Vaporization products of transition-metal and rare-earth complex fluorides studied by high-temperature mass spectrometry

Article abstract of DOI:10.1007/s10789-005-0310-y

Gaseous products of the thermal decomposition of Ni(IV), Tb(IV), Mn(IV), and Pt(IV) complex fluorides were studied by high-temperature mass spectrometry. The results demonstrate that the decomposition of potassium hexafluoronickelate and potassium heptafl

Full text of DOI:10.1007/s10789-005-0310-y

Inorganic Materials, Vol. 41, No. 12, 2005, pp. 1327–1333. Translated from Neorganicheskie Materialy, Vol. 41, No. 12, 2005, pp. 1502–1509.
Original Russian Text Copyright © 2005 by Leskiv, Abramov, Oleinik, Kepman, Sukhoverkhov, Mazej, Rau, Chilingarov, Sidorov.
Vaporization Products of Transition-Metal
and Rare-Earth Complex Fluorides
Studied by High-Temperature Mass Spectrometry
M. S. Leskiv*, S. V. Abramov*, L. I. Oleinik*, A. V. Kepman*, V. F. Sukhoverkhov**,
Z. Mazej***, D. V. Rau*, N. S. Chilingarov*, and L. N. Sidorov*
* Faculty of Chemistry, Moscow State University, Vorob’evy gory 1–3, Moscow, 119992 Russia
** Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia
*** Inorganic Chemistry and Technology Division, Joz^ ef Stefan Institute, Slovenian Academy of Sciences,
Jamova 39, 61111 Ljubljana, Slovenia
e-mail: nsc@phys.chem.msu.ru
Received January 31, 2005
Abstract—Gaseous products of the thermal decomposition of Ni(IV), Tb(IV), Mn(IV), and Pt(IV) complex
fluorides were studied by high-temperature mass spectrometry. The results demonstrate that the decomposition
of potassium hexafluoronickelate and potassium heptafluorotherbate is accompanied by atomic fluorine release
and that K2NiF6 decomposes in two steps, at 570–670 and 700–850 K. p(F) and p(F2) measurements indicate
that the reaction 2F = F2 does not reach equilibrium. Data on K3TbF7 vaporization suggest that the vapor phase
contains complex Tb(IV) molecules. Comparison of the decomposition temperatures of TbF4 and K3TbF7 indi-
cates that the complex fluoride is more stable. The thermolysis of cesium hexafluoromanganate and potassium
hexafluoroplatinate is accompanied by no fluorine release. Experimental data attest to the presence of Cs2MnF6
in the vapor phase. In addition, Cs2MnF6 decomposes to form Mn(III) complexes. K2PtF6 sublimes with
decomposition to form KF and Pt metal.
INTRODUCTION
Cs2MnF6(s), and potassium hexafluoroplatinate
K2PtF6(s) by mass spectrometry.
Higher fluorides of transition metals and rare earths
have long been used on a laboratory scale as fluorina-
tion agents. Among the available data on the physico-
chemical properties of such compounds, information
concerning their thermal stability is of special interest.
The point is that thermal stability was studied earlier for
the most part by thermogravimetry, a method which
allows one to establish the temperature range of decom-
position and to assess, very approximately, the compo-
sition of possible gaseous products from weight loss
data. Given that weight losses may be due to side pro-
cesses (e.g., hydrolysis) accompanying thermal decom-
position proper, it is clear that available data on the
composition of gaseous products of thermolysis need
careful verification.
EXPERIMENTAL
In our measurements, we used an MI-1201 single-
focusing magnetic mass spectrometer with an ion
source designed for high-temperature measurements.
The experimental procedure was described in detail
elsewhere [1]. Fluoride samples were evaporated from
prefluorinated nickel effusion cell (Deff = 0.3 mm). The
neutrals in the molecular beam were ionized by elec-
trons (Vi = 65–70V), and the forming positive ions were
focused, accelerated (Vacc = 3 kV), and then separated
in a uniform magnetic field according to the
mass/charge quotient. The ion currents were measured
on the collector using an electrometer (Rinput = 1012 Ω).
The “useful” signal was identified with the use of a
movable shutter situated between the effusion cell and
ionization chamber. From the fraction of the signal
intercepted by the shutter, we calculated the partial
pressures pj of vapor species by the formula
One effective tool for determining the qualitative
and quantitative compositions of the saturated vapor
over low-volatility inorganic compounds is high-tem-
perature mass spectrometry, which has been used for
this purpose for more than 40 years now. The objective
of this work was to analyze the gaseous thermolysis
products of several complex fluorides: potassium
pj = k(Qj)–1Σ(Iij)T,
(1)
hexafluoronickelate K2NiF6(s), potassium heptafluo- where k is the sensitivity factor, Qj is the total ionization
rotherbate K3TbF7(s), cesium hexafluoromanganate cross section of the jth molecular species, Σ(Iij) is the
0020-1685/05/4112-1327 © 2005 Pleiades Publishing, Inc.

1328
p(F), Pa
LESKIV et al.
XRD results, the sample consisted of potassium hep-
tafluorotherbate and a small amount of potassium
hexafluoronickelate, which was probably formed
because of the severe fluorination conditions. Cs2MnF6
was synthesized by fluorinating a mixture of CsF and
MnF2 with molecular fluorine in hydrofluoric acid as
described elsewhere [4]. K2PtF6 was synthesized by flu-
orinating K2PtCl6 with chlorine trifluoride and was then
characterized by XRD.
3.5
3.0
2.5
2.0
1.5
0.5
0
RESULTS AND DISCUSSION
Potassium
hexafluoronickelate,
K2NiF6(s).
According to our results, potassium hexafluoronicke-
late decomposes in two steps (Fig. 1), with atomic flu-
orine release in the ranges 570–670 (step I) and 700–
850 K (step II). Note that Ni(IV) fluoride is only stable
below –60°C. At higher temperatures, it decomposes to
form NiF3 and NiF2, thermodynamically more stable
compounds [5].
550
600 650
700 750
800
850
T, K
Fig. 1. Temperature variation of the atomic fluorine pres-
sure during K NiF decomposition.
2
6
According to XRD results, one of the K2NiF6(s)
decomposition products is nickel difluoride. The partial
pressures of the gaseous products of K2NiF6 decompo-
sition were calculated using the sensitivity factor eval-
sum of the ion currents arising from ionization of mol-
ecules j, and T is the absolute temperature.
To preclude contact with atmospheric moisture, the
sample was loaded into the effusion cell in a “dry”
chamber under a high-purity argon atmosphere
(≥99.998 vol %Ar, ≤2 × 10–4 vol % O2, ≤3 × 10–4 vol %
H2O). Nevertheless, in the initial stages of vaporization
the pressure in the source region increased at cell tem-
peratures in the range 330–450 K. As a result, the mass
uated from the measured NiF+n (n = 0–2) currents.
Table 1 presents the p(F) and p(F2) data for the two
steps of thermolysis, the present and earlier reported [6]
equilibrium constants Kp of the reaction
spectrum showed an increase in HF+ and O2+ signal
F2 = 2F,
(2)
intensities owing to HF and O2 release (gaseous prod-
ucts of partial hydrolysis). Usually, after the gas release
terminated the source region was evacuated for 5 h by a
mercury diffusion pump (50 hp, 1.5 × 10–4 Pa) at a cell
temperature at least 100 K below the decomposition
onset temperature of the fluoride. This procedure sig-
nificantly reduced the background at m/z = 19 (F+).
Next, the temperature was raised, and the mass spec-
trum was recorded over a broad range of mass numbers.
and the measured and “equilibrium” p(F)/p(F2) ratios.
Hereafter, the standard state of atomic and molecular
fluorine is taken to be the state of an ideal gas at a pres-
sure of 1 atm = 101325 Pa. The “equilibrium” pressures
of F and F2 were calculated under the assumption that
the reaction system (2) is at equilibrium:
K2p + 4pKp – Kp
-------------------------------------------
2
peq(F) =
,
(3)
(4)
In calculating the atomic fluorine partial pressure,
we took into account the fractions of the HF+, F2+ , and
p2eq(F)
Kp
F+ signals intercepted by the shutter. Using the mass
---------------
peq(F2) =
.
spectra of HF and F2 reported earlier [2], we calculated
the contributions of the F+ fragment ions originating
from these molecules to the ion current at m/z = 19. The
rest of the signal was regarded as the F+ current result-
ing from the ionization of F atoms.
The total pressure p in the system was taken to be equal
to the sum of the measured partial pressures of atomic
and molecular fluorine: p = p(F) + p(F2). As can be seen
from Table 1, the experimentally determined values of
Kp(2) and p(F)/p(F2) notably exceed the corresponding
equilibrium values, indicating that the reaction system
(2) is out of equilibrium.
Potassium hexafluoronickelate was prepared by flu-
orinating a mixture of KCl and NiCl2 (2 : 1 molar ratio)
with molecular fluorine. The sample was then charac-
terized by x-ray diffraction (XRD). Potassium heptaflu-
orotherbate was prepared by fluorinating a stoichiomet-
Potassium heptafluorotherbate, K3TbF7. Up to
750 K, we observed atomic and molecular fluorine
ric mixture of KCl and Tb4O7 with molecular fluorine release due to the thermal decomposition of K2NiF6
in a nickel reactor as described in [3]. According to impurities. To fully decompose the impurity phase, the
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VAPORIZATION PRODUCTS OF TRANSITION-METAL
1329
I, mV
120
39K+
41K+
F+
(I = 2 V) (I = 0.15 V)
100
80
60
40
20
0
HF+
K2F+
K++
F+2
NiF+
80
KF+ Ë Ni+
60
20
30
40
50
70
90
100
m/z
Fig. 2. Mass spectrum of saturated vapor over K TbF (s) during thermolysis at 993 K.
3
7
sample was heat-treated at 750 K until the F+ and
F+2 signal intensities dropped to the background level.
The temperature was then raised, and the mass spec-
trum taken at 873 K showed the F+, HF+, F2+ , and K+
signals. Starting at 933 K, the K2F+ ion was present.
The 993-K mass spectrum of the saturated vapor over
K3TbF7(s) is displayed in Fig. 2.
evaluated from the Kp of KF dimerization,
2KF(g) = K2F2(g),
(5)
expressed through ion currents using Eq. (1), with KF
and K2F2 as the molecular precursors of K+. To identify
the I(ä+) doublet, we used earlier results [7]. The value
of Kp(5) was taken from [6]. The partial pressures of
vapor species were calculated using the sensitivity fac-
tor averaged over the range 1013–1113 K: k = 2.1 ×
10−3 Pa/(V K). Note that this k value fits well with
results for other fluorides.
The mass spectrum was identified under the
assumption that the ä+ and K2F+ ions originated from
KF and K2F2 molecules. The sensitivity factor k was
Table 1. Measured and “equilibrium” pressures of atomic and molecular fluorine for potassium hexafluoronickelate decom-
position
T, K
p(F)meas, Pa p(F2)meas, Pa (p(F)/p(F2))meas (×102) (p(F)/p(F2))“eq” (×102)
Kp(2)meas
Kp(2) [6]
561
598
615
679
701
756
781
815
846
1.2 × 10–2
6.2 × 10–1
1.4
2.3 × 10–1
2.3 × 10–2
2.3 × 10–1
7.1 × 10–1
2.0
3.3 × 10–2
2.1
36
30
6.9
2.6
2.4
4.3 × 10–8
1.8 × 10–6
3.1 × 10–6
1.3 × 10–6
1.6 × 10–6
2.6 × 10–5
9.3 × 10–5
3.7 × 10–4
6.2 × 10–4
2.0 × 10–9
1.8 × 10–8
4.3 × 10–8
8.6 × 10–7
2.2 × 10–6
1.7 × 10–5
3.8 × 10–5
1.1 × 10–4
2.4 × 10–4
6.4
22
4.0 × 10–1
3.3 × 10–3
2.1 × 10–2
5.5 × 10–2
1.1 × 10–1
3.0 × 10–3
58
45
700
1000
1300
1800
1500
900
740
570
590
4.3 × 10–1
5600
INORGANIC MATERIALS Vol. 41 No. 12 2005

1330
p(F), Pa
LESKIV et al.
assumption because Tb(IV) fluorides have not been
detected previously in the vapor phase.
0.20
0.15
0.10
0.05
0
2. At the beginning of the vaporization process
(≤568 K), the HF/O2 ratio notably exceeded the value
typical of partially hydrolyzed higher fluorides of tran-
sition metals and rare earths. Moreover, HF release
continued even after oxygen release stopped, in the
ranges 573–666 and 873–993 K. It cannot be ruled out
that the effect was due to the presence of HF molecules
in the structure of the complex fluoride. Such an adduct
might be formed during fluorination of the KCl + Tb4O7
mixture if one of these compounds contained water of
hydration.
600
700
800
900
1000
Table 3 compares the thermolysis parameters for
TbF4 and K3TbF7.
T, K
Fig. 3. Temperature variation of the atomic fluorine pres-
sure during K TbF decomposition.
For either fluoride, we indicate two temperatures:
the first temperature corresponds to the onset of thermal
decomposition, and the second, to the highest decom-
position rate (the temperature at which p(F) reaches a
maximum). These data confirm the higher thermal sta-
bility of the complex fluoride.
3
7
Figure 3 shows the temperature-dependent partial
pressure of atomic fluorine. There are two well-
resolved peaks, one due to the second step of K2NiF6(s)
impurity decomposition (Ӎ740 K), and the other to
K3TbF7(s) decomposition (Ӎ990 K).Above 1000 K, the
mass spectrum showed a weak signal at m/z = 19, which
could be intercepted by the shutter and was attributed to
the F+ fragment ion originating from KF.
Cesium hexafluoromanganate, Cs2MnF6(s).
Thermolysis of this fluoride was studied in the range
573–1013 K. The Cs+ signal emerged at a temperature
as low as 600 K. Below 750 K, I(Cs+) showed a number
of noteworthy features: it increased sharply during
heating and gradually decreased to its initial level dur-
ing subsequent isothermal holding. The partial pressure
calculated from that signal under the assumption that
Cs+ originated from CsF molecules exceeded the satu-
rated CsF vapor pressure. That the vapor phase con-
tained no CsF molecules was also evidenced by the
absence of a Cs2+ signal in the mass spectrum. Note
that, in control experiments with pure CsF, the mass
spectrum always showed a Cs2+ signal, and we were
able to establish the temperature dependence of the
I(Cs+)/I(Cs2+) ratio. It seems likely that the Cs+ signal
detected below 750 K was due to the complex molecule
Cs2MnF6. The existence of that molecule was con-
Table 2 compares the experimentally determined
and “equilibrium” values of Kp(2) and p(F)/p(F2).
Several features of our experimental data on potas-
sium heptafluorotherbate vaporization warrant special
attention.
1. Using the K2F+ signal intensity measured in the
range 933–993 K, average sensitivity factor, and Kp of
reaction (5), we calculated the K+ current correspond-
ing to the equilibrium KF pressure. The I(K+/KF) val-
ues were about 2.5 times lower than the I(K+) signal
intensities measured in the temperature range in ques-
tion. It cannot be ruled out that the rest of the K+ ions
originated from the ionization of complex molecules firmed indirectly using a combination ion source. The
containing Tb(IV). It is of interest to check this 950-K mass spectrum showed a well-defined thermal
Table 2. Measured and “equilibrium” pressures of atomic and molecular fluorine for K3TbF7(s) decomposition
T, K
p(F)meas, Pa
p(F2)meas, Pa
(p(F)/p(F2))meas (p(F)/p(F2))“eq”
K(2)meas
K(2) [6]
710
713
6.6 × 10–3
5.9 × 10–3
3.6 × 10–2
2.6 × 10–2
1.2 × 10–2
4.1 × 10–2
4.0 × 10–3
8.1 × 10–3
5.0 × 10–2
5.0 × 10–3
9.9 × 10–4
1.1 × 10–3
1.6
0.7
0.7
5
30
25
1.1 × 10–7
4.3 × 10–8
2.6 × 10–7
1.4 × 10–6
1.6 × 10–6
1.5 × 10–5
3.0 × 10–6
3.4 × 10–6
7.2 × 10–6
5.4 × 10–4
5.4 × 10–4
9.5 × 10–3
733
9.3
873
1700
4000
873
13
36
1003
22000
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VAPORIZATION PRODUCTS OF TRANSITION-METAL
1331
I, mV
240
220
200
180
160
140
120
100
80
(a)
60
40
20
0
180
160
140
120
100
80
(b)
60
40
20
0
125 150 175 200 225 250 275 300 325 350 375 400
m/z
Fig. 4. Mass spectra of the vapor phase measured at a 5-h interval during Cs MnF (s) thermolysis at 993 K.
2
6
Cs3MnF+6 signal, which disappeared upon Cs2MnF6(s)
decomposition into Mn(III) complexes.
To calculate the CsF pressure, we used the Cs2+ sig-
nal and the I(Cs+)/I(Cs2+) ratio found for the saturated
CsF vapor. The Cs2F2 pressure was evaluated from the
Above 750 K, isothermal holding produced signifi-
cant changes in the mass spectrum of the vapor phase.
For example, after holding at 993 K for 5 h (Fig. 4) the
mass spectrum showed no ions originating from com-
plex Mn(III) molecules. Moreover, we identified MnFn+
(n = 0–2) ions, whose relative intensities differed from
those reported in [8] for MnF2 by a higher Mn+ intensity
(Table 4).
Table 3. Data on TbF4 and K3TbF7 thermolysis
Fluoride
T, K
p(F), Pa
TbF4
597
675
873
993
1.5
1.0 × 102
2.5 × 10–2
2.1 × 10–1
K3TbF7
Our results showed clear evidence that the vapor
phase contained no atomic fluorine.
INORGANIC MATERIALS Vol. 41 No. 12 2005

1332
LESKIV et al.
I, mV
70
Pt+
60
50
40
30
20
10
KPt+
PtF+2
KPtF+
PtF+3
PtF+
K2PtF+4
350
+
KPtF+3
K2PtF5
K2PtF+6
PtF+4
200
250
300
400
m/z
Fig. 5. Mass spectrum (m/z = 190–400) obtained during potassium hexafluoroplatinate vaporization at 1043 K.
CsF and Cs2F2 may correspond to the sum of the Cs+
signals from the complex Mn(III) molecules CsMnF4,
Cs2MnF5, Cs3MnF6, and Cs2Mn2F8. The presence of
these vapor species was inferred from data on thermal
experimental p(CsF) data and the Kp(6) given in [6] for
the reaction
2CsF(g) = Cs2F2(g).
(6)
positive ions in the vapor phase: Cs2MnF+4 , Cs3MnF5+ ,
Cs3Mn2F+8 , and Cs4MnF+6 (experiments were per-
The p(CsF) and p(Cs2F2) thus determined are pre-
sented in Table 5, together with CsF activity data.
The difference between the total Cs+ signal and the
formed with a combination ion source). Clearly, elec-
tron-impact mass spectra could not be identified with
certainty in that case.
contribution of the fragment Cs+ ions originating from
Table 4. Relative MnF+n (n = 0–2) signal intensities ob-
served in this study and reported for MnF2
Thus, we are led to conclude that Cs2MnF6 decom-
poses rapidly below 750 K to form Mn(III) complexes.
Rough estimates indicate that the weight ratio of sub-
limed Cs2MnF6 to Mn(III) complexes in the vapor
phase is 1 : 45. In addition to Cs2MnF6 decomposition
and fluorination of the cell material (the only likely rea-
son for the absence of fluorine in the gas phase),
Mn(III) complexes may originate from partial sample
hydrolysis at low temperatures.
Relative intensity
T, K
Source
MnF+2
Mn+
MnF+
951
973
0.65
0.57
0.22
1.00
1.00
1
0.29
0.42
0.35
This work
This work
[8]
Potassium hexafluoroplatinate, K2PtF6(s). Ther-
molysis of this fluoride was studied in the range 600–
1063 K. Below 900 K, the mass spectrum showed no
intercepted signals. At 923 K, a I(K+) signal emerged.
Increasing the temperature to 1023 K made it possible
to detect complex ions.
1100
Table 5. CsF and Cs2F2 partial pressures and CsF activity
for Cs2MnF6(s) thermolysis
T, K
p(CsF), Pa
a(CsF)
p(Cs2F2), Pa
The mass spectrum shown in Fig. 5 is, on the whole,
similar to that reported earlier [9]. In this study, how-
951
973
989
0.33
0.72
1.1
3.2 × 10–4
1.0 × 10–3
1.6 × 10–3
0.006
0.007
0.008
ever, we also detected KPtF+n (n = 0–3) ions. The nature
of the molecular precursor to the Pt+ ion, whose signal
was the strongest among Pt-containing ions, is not yet
INORGANIC MATERIALS Vol. 41 No. 12 2005

VAPORIZATION PRODUCTS OF TRANSITION-METAL
1333
Table 6. K2F2, KF, and K2PtF6 partial pressures and KF activity for K2PtF6(s) thermolysis
T, K
p(K2F2) × 102, Pa
p(KF), Pa
a(KF)
p(K2PtF6), Pa
1043
1063
1073
1093
1.4
2.1
2.7
1.2
0.68
1.0
1.3
1.1
0.11
0.10
0.10
0.05
0.34
0.34
0.61
0.67
clear. We also identified the F+, K+, K2+, and K2F+ ions.
During fluoride thermolysis accompanied by fluorine
release, the vapor phase typically contains F atoms and
F2 molecules. The relatively low F+ signal intensity and
sublimation is accompanied by decomposition and Pt
metal formation, without atomic fluorine release.
ACKNOWLEDGMENTS
the absence of F+2 suggest that the F+ ions originate,
This work was supported by the Russian Foundation
for Basic Research, grant no. 05-03-32916a.
most likely, from KF and K2PtF6 molecules.
During isothermal holding, all of the signals that
could be intercepted by the shutter gradually became
weaker, which is believed to be due to the gradual
blinding of the effusion orifice as a result of Pt metal
deposition. Platinum release during thermolysis was
also evidenced by XRD results for the evaporation res-
idue. Note that platinum was deposited for the most
part onto the sample surface and near the effusion ori-
fice, i.e., where KF was removed most rapidly. In the
sample bulk, no Pt metal was detected.
We were able to identify the mass spectrum and
evaluate partial pressures only under assumption that
the entire K2F+ signal was due to the K2F2 dimer. We
also assumed that the reaction system (5) was at equi-
librium and that the only vapor species present was
K2PtF6. The calculated partial pressures of vapor spe-
cies are listed in Table 6.
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Our results indicate that K2NiF6(s) decomposition is
accompanied by two steps of atomic fluorine release, at
570–670 and 700–850 K.
During K3TbF7(s) thermolysis, atomic fluorine is
released in the range 873–1000 K, that is, well above
the temperature range of TbF4 decomposition.
Cs2MnF6(s) vaporization is accompanied by decom-
position in the range 600–750 K and the formation of
Mn(III) fluoro complexes, without atomic fluorine
release.
The major species in the saturated vapor over potas-
sium hexafluoroplatinate are KF and K2PtF6. K2PtF6
INORGANIC MATERIALS Vol. 41 No. 12 2005