂WF₇ were orthorhombic, space group Pnma (No. 62) with a = 9.800(2) A, b = 5.7360(11) A, c = 11.723(2) A, and Z = 4. Crystals of K₃WF₆ were cubic, space group Fm3 (No. 225) with a = b = c = 8.9160(10) A, Z = 4. Electrodeposition of tungsten metal on Pt from FLINAK, prepared by the addition of WF₆ gas and metallic tungsten to the melt, is suggested to result from reduction of an equilibrium mixture of WF₈^3- and WF₆^3-.

Full text of DOI:10.1021/ic000491o

Inorg. Chem. 2001, 40, 715-722
715
Tungsten Fluorides: Syntheses and Electrochemical Characterization in the FLINAK
Molten Salt Eutectic
Sven E. Eklund, James Q. Chambers,* Gleb Mamantov,‡ Jon Diminnie, and Craig E. Barnes
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996
ReceiVed May 8, 2000
The following tungsten fluorides have been synthesized by simple addition reactions or by reduction with tungsten
metal at elevated temperature: KWF7, K2WF8, MWF6 (M ) K, Na, Rb, Cs), K2WF7, M3WF8 (M ) K, Na, Rb),
and K3WF6. The compounds were characterized by their Raman spectra and by cyclic voltammetry in the molten
FLINAK eutectic melt (46.5, 11.5, and 42.0 mol % of LiF, NaF, and KF, respectively) at 475-800 °C. X-ray
crystal structures are reported for two new compounds K2WF7 and K3WF6. The crystals of K2WF7 were
orthorhombic, space group Pnma (No. 62) with a ) 9.800(2) Å, b ) 5.7360(11) Å, c ) 11.723(2) Å, and Z )
4. Crystals of K3WF6 were cubic, space group Fm3h (No. 225) with a ) b ) c ) 8.9160(10) Å, Z ) 4.
Electrodeposition of tungsten metal on Pt from FLINAK, prepared by the addition of WF6 gas and metallic
3-
3-
tungsten to the melt, is suggested to result from reduction of an equilibrium mixture of WF8 and WF6
.
High-temperature molten salt media are well suited to the
electrolytic production of the refractory metals in groups IVA-
VIA. In particular, the eutectic alkali fluoride salt mixture of
LiF, NaF, and KF (46.5, 11.5, and 42.0 mol %, respectively;
mp, 454 °C), also known as FLINAK, has been employed to
electrodeposit all of the metals in these groups except for
titanium.1,2 Possible reasons for this behavior are the lability of
the fluoride ligands and that the fluoride salt environment
minimizes the formation of cluster species, as is observed when
chloride is present. Molten fluoride salts have played important
roles in other applications as well, including the electrowinning
of aluminum from cryolite melts3 and as homogeneous fluid
fuels in molten salt nuclear reactors.4
Previously tungsten has been electrodeposited from FLINAK
solutions containing about 1-10 wt % W that were prepared
by the addition of WF6 gas to the molten salt medium under
reducing conditions.5-7 The mean oxidation state of the elec-
troactive species, which was reported to be +4.51 by Senderoff
and Mellors6 and +4 to +4.5 by Broc,5 suggests the formation
of either mixed-valent polynuclear species or multiple species
with different oxidation states at equilibrium. A simple WF4
species is unlikely to exist in these melts because this species
is known to disproportionate to W metal and WF6 gas at
temperatures above 200-300 °C.8
of six-, seven- and eight-coordinate tungsten(VI) and tungsten-
(V) fluorides were achieved in excellent yields by simple high-
temperature addition and reduction reactions using tungsten
metal as the reductant. Previous synthetic methods reported for
these compounds have employed iodide salts as reductants and
reagents such as IF5,9-12 or metallocene reductants,13 or NOF
in CH3CN.14,15 The high-temperature routes reported here, which
employ neat molten salt reactants, avoid complications due to
impurities introduced by side reactions and the formation of
oxyfluoride products. Raman spectra for the tungsten fluorides
and a preliminary report of their electrochemical behavior in
the FLINAK eutectic are presented below.
Experimental Section
General Information. All manipulations of purified chemicals and
reaction products were performed in an inert atmosphere glovebox
(VAC, model HE 43-2) filled with purified nitrogen and operated at
<2 ppm of H2O. KF and NaF (Fluka, >99%) were first dried at 300
°C overnight under vacuum and then recrystallized from the molten
state in a glassy carbon crucible. Infrared spectra of the purified salts
did not contain bands in the 3550-3700 cm-1 region that could be
assigned to hydroxide ion. RbF(Alfa/Aesar, 99.98%), CsF(Alfa/Aesar,
99.9%), and Na2O (Ventron, Alfa division, 98%) were used without
further purification except that the former materials were dried in situ
under vacuum at 600 °C for 5 h before addition of the WF6 in the
synthesis procedures. WF6 gas (Cerac, 99.9%) and tungsten metal
powder (Aesar, 99.0%, 100 and 300 mesh) were used without further
purification. Elemental analysis for K was performed by E&R Mi-
croanalytical Laboratory, Inc. W was analyzed by inductively coupled
plasma atomic absorption spectroscopy (ICP AAS) using Na2WO4‚
2H2O in a NaOH solution as a standard.
In the course of the investigation of the tungsten electrodepo-
sition process in FLINAK, the syntheses of several alkali salts
‡ Deceased, March 11, 1995.
(1) Mellors, G. W.; Senderoff, S.; Canadian Patent No. 688546, 1964.
(2) Robin, A.; Ribeiro, R. B. J. Appl. Electrochem. 2000, 30, 239-246.
(3) Grojtheim, K.; Krohn, C.; Malinovsky, M.; Matiasovsky, K.; Thonstad,
J. Aluminum Electrolysis, 2nd ed.; Aluminum-Verlag: Dusseldorg,
1982.
(4) Lane, J. A.; MacPherson, H. G.; Maslan, F. Fluid Fuel Reactors;
Addison-Wesley Publishing Co., Inc.: Reading, MA, 1958.
(5) Broc, M. Etude de revetements metalliques prepares par electrolyse
ignee (A Study of Metallic Platings Prepared by Fused Salt Electroly-
sis). Ph.D. Thesis, Universite´ Pierre et Marie Curie, Paris, 1977. See
also Central European Agency Report, No. 4921, 1978.
(6) Senderoff, S.; Mellors, G. W. J. Electrochem. Soc. 1965, 112, 841-
843.
The FLINAK eutectic mixture, which was supplied by L. M. Toth
(ORNL), was purified by bubbling HF through the molten material at
550 °C for 3-4 days, followed by an H2/He sparge to remove excess
(9) Burgess, J.; Peacock, R. D. J. Fluorine Chem. 1977, 10, 479-486.
(10) Beuter, A.; Kuhlman, W.; Sawodny, W. J. Fluorine Chem. 1975, 6,
367-368.
(11) Hargreaves, G. B.; Peacock, R. D. J. Chem. Soc. 1957, 4212-4214.
(12) Hargreaves, G. B.; Peacock, R. D. J. Chem. Soc. 1958, 2170-2175.
(13) Moock, K. H.; Rock, M. H. J. Chem. Soc., Dalton Trans. 1993, 2459-
2463.
(14) Sunder, W. A.; Wayda, A. L.; Distefano, D.; Falconer, W. E.; Griffiths,
J. E. J. Fluorine Chem. 1979, 14, 299-325.
(15) Adam, S.; Ellern, A.; Seppelt, K. Chem.sEur. J. 1996, 2, 398-402.
(7) Baraboshkin, A. N.; Saltykova, N. A.; Semenov, B. G. Tr. Inst.
Elektrochim., Ural’. Nauchn. Tsentr, Akad. Nauk SSSR 1976, 24, 28-
31.
(8) Butskii, V. D.; Pervov, V. S.; Sevast’yanov, V. G. Russ. J. Inorg.
Chem. 1977, 22, 771-773.
10.1021/ic000491o CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/12/2001

716 Inorganic Chemistry, Vol. 40, No. 4, 2001
Eklund et al.
and 5/8 to 1/4 in.) and cone-shaped Teflon bushings to provide vacuum-
tight connections as well as electrical insulation of the electrodes from
the cell. The length of the tubes was sufficient to minimize the heat
conduction from the cell during operation, which prevented melting of
the Teflon bushings. Two Monel diaphragm valves (Hoke) were silver-
1
soldered to /4 in. Monel tubing, which was in turn welded to the cell
top flange in order to allow gas inlet/outlet flow and regulate the
pressure/evacuation of the cell. All connections to the vacuum/pressure
system were made with Monel or SS Swagelock fittings and Monel
tubing (1/4 in.). The top of the cell usually remained cool enough that
additional cooling was not necessary to prevent overheating and seizing
of the bolts. The system was pressurized with helium gas at all times
except when a vacuum was applied to remove residual hydrogen
fluoride gas dissolved in the melt.
Working electrodes for the electrochemical measurements were fab-
ricated by attaching either platinum wire (Aesar, 0.5 mm diam, 99.95%),
platinum foil (Aesar, 0.05 and 0.1 mm, 99.95%), tungsten wire (Aesar,
1
1.0 mm diam, 99.98%), or polished Ni plate to the end of a /8 in.
nickel rod, which provided exterior electrical connection when inserted
through the Teflon bushing. The connections were made by crimping
the electrodes into a V-shaped cut in the Ni rod. Immersion depth in
the molten FLINAK was determined by measuring the resistance
between the glassy carbon crucible and the electrode as the latter was
slowly lowered into the cell to determine the point of contact with the
melt; upon contact with the melt the resistance dropped to ca. 1.2 Ω.
After the electrodes were removed from the melt, the distance was con-
firmed by measurement of the discoloration on the electrode. The elec-
trodes were generally 2 cm in length or more to avoid contact of the
melt with the nickel support rod. Either a glassy carbon rod (Sigridur,
grade K, 1 mm diam) or the crucible was the counter electrode. When
the glassy carbon crucible was used as the counter electrode, electrical
connection was made through the main body of the cell.
Reference electrodes either were platinum foil connected to a nickel
rod or were constructed from boron nitride (BN, Advanced Ceramics
Corporation and Union Carbide, 1/2 in. diam) as described by Jenkins.17
However, it was found that BN often gave unreliable potential
measurements when tungsten fluorides were added to the melt. This
was most likely due to the binder used to hold the BN material together,
which was an oxide-containing ceramic. Since the tungsten fluorides
were sensitive to the presence of oxide impurities, the BN reference
electrodes were subsequently only used to establish the viability of the
electrochemical cell apparatus by observing known potentials, such as
the Ni2+/Ni couple, and to make reasonable estimates of the metal
impurities in the melt before and after electrolysis at a nickel electrode.
X-ray Crystallography. The X-ray crystal structures were deter-
mined using a Siemens R3m/V diffractometer fitted with a Nicolet LT-2
low-temperature device. The intensity data were measured with gra-
phite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The
SHELXTL 93 (versions 5.0 and 5.10) software packages were used to
solve (direct methods) and refine the crystal structures. Low-temperature
X-ray data were collected on a crystal of K3WF6 mounted in paratone
oil (Exxon). Crystals of K2WF7 were observed to shatter at approx-
imately -25 °C, and thus, room-temperature data were obtained on a
crystal encased in epoxy. Although both tungsten salts have large X-ray
absorption cross sections, it was not possible to perform analytical
corrections to diffraction data based on indexed faces. In the case of
K3WF6, the low-temperature transfer arms attached to the ø circle pre-
vented orientation of the crystal for face indexing. All attempts to correct
for absorption based on faces that were indexed and best estimates of
the indices for other faces were not successful. In the case of K2WF7,
it was very difficult to find a suitable crystal for diffraction studies of
this extremely sensitive complex. The crystal chosen was very irregu-
larly shaped, and no analytical absorption model based on face indexing
could be found before the crystal decomposed. Therefore, in both cases
correction for absorption was made using empirical scan data.
Figure 1. Reaction apparatus for tungsten compounds: (A) Monel
gauge for vacuum/pressure; (B) He-N2 vacuum/pressure inlet; (C)
Monel tubing; (D) WF6 gas cylinder; (E) Monel valves; (F) Inconel
cell; (G) glassy carbon crucible.
HF and reduce metal ion impurities. This produced FLINAK containing
<12 ppm oxide, as determined by the apparatus described previously.16
Prior to electrochemical measurements, the FLINAK melt was further
purified by electrolysis at a Ni plate cathode with an applied potential
of -1.5 to -1.8 V. The glassy carbon crucible containing the solution
acted as the counter electrode. This procedure generally produced
smooth voltammograms with background currents at least 2 orders of
magnitude less than the currents due to the tungsten fluorides. When
the background currents were large enough to cause interference and
could not be removed, the FLINAK was discarded. The observed redox
potentials (vs a BN reference electrode containing the Ni2+/Ni couple)
indicated that iron (Fe3+/Fe2+, E0 ) -0.2 V; Fe2+/Fe, E0 ) -0.39 V),
chromium (Cr2+/Cr, E0 ) -0.681 V), and sometimes nickel ions (E0
) 0.0 V) might be the major sources of contamination.
Reaction/Electrochemical Cell. The nickel/stainless steel reaction
cell (ca. 90 mL volume) was made from Inconel pipe to which was
welded an Inconel plate to form the cell bottom (Figure 1). Stainless
steel (SS) knife-edge flanges (Huntington) were used to cap the cell,
and copper or nickel-plated copper gaskets were used to seal the cell
at the knife edges. Hoke Inconel diaphragm valves in either straight or
tee patterns were used throughout the system except for the WF6
cylinder valve that was supplied by the vendor. The vacuum/pressure
gauge (Ashcroft, 30 in. Hg to 30 psi) was also Monel, which resisted
corrosion by the WF6 gas and byproducts such as HF. An Inconel
sheathed chromel-alumel thermocouple was placed on the external
face of the bottom of the cell to measure the temperature. The interior
cell temperature was assumed to be only 2-3 °C lower than the exterior
cell reading, since this temperature difference was found with similar
cells in which a thermocouple was inserted directly into the melt.
The crucibles used to contain the reactants at the higher temperatures
were in all cases made from glassy carbon (Sigridur G, obtained from
EMC Industries, Flemington, NJ) and were from 30 to 50 mL in
volume. Helium gas was used to pressurize the system except when
the reaction was performed without release of the pressure, in which
case the cell contained nitrogen from the inert atmosphere glovebox.
All connections were made by either silver-soldering or using Monel
Swagelock unions and connectors with Monel ferrules, except where
Teflon tubing was used as mentioned in the synthesis procedures.
The cell used for the electrochemical measurements was of similar
design except that the volume was ca. 300 mL and that larger glassy
carbon crucibles were used. Stainless steel tubes were welded into the
top flange to allow the insertion and removal of electrodes, the
measurement of temperature, and the addition of chemical samples.
These tubes were capped with Swagelock reducing unions (1 to 1/4 in.
Difference Fourier maps for K2WF7 reproducibly showed two areas
of low electron density perpendicular to the plane defined by W, F(2),
and its symmetry equivalent (Figure 4) in the structure. Thermal
(16) Eklund, S. E.; Toth, L. M.; Chambers, J. Q.; Mamantov, G. Anal.
Chem. 1999, 71, 539-543.
(17) Jenkins, H. W., Jr. Electrochemical Measurements in Molten Fluorides.
Ph.D. Thesis, The University of Tennessee, Knoxville, TN, 1969.

Tungsten Fluorides
Inorganic Chemistry, Vol. 40, No. 4, 2001 717
Table 2. Raman Bands for Compounds of This Study
Table 1. Syntheses of Alkali Metal Tungsten Fluorides
product
reactants (amount)
conditions
yield (%)
compound
frequencies (cm-1
)
K2WF8
KWF7
K2WF7
Rb2WF7
KWF6
NaWF6
RbWF6
CsWF6
K3WF8
Na3WF8
Rb3WF8
K3WF6
Na3WF6
667 (strong), 419, 389
K2WF8 KF (5.0 g, 86 mmol)
WF6 gas
870 °C (1 h)
g97
716 (strong), 665, 615, 535, 435, 327, 220 (weak)
642 (strong), 398 (weak), 250 (broad)
637 (strong)
KWF7
KF (5.0 g, 86 mmol)
WF6 gas
750 °C (2-3 days)
850 °C (30 min)
>60
g97
702 (strong), 225 (broad, weak)
712 (strong)
696 (strong), 212 (weak)
692 (strong), 209 (weak)
KWF6
KF (7.293 g, 25.46 mmol)
W (4.680 g, 125.5 mmol)
WF6 gas
NaWF6 NaF (4.6 g, 110 mmol)
W (3.752 g, 20.41 mmol)
WF6 gas
RbWF6 RbF (5.21 g, 49.9 mmol)
W (5.21 g, 28.3 mmol)
1050 °C (1 h)
850 °C (2 h)
g98
598 (strong), 435 (weak), 362
614 (strong), 411 (weak), 372 (weak)
595 (strong), 428 (weak), 335 (weak)
565 (strong), 198 (weak)
800 °C (2 h)
750 °C (2 h)
g96
g94
585 (strong)
CsWF6 CsF (6.03 g, 39.7 mmol)
W (3.21 g, 17.4 mmol)
Conditions are given in Table 1 and elsewhere.18 These products, which
were yellow, deomposed slowly with trace moisture in the glovebox
to a dark-blue material indicative of the well-known “tungsten blues”,
such as W2O5 and W4O11. The yellow material did not discolor when
it was carefully stored in sealed glass vials. In addition to the KWF6,
a small amount of K2WF8 (identified by Raman spectrum) condensed
on the cell top. The Raman spectra (Figure S2 of Supporting
Information, Table 2) of these compounds exhibited strong V1 bands
in the region of 690-700 cm-1 and much weaker V5 bands at 210-
225 cm-1, in agreement with the literature.19,20 Weak bands around
990 cm-1 in the spectra are attributed to oxide impurities.21
WF6 gas
K3WF8 KF (4.258 g, 73.29 mmol)
KWF6 (12.324 g, 36.58 mmol) 800 °C (30 min)
Na3WF8 NaF (12.43 g, 296.0 mmol) 1000 °C (30 min)
870 °C (15 min)
a
<50
NaWF6 (23.63 g, 73.65 mmol)
W (9.44 g, 51.34 mmol)
a A yield of K3WF8 was not calculated because the isolated material
contained K2WF7, as indicated by the Raman spectra.
ellipsoids for F(2) were severely elongated in the direction of these
positions when they were not incorporated into the model. A disorder
model involving a second pair of fluoride ligands (F(5) and F(6)) at
the sites of low electron density was developed. Refinement of the
fractional occupancy between these two sets of sites (taking into account
special position constraints on site occupation) gave rise to a 0.75/
0.25 split. In the disorder model, the thermal ellipsoid parameters for
F(2), F(5), and F(6), although still large, were more reasonable than
when the disorder was not accounted for in the model. More
complicated disorder models did not improve the figures of merit for
the overall structure. All residuals and goodness-of-fit parameters
improved slightly in the disorder model describe above.
Raman Spectroscopy. All Raman spectra were recorded with a Dilor
XY CCD detector multichannel spectrometer using the 514 nm line of
an Ar ion laser as an excitation source and a laser power between 5
and 100 mW. The samples were sealed in 3 mm square Pyrex tubing
(Vitro Dynamics) under vacuum. By careful focusing through a Spex
microscope fitted to the Dilor instrument using 10×, 80×, and 100×
objectives, the broad background signals at 800 and 450 cm-1 due to
the glass could be minimized.
Other Instrumentation. The electrochemical instrument used to
acquire and record the electrochemical data was an EG&G Princeton
Applied Research (PAR) model 273 potentiostat/galvanostat controlled
by the PAR M270 Electrochemical Research Software. XPS spectra
were obtained using a Perkin-Elmer PHI5000 instrument, which was
calibrated using gold and copper standards.
Synthesis of Potassium Octafluorotungstate(VI), K2WF8, and
Potassium Heptafluorotungstate(VI), KWF7. These compounds were
prepared by the reaction of molten KF with WF6 gas. Reaction
conditions are given in Table 1, and further details can be found
elsewhere.18 The initial rate of reaction was fast because the pressure
in the system dropped to 0.0 in. of Hg in less than 2 s when the WF6
cylinder was closed. The Raman spectra (Figure S1 of Supporting
Information and Table 2) of the products were in agreement with the
data of Beuter et al.10 A slight pink coloration, which was evident on
occasion, was taken to be indicative of an oxide contamination.
Synthesis of Potassium, Sodium, Rubidium, and Cesium Hexaflu-
orotungstate(V), MWF6 (M ) K, Na, Rb, Cs). This series of
compounds was prepared by the reaction of WF6 with metallic tungsten
in a limiting amount of molten MF according to the following reaction:
Synthesis of Potassium Heptafluorotungstate(V), K2WF7. This
compound, which was obtained by the addition reaction of KWF6 and
KF, has not been synthesized previously and represents the first MF7
2-
complex of the group VI transition metals. In a typical procedure, 2.032
g (34.97 mmol) of KF and 12.45 g (36.95 mmol) of KWF6 were added
to the reaction cell, which was heated under vacuum at 850 °C for 1
h. The temperature was then decreased to 800 °C, while the vacuum
was maintained for approximately 14 h. Then the temperature was
decreased to 600 °C over 8 h, followed by rapid cooling to room
temperature. Orange crystals (density 3.87 g/cm3) were isolated from
the solidified reaction mixture in approximately 40-50% yield. Anal.
Calcd for K2WF7: K, 19.79; W, 46.54. Found: K, 20.05; W, 47.55.
The Raman spectrum of the orange K2WF7 crystals is shown in
Figure 2 along with the Raman spectra of Rb2WF7 and K2TaF7 for
comparison. The principal band appears at 642 cm-1; weaker bands at
398 and 250 cm-1 can be assigned to W-F bending modes by analogy
to the K2TaF7 spectrum.22 Suitable crystals for an X-ray structure
determination were mounted at room temperature on a glass fiber after
coating them with epoxy glue. The crystal data for K2WF7 are collected
in Tables 3-5.
When the synthesis of Rb2WF7 was carried out at 800 °C in the
same manner as above, a light-pink crystalline material was obtained
that appeared to be a mixture of compounds. In addition to the pink
material, a green material (possibly Rb3WF8; see below) along with
some orange-brown material was formed in a pocket of the product
boule. The Raman spectrum of the pink material featured a prominent
band at 637 cm-1, which is in the range expected for Rb2WF7(vide
infra), along with a significant oxide band at 973 cm-1
.
Synthesis of Potassium, Sodium, and Rubidium Octafluorotung-
state(V), M3WF8 (M ) K, Na, Rb). These compounds were obtained
by the addition reaction of molten MF and MWF6 (see Table 1) via
the following reaction:
MWF6 + 2MF a M3WF8
(2)
The isolated potassium salt of the eight-coordinate tungstate(V) fluoride
contained some seven-coordinate WF72- ions, as indicated by the Raman
spectra, and the Rb3WF8 was obtained as a byproduct (yield, ca. 5%)
of the Rb2WF7 procedure described above.
6MF + W + 5WF6 f 6MWF6
(1)
(19) Anderson, G. M.; Iqbal, J.; Sharp, D. W. A.; Winfield, J. M.; Cameron,
J. H.; McLeod, A. G. J. Fluorine Chem. 1984, 24, 303-317.
(20) Shamir, J.; Malm, J. G. J. Inorg. Nucl. Chem. Suppl. 1976, 107.
(21) Beuter, A.; Sawodny, W. Z. Anorg. Allg. Chem. 1976, 424, 37-44.
(22) Torardi, C. C.; Brixner, L. H.; Blasse, G. J. Solid State Chem. 1987,
67, 21-25.
(18) Eklund, S. E. Tungsten fluorides: synthesis and characterization, and
electrochemical investigations in the FLINAK molten salt eutectic.
Ph.D. Thesis, University of Tennessee, Knoxville, TN, 2000.

718 Inorganic Chemistry, Vol. 40, No. 4, 2001
Eklund et al.
Table 4. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2 × 103) for K2WF7
a
x
y
z
U(eq)
W(1)
K(1)
K(2)
F(1)
F(4)
F(3)
F(2)
F(5)
F(6)
7544(1)
2500
3709(1)
5603(4)
6684(5)
3700(9)
4551(13)
5253(16)
2593(15)
2170(50)
3130(50)
23(1)
30(1)
8637(5)
7500
5519(6)
2500
44(1)
9003(11)
6476(14)
8270(30)
6890(30)
8280(80)
5680(80)
220(15)
260(20)
2500
57(3)
103(5)
119(9)
154(18)
130(40)
130(40)
420(30)
2500
2500
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
Table 5. Selected Bond Lengths [Å] and Angles [deg] for K2WF7
Bond Distances
W(1)-F(1)
W(1)-F(2)
W(1)-F(3)
W(1)-F(4)
W(1)-F(5)
W(1)-F(6)
W(1)-K(1)
W(1)-K(1)#1
W(1)-K(1)#2
K(1)-F(5)#3
K(1)-F(2)#3
K(1)-F(1)#4
K(1)-F(1)#5
1.937(10)
1.882(16)
1.943(19)
1.929(12)
1.94(6)
K(1)-F(4)#4
2.917(16)
2.920(4)
3.03(6)
K(1)-F(3)
K(1)-F(6)#3
K(2)-F(1)#6
K(2)-F(4)#7
K(2)-F(4)
2.870(11)
2.902(16)
2.965(16)
2.62(7)
1.95(7)
3.782(3)
3.782(3)
3.820(5)
2.62(7)
2.670(16)
2.747(10)
2.780(12)
K(1)-F(5)#3
K(2)-F(2)#7
K(2)-F(6)#11
K(2)-F(5)#3
F(3)-K(1)#5
F(2)-K(2)#11
F(2)-K(2)#13
3.02(3)
3.11(3)
3.15(3)
3.20(3)
3.02(3)
3.22(3)
Angles
78.6(10) F(1)#4-W(1)-F(1)
75.5(9) F(2)-W(1)-F(3)
126.4(11) F(4)#4-W(1)-F(3)
F(2)-W(1)-F(2)#4
F(2)-W(1)-F(4)
84.9(6)
140.5(5)
73.8(8)
F(2)-W(1)-F(4)#4
F(2)#4-W(1)-F(4)#4
F(4)-W(1)-F(4)#4
F(2)-W(1)-F(1)#4
F(2)#4-W(1)-F(1)#4
F(4)-W(1)-F(1)#4
F(4)#4-W(1)-F(1)#4
75.5(9)
83.6(8)
132.4(9)
79.6(9)
148.5(6)
87.3(5)
F(1)#4-W(1)-F(3)
F(2)#4-W(1)-F(5)
F(4)-W(1)-F(5)
F(3)-W(1)-F(5)
F(2)-W(1)-F(6)
74.7(7)
58.6(16)
132.5(10)
137(2)
55.9(14)
Figure 2. Raman spectra of Rb2WF7, K2WF7, and K2TaF7.
WF6 below). The Raman spectrum (Figure S3), which indicated the
salt to be quite pure, was strikingly similar to that of Na3TaF8.23
The material that gave the Raman spectrum in Figure S3, assigned
to the Rb3WF8 compound, was isolated as a byproduct of the preparation
of Rb2WF7 in less than 5% yield. The isolated material contained a
significant fraction of seven-coordinate WF72- and a significant oxide
Table 3. Crystal Data and Structure Refinement for K2WF7 and
K3WF6
K2WF7
395.05
9.800(2)
5.7360(11)
11.723(2)
90
K3WF6
415.15
8.9160(10)
8.9160(10)
8.9160(10)
90
fw
impurity indicated by the presence of a band at 956 cm-1
.
a (Å)
b (Å)
c (Å)
R ) â ) γ (deg)
V (Å3)
Z
Synthesis of Potassium Hexafluorotungstate(III) Fluoride, K3WF6.
The synthesis of this compound was attempted by reduction of K3WF8
with tungsten metal via the following reaction:
659.0(2)
4
708.78(14)
4
space group
λ (Å)
Pnma (No. 62)
0.710 73
293(2)
Fm3h (No. 225)
0.710 73
173(2)
3K3WF8 + 2W + 6KF f 5K3WF6
(3)
temp (K)
calc (Mg/m3)
3.982
3.890
Although the bulk of the isolated material was a mixture of K3WF8
and K3WF6, a small amount of a crystalline product, suitable for X-ray
structure determination, was isolated and identified as K3WF6. Two
procedures were followed: in one the reaction mixture was under
vacuum, and in the other the pressure was allowed to build up. In the
former, 5.03 g (11.1 mmol) of K3WF8, 1.32 g (22.7 mmol) of KF, and
4.57 g (24.9 mmol) of W was heated to 810 °C for approximately 16
h. The bulk of the product isolated from the cell at room temperature
was a dark-brown solid that was shown by its Raman spectrum to be
close to a 50-50 mixture of K3WF8 and some other compound. Above
the main boule of brown material, at the ends of KF dendrites that had
condensed from the vapor state, translucent green crystals had formed
(yield, <1%). This compound was shown to be K3WF6 by its X-ray
crystal structure. The crystal data and summaries of atomic coordinates,
bond lengths, and bond angles are given in Tables 3, 6, and 7.
In the second procedure, a mixture of 14.80 g (32.7 mmol) of K3-
WF8, 5.268 g (90.67 mmol) of KF, and 5.20 g (28.3 mmol) of tungsten
powder was heated to 800 °C for 3 days without relieving the pressure
buildup. When the reaction vessel was opened after slow cooling to
room temperature, three distinct layers were present. The bottom two
F
µ (cm-1
)
18.854
18.093
R a (Fo ; all data)
0.0539
0.0435
2
Rw2 a (Fo ; all data)
0.0964b,d
0.1304c,d
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. Rw2 ) [∑w(|Fo| - |Fc|)2/∑w|Fo| ]1/2
.
b w
2
) 1/[σ2(Fo ) + (0.0277P)2 + 17.23P]. c w ) 1/[σ2(Fo ) + (0.0797P)2
2
2
+ 12.15P]. d P ) (Fo + 2Fc2)/3.
2
These compounds, which were green, exhibited a principal strong
Raman band in the region 595-614 cm-1 and two weaker bands at
411-435 and 335-372 cm-1 (Figure S3 of Supporting Information
and Table 2). The Raman spectrum of the K3WF8 contained a band at
642 cm-1, with an intensity of approximately 20% of the 598 cm-1
band, at the same frequency as the V1 band for the WF72- species (Figure
2). This latter band was not present in the room-temperature spectrum
of frozen FLINAK containing the green solid, indicating the presence
2-
of WF8 alone in the presence of excess MF.
The salt Na3WF8 was actually isolated from a reaction mixture
containing metallic tunsgten powder and excess NaF in an attempt to
prepare lower oxidation state tungsten fluorides (see synthesis of K3-

Tungsten Fluorides
Inorganic Chemistry, Vol. 40, No. 4, 2001 719
Table 6. Atomic Coordinates (×104) and Equivalent Isotropic
Displacement Parameters (Å2 × 103) for K3WF6
a
x
y
z
U(eq)
K(2)
W
K(1)
F(1)
5000
10000
5000
7500
5000
0
0
71(6)
48(2)
5000
7500
2970(40)
2500
0
83(5)
250(30)
a U(eq) is defined as one-third of the trace of the orthogonalized Uij
tensor.
Table 7. Selected Bond Lengths [Å] and Angles [deg] for K3WF6
Bond Distances
K(2)-F(1)#1
K(2)-K(1)
W-F(1)
2.65(3)
W-K(1)
3.8607(4)
3.180(4)
2.65(3)
3.8607(4)
1.81(3)
K(1)-F(1)#2
F(1)-K(2)#3
Bond Angles
F(1)#4-K(2)-F(1)#1
90.0
F(1)#10-W-F(1)
F(1)#11-W-F(1)
F(1)#10-W-K(1)
180.0
90.0
K(1)#5-K(2)-K(1)#6 180.0
F(1)#1-K(2)-K(1)#7
F(1)#8-K(2)-K(1)
K(1)#6-K(2)-K(1)
K(1)#9-K(2)-K(1)
54.7
54.7
54.736(2) F(1)-W-K(1)
109.5
70.5
125.3
54.7
F(1)-W-K(1)#10
F(1)#11-W-K(1)#12 125.264(1)
layers were brown, the bottom darker than the middle layer, and the
top layer was a cloudy white with a greenish tint (possibly phase-
separated KF and green K3WF8). Unreacted shiny, flat tungsten metal
layers (identified by XPS spectra) were embedded throughout the brown
material, which indicated the disproportionation of an intermediate to
tungsten metal and a higher oxidation state.
In a similar procedure, 7.17 g of a dark-brown product was obtained
when 5.30 g (13.1 mmol) of Na3WF8 and 3.10 g (16.9 mmol) of NaF
were heated at 850 °C for 5 h. The major loss of material was due to
a condensate of NaWF6 (identified by its Raman spectrum) that had
formed on the cell top consistent with the following reaction:
Figure 3. Raman spectra of Na3WF8/Na3WF6 and K3WF8/K3WF6
mixtures: (O) K3WF8; (/) K3WF6.
6Na3WF8 + 2W f 5Na3WF6 + 3NaWF6
(4)
The Raman spectra of the brown products of these reactions (Figure
3) revealed the fingerprint bands for K3WF8 and Na3WF8 at 599 and
361 cm-1 and 615 and 375 cm-1, respectively. A second set of bands
associated with an unknown component can be seen at 565 and 198
cm-1 for the potassium salt mixture and one band at 585 cm-1 for the
sodium salt mixture. In addition bands in the 960-980 cm-1 region,
which were always present in the spectra of the brown material,
indicated a minor oxide impurity. As discussed below, the unknown
component of these mixtures is likely to be M3WF6. An X-ray powder
diffraction pattern of the brown material showed a slight splitting of
the peaks, indicating the presence of two distinct unit cells, the larger
agreeing with the cell size of K3WF8.
Results and Discussion
Crystal Structures. ORTEP drawings of the structures of
the two new potassium tungsten fluorides prepared in this study
are shown in Figures 4 and 5. In the K2WF7 structure, the
tungsten atom is surrounded by an array of seven fluoride
ligands in a distorted trigonal prism where one of the rectangular
faces is capped by the seventh fluoride ion. Two of the fluoride
ligands are unequally disordered between two sets of positions
{F(2)/F(2′) and F(5)/F(6)} around the tungsten and exhibit large
thermal parameters (see Experimental Section). The seven W-F
bond lengths are in the range 1.88(2)-1.94(2) Å. Each WF7
unit is accompanied by two crystallographically nonequivalent
11-coordinate K+ ions, with K-F distances ranging from 2.67-
Figure 4. ORTEP diagrams for K2WF7 and K3WF6. Thermal ellipsoids
represent 50% probabilities. In the structure of K2WF7, F(2)/F(2′)
(major) and F(5)/F(6) (minor) constitute a set of two disordered fluoride
ligands around the tungsten. See Experimental Section for details
concerning the structural models and refinement results.
was based on the coordinates of the molybdenum species. The
fluorine anisotropic temperature factors normal to the W-F
bond are abnormally large (Table S5 of Supporting Information).
Similar behavior has been observed in the structures of
isomorphous analogues to K3WF6. The possibility that such large
22
(2) to 3.22(3) Å. This is in contrast to the K2TaF7 and K2-
NbF724 structures, both of which contain 9-coordinate potassium
ions.
The structure of K3WF6 is isomorphous with that of the
molybdenum analogue K3MoF6 previously reported by Toth et
al.;25 the initial model for refinement of the K3WF6 structure
(23) Hartman, K. O.; Miller, F. A. Spectrochim. Acta A 1968, 24, 669.

720 Inorganic Chemistry, Vol. 40, No. 4, 2001
Eklund et al.
Figure 5. ORTEP diagram for K3WF6. Thermal ellipsoids represent
50% probability.
thermal parameters represent a random spatial disorder of the
fluorine around the special position it resides on has been
discussed.25 As observed previously, attempts to model such
disorder were unsuccessful in the case at hand, and the simplest
model for the system was chosen.
Figure 6. Main band frequency shift with change in oxidation state
and fluoride coordination number.
The tungsten atom coordination is a perfect octahedron with
1.81(3) Å W-F bond distances. The unit cell contains alternat-
ing WF6 and KF6 octahedrons in a NaCl-like arrangement. The
K-F bond distances are 2.65(3) Å, and there are an additional
eight K+ ions per unit cell, which are coordinated to 12 F-
ions in a distorted cubic octahedral array with K-F distances
of 3.8607(4) Å.
Raman Spectra. The Raman spectra of the tungsten fluorides
prepared in this study (Figures 2, 3, and S1-S3) feature a
prominent, intense band in the region 565-715 cm-1 that can
be assigned to a symmetric stretching mode of the complex ion.
For the six-coordinate MWF6 compounds this band can be
readily assigned to the V1(A1g) breathing mode of an octahedral
complex. In addition, a much weaker V5(F2g) scissoring mode
was present in the spectra in the 200 cm-1 region. Polarized
Raman spectra (not shown) indicated that this band was buried
under the broad band seen in the 200-225 cm-1 in the KWF6
spectrum of Figure S2. The V2(Eg) vibration, which has been
reported at 470 cm-1 for the gas-phase spectrum of WF6-,9 was
not seen presumably because of suppression by solid-state
effects.
Identification of the unknown components of the solid-state
mixtures that gave the spectra of Figure 3 as M3WF6 is
supported by the correlations seen in Figures 6 and 7. In Figure
6, the correlation among coordination number, oxidation state,
and the frequency of the principal stretching mode of the
complex is seen. Figure 7 shows that the V1, V2, and V5 bands
for the presumed K3WF6 compound are in complete accord with
the frequencies of the bands for known WF6 and KWF6
compounds. This strongly supports our assignment of M3WF6
(M ) Na, K) to the unknown component of the brown product
mixtures.
Figure 7. Relationship of main band Raman frequencies for octahe-
drally coordinated tungsten fluorides: (b) V1 band; (O) V2 band; (1)
V5 band.
plexes, which produced the most clearly defined electrochemical
responses. Since all of the voltammetry in FLINAK was plagued
by the presence of oxides, the effect of added Na2O was also
determined.
Raman spectra of all of the W(V) species in FLINAK were
nearly identical, showing the main band at 598 cm-1 for the
3-
2-
WF8 species. The band for WF7 at 637 cm-1 was barely
-
discernible, there was no evidence for the WF6 species, and
all three W(V) compounds (K3WF8, K2WF7, and KWF6) gave
similar cyclic voltammetry. These observations indicate that
3-
WF8 is the principal W(V) species present in the FLINAK
melt.
The voltammogram of Figure 8 exhibits at least two reduction
waves (R1 and R2) and a sharp W metal anodic stripping wave
in the region 0.3-0.4 V vs the quasi-Pt reference. The
voltammetry of the tungsten fluorides in FLINAK is remarkably
similar to previously reported voltammograms for other refrac-
tory metals in this medium.26-29 Assignment of the anodic
process in Figure 8 to oxidation of electrodeposited tungsten is
consistent with these previous studies and with the voltammetric
corrosion of a tungsten wire electrode depicted in Figure S4 of
Supporting Information. Besides showing that the anodic
dissolution of W occurs in the 0.4 V region, the voltammogram
of Figure S4 also provides a rough calibration of the Pt quasi-
reference electrode because the Li+/Li metal couple is seen at
-1.8 V.
Electrochemistry. Cyclic voltammograms were obtained for
all of the above compounds in molten FLINAK at platinum
working electrodes. In addition Raman spectra were obtained
on the frozen FLINAK mixtures in order to ascertain the
principal species in the melt by comparison with the spectra of
the known compounds in Figures 2, 3, S1, and S2. Only the
potassium and sodium tungsten fluoride salts were used in these
studies so that no new cations were introduced into the melt.
Emphasis was placed on the voltammetry of the W(V) com-
(24) Brown, G. M.; Walker, L. A. Acta Crystallogr. 1966, 20, 220-229.
(25) Toth, L. M.; Brunton, G. D.; Smith, G. P. Inorg. Chem. 1969, 8, 2694-
2697.

Tungsten Fluorides
Inorganic Chemistry, Vol. 40, No. 4, 2001 721
Figure 9. Voltammograms showing increasing current with increasing
temperature: T ) 475, 535, 550, 575, 600, 625, 650, 675, 700, 725,
750, 775, and 800 °C; Apt ) 0.157 cm2; scan rate ) 100 mV/s; CW(V)
(at 500 °C) ) 5.20 × 10-5 M (0.0990 mol %). Inset: variation of
ln IpT 1/2 with T-1; Eact ) 58.9 ( 1.4 kJ/mol.
Figure 8. Cyclic voltammogram of K3WF8 in FLINAK, 540 °C: APt
) 0.157 cm2, scan rate ) 100 mV/s, Pt quasi-reference electrode, glassy
carbon crucible counter electrode.
When the negative potential sweep was restricted to the R1
wave, integration of the cathodic current in all instances gave
a cathodic charge equal to the anodic charge under the tungsten
metal stripping wave. This indicates that the R1 current is due
to a five-electron reduction to tungsten metal:
activation energy of diffusion equal to 58.9 ( 1.4 kJ/mol for
the data of Figure 9.
For the K3WF8/FLINAK system, temperatures between 600
and 700 °C produced cyclic voltammograms with the “cleanest”
anodic stripping pattern and minimal distortion of the R1 wave
by the R0 prewave. On this basis, this temperature range is
suggested for electrodeposition of tungsten on platinum.
Addition of the brown solid believed to be a mixture of K3-
WF8 and K3WF6 to FLINAK produced cyclic voltammograms
very much identical to those obtained with the pure green K3-
WF8 compound. This is consistent with the Raman spectra
obtained on FLINAK melts containing increasing amounts of
the brown solid (Figure 10). At low concentration the peaks
assigned to K3WF8 and K3WF6 were present in roughly equal
amounts, and as the concentration was increased, the K3WF8
peak is seen to dominate the K3WF6 peak. This indicates that
WF83- + 5e- f W + 8F-
(5)
A similar conclusion was reached by Polyakova et al. for the
reduction of K2TaF7 in FLINAK.27
Assignment of wave R2 in the voltammogram of Figure 8 to
reduction of an oxide species is supported by the voltammo-
grams obtained when Na2O was purposefully added to the melt.
As the oxide content was increased, wave R2 increased at the
expense of R1. This behavior also has precedence in the Ta(V)
FLINAK system.27 An example is shown in Figure S5 of
Supporting Information where the oxide/W ratio is 0.76. At
higher oxide concentration, R1 is difficult to distinguish from
the presumed tungsten oxide reduction current and the anodic
stripping wave is diminished and broadened considerably.
When the temperature of the WF83-/FLINAK melts was
increased, a prewave (R0) became evident at potentials positive
of the R1 wave (Figure 9). Concurrent with the growth of the
prewave, the anodic stripping wave became much more
structured with up to three peaks evident at potentials positive
of the main anodic peak. Formation of a W(Pt) alloy layer in
the electrodeposition process is a possible explanation for this
behavior. In this case, the overpotential barrier(s) that gives rise
to the stripping wave structure is due to the energy required to
break the Pt-W bonds. The inset in Figure 9 shows a plot of
ln(IpkT1/2) where Ipk is the maximum current and T is the absolute
temperature. If the temperature dependence of the current is
primarily due to diffusion of the tungsten fluoride complex, the
slope of this plot gives an estimate of the activation energy for
diffusion. For an irreversible reaction that obeys the Randles-
Sevcik equation,
3-
comproportionation of WF8 and metallic W occurred in the
3-
fluoride-rich melt to form WF6
:
3WF83- + 2W + 6F- a 5WF6
(7)
3-
and that a [WF63-]/[WF83-] equilibrium ratio close to unity
existed under these conditions. Accordingly, the potentials of
the W(V/III) and W(III/0) couples will be close to each other
in the melt. Note that excess W metal is likely to be present as
an impurity in the brown solid and that for the spectra of Figure
10 excess W metal was added on purpose. Also, the concentra-
tion dependence seen in Figure 10 is in accord with the law of
mass action for the above equilibrium if the fluoride activity is
constant in the FLINAK melt. This equilibrium would also
account for the noninteger oxidation states seen by previous
workers in the tungsten/FLINAK system.
The voltammetry of the brown K3WF8/K3WF6 material in
FLINAK again revealed the problem of oxide impurity. In the
absence of added Na2O, a poorly formed reduction wave was
observed with R1 and R2 components similar to those of Figures
8 and S5. Identical behavior was seen when the sodium salt
mixture was used. Upon addition of oxide to the melt, wave
R1 was suppressed and the anodic stripping wave broadened
and became less intense. At a 2:1 molar ratio of oxide/W, no
Faradaic current was seen in the region of 0.0 to -1.0 V,
indicating that the tungsten oxides are electroinactive at these
potentials in FLINAK.
IpkT1/2 ) 0.496nFACbV1/2(RnaF/R)1/2D1/2
(6)
the slope of the log plot is Eact/(2RT), which results in an
(26) Van, V.; Silny, A.; Hives, J.; Danek, V. Electrochem. Commun. 1999,
1, 295-300.
(27) Polyakova, L. P.; Polyakov, E. G.; Matthiensen, F.; Christensen, E.;
Bjerrum, N. J. J. Electrochem. Soc. 1994, 141, 2982-2988.
(28) Clayton, F. R.; Mamantov, G.; Manning, D. L. J. Electrochem. Soc.
1974, 121, 86-90.
Summary
As shown in this study, molten fluorides are a viable medium
for syntheses of refractory metal complexes and their alkali
(29) Taxil, P.; Mahenc, J. J. Appl. Electrochem. 1987, 17, 261-269.

722 Inorganic Chemistry, Vol. 40, No. 4, 2001
Eklund et al.
by the facile kinetics, high yields, and simple isolation proce-
dures that are realized. Care must be taken, however, to
minimize the level of oxide impurities, which affects both the
synthetic methods and the electrochemical behavior.
While the voltammetry of the tungsten electrodeposition
process in FLINAK suggested that reduction of W(V) proceeded
via an overall five-electron process, it is likely that a W(III)
species such as WF63- is formed as a two-electron intermediate
in this process. Since the latter species was indicated as an
equilibrium component of the K3WF8/W mixture in FLINAK
by the Raman spectra, it is also likely to be present in the
diffusion layer at a freshly deposited tungsten metal layer in
this same medium. Further studies, e.g., rotating ring-disk
voltammetry, are indicated in order to more fully understand
3-
the role of WF6 in the electrodeposition process.
Acknowledgment. This work was supported by a contract
with Lockheed Martin Energy Systems, Inc. and AFOSR for a
fellowship to Sven Eklund. Dr. L. M. Toth (ORNL) provided
much useful guidance on the high-temperature molten salt
chemistry. We also gratefully acknowledge several critical
insights provided by a reviewer and Dr. Jeff C. Bryan (ORNL)
in developing the model for the structure of K2WF7. Charmaine
Mamantov and Haiming Xiao are also thanked for many helpful
discussions.
Supporting Information Available: Listings of bond lengths and
angles, anisotropic displacement parameters, structure factors, and X-ray
crystallographic files in CIF format for the structure determinations of
K2WF7 and K3WF6; Raman spectra of K2WF8, KWF7, MWF6 (M )
Na, K, Rb and Cs), and M3WF8 (M ) Na, K and Rb); cyclic
voltammograms of (1) a tungsten wire and (2) K3WF8 upon addition
of oxide in FLINAK. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 10. Variation in K3WF8 (598 cm-1) and K3WF6 (565 cm-1
Raman band intensities in a solidified FLINAK melt with increasing
addition of WF6 in a bath containing excess W metal: (A) 1.12, (B)
4.21, and (C) 6.29 mol % WF6.
)
metal salts. While high-temperature conditions and inert atmo-
sphere manipulations are required, these difficulties are offset
IC000491O