Thermal behavior of ammonium-containing titanium and germanium fluoride complexes (NH4)4Li2(AF6)3 and NH4NaAF6 (A = Ge or Ti)

Article abstract of DOI:10.1134/S0036023608040177

The compounds (NH₄)₄Li₂A₃F ₁₈ and NH₄NaAF₆ (A = Ge or Ti) were studied thermogravimetrically. These fluoro complexes are thermolyzed in the range 230-450°C. Measurements on a differential scanning microcalorimeter revealed reversible phase transitions in NH₄NaAF₆ at T ₁ = 126°C and T ₂ = 111°C.

Full text of DOI:10.1134/S0036023608040177

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 4, pp. 583–587. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © T.F. Antokhina, I.N. Flerov, N.N. Savchenko, T.A. Kaidalova, E.B. Merkulov, L.N. Ignat’eva, V.D. Fokina, 2008, published in Zhurnal Neorgan-
icheskoi Khimii, 2008, Vol. 53, No. 4, pp. 639–643.
PHYSICAL METHODS
OF INVESTIGATION
Thermal Behavior of Ammonium-Containing Titanium
and Germanium Fluoride Complexes (NH4)4Li2(AF6)3
and NH4NaAF6 (A = Ge or Ti)
T. F. Antokhinaa, I. N. Flerovb, N. N. Savchenkoa, T. A. Kaidalovaa, E. B. Merkulova,
L. N. Ignat’evaa, and V. D. Fokinab
a Institute of Chemistry, Far-East Division, Russian Academy of Sciences,
pr. Stoletya Vladivostoka 159, Vladivostok, 690022 Russia
b Kirenskii Institute of Physics, Siberian Branch, Russian Academy of Sciences,
Akademgorodok, Krasnoyarsk, 660036 Russia
Received December 7, 2005
Abstract—The compounds (NH4)4Li2A3F18 and NH4NaAF6 (A = Ge or Ti) were studied thermogravimetri-
cally. These fluoro complexes are thermolyzed in the range 230–450°ë. Measurements on a differential scan-
ning microcalorimeter revealed reversible phase transitions in NH4NaAF6 at T1 = 126°C and T2 = 111°C.
DOI: 10.1134/S0036023608040177
In [1], we described ammonium-containing com- transformations during heating. Each compound fol-
plexes of germanium and titanium fluorides lows its own route, and it is difficult to predict the trans-
(NH4)4Li2A3F18 and NH4NaAF6 (A = Ge or Ti). The formation character. This work describes the thermal
stability and the mechanism of thermal transformations
in the compounds studied.
structural study of (NH4)4Li2(GeF6)3 [2] showed that
the structural motif of this compound had not been
encountered in mixed-cation hexafluoro complexes.
The crystal structure of (NH4)4Li2(GeF6)3 can conven-
tionally be divided into alternating NH4LiGeF6 and
(NH4)2GeF6 layers. The coordination polyhedra around
Ge(1) and Ge(2) atoms are octahedra with fluorine
atoms in their vertexes; the coordination polyhedron
around a lithium atom is a tetrahedron. Layers lying
parallel to plane (100) and built of tetranuclear frag-
ments are distinguished in the three-dimensional
framework of the (NH4)4Li2(GeF6)3 crystal structure.
These fragments are built of two Ge(1) octahedra and
two Li tetrahedra, which share corners. Ge(2) octahedra
link layers into a continuous framework. Ammonium
cations reside in channels that penetrate the crystal
architecture.
EXPERIMENTAL
A Paulik-Paulik-Erdey derivatograph was used to
study the thermal behavior of ammonium complexes of
germanium fluoride and titanium fluoride in labyrinth
platinum crucibles in air at a heating rate of 10 K/min.
Alumina calcined at 1000°ë was the reference. The
temperature measurement accuracy was 3°ë K. The
sample size was 200 mg. The TG error was 1 mg.
All X-ray diffraction patterns of intermediates and
final products were recorded on a DRON-2 diffracto-
meter (graphite monochromator, CuKα radiation),
High-temperature experiments were carried out on an
Advance D8 diffractometer (CuKα radiation).
We described the NH4NaTiF6 structure in [3]. This
compound crystallizes in an NH4NaTiF6-type orthor-
hombic structure with the following unit cell parame-
ters: a = 8.034 Å, b = 12.953 Å, c = 10.755 Å [4].
NaRbSnF6 crystals have a continuous three-dimen-
sional framework built of pairs of edge-sharing dis-
torted NaF–6 and TiF–6; octahedra; pairs are linked via
Calorimetric measurements were carried out on a
DSM-2M differential scanning calorimeter at 60–
210°ë in heating and cooling modes at a rate of
8 K/min. Heating/cooling cycles were repeated seve-
ral times to improve the ruggedness of the results. Sam-
ples were fine powders ground with an agate mortar.
Sample sizes were 0.13–0.16 g.
polyhedral corners into zigzag ribbons running along
the direction [100]. Interstices are occupied by ammo-
nium cations, which additionally link architecture ele-
ments via hydrogen bonds N–H···F.
IR absorption spectra were recorded on an IFS Equi-
nox 55S instrument (350–4000 cm–1; wavenumber
accuracy, 0.5 cm–1). Test samples were prepared by a
routine procedure: they were finely ground with an
Coordination compounds of complex composition agate mortar and spread as Nujol mull onto KBr sub-
typically experience complex chemical and structural strates.
583

584
ANTOKHINA et al.
In the thermal curve for NH4NaGeF6 (Fig. 1b), there
∆m, mg
0
is an endotherm at 340–420°ë with the attendant
weight loss exceeding removal of one NH4F molecule.
The sample calcined to 420°ë has X-ray diffraction
reflections from Na2GeF6 and an unknown phase. We
carried out IR spectroscopic analysis in order to iden-
tify this phase. The IR absorption spectrum contains a
strong band with a peak at 610 cm–1; we assigned it to
the stretching vibrations of the hexafluoride ion GeF62–
in the compound Na2GeF6 [5, 6]. A broad band in the
region 800–1000 cm–1 and a band at 3250–3300 cm–1
imply the existence, along with Na2GeF6, of a phase
containing an OH group. According to [6], the spectral
manifestations of M–OH groups are in the aforemen-
tioned regions. From the above, we infer the following
scheme of NH4NaGeF6 degradation:
(a)
(c)
TG
DTA
TG
20
DTA
40
60
80
390
390
525
490
435
540
440
810
395
100
120
140
0
(d)
(b)
TG
TG
20
40
515
DTA
740
600
575
480
DTA
2NH4NaGeF6
GeF4 + 2H2O
2NH4F↑ + GeF4 + Na2GeF6,
Ge(OH)2F2 + 2HF↑.
605
555
60
143
450
415
420
80
According to this scheme, removal of NH4F and HF
is 25.1%, which virtually coincides with the weight loss
derived from the TG curve in the range 320–420°ë. In
the sample calcined to 560°ë, X-ray diffraction reflec-
tions from Na2GeF6 and several reflections from an
unknown phase are observed. The final products (at
760°ë) are Na2GeF6 and tetragonal GeO2.
100
100 300 500 700
200 400 600
100 300 500 700
200 400 600 800
T, °C
Fig. 1. DTA and TG curves for ammonium titanium fluoride
and germanium fluoride complexes: (a) (NH ) Li (GeF ) ,
4 4
2
6 3
(b) NH NaGeF , (c) (NH ) Li (TiF ) , and (d) NH NaTiF .
The thermal curves for NH4NaTiF6 and
(NH4)4Li2Ti3F18 (Figs. 1c, 1d) show that ammonium
fluorotitanate complexes are stable up to 230–250°ë.
Prior to thermolysis, the thermal curve for NH4NaTiF6
shows an endotherm, which is not accompanied by
weight low and is due to reversible polymorphic transi-
tion. The X-ray diffraction pattern for the product of
NH4NaTiF6 heated to 150°ë and then cooled to room
temperature coincides with that of the starting com-
pound. To prove the existence of a high-temperature
NH4NaTiF6 phase, we recorded X-ray diffraction pat-
terns of a sample at 130–150°ë. The table contains
X-ray diffraction data for the β-NH4NaTiF6 phase and,
as a reference, our previous data for the starting com-
pound [1, 2]. The X-ray diffraction pattern for
β-NH4NaTiF6 slightly differs from that for the starting
compound. The unit cell parameters of β-NH4NaTiF6
were determined by indexing its powder pattern by iter-
ation with reference to the unit cell parameters of the
starting NH4NaTiF6, found from a single-crystal exper-
iment, with the doubled parameter a (table).
4
6
4 4
2
6 3
4
6
RESULTS AND DISCUSSION
Data on the thermal stability of the complexes
(NH4)4Li2Ge(Ti)3F18 and NH4NaGe(Ti)F6 were
obtained from thermal analysis (Fig. 1) and X-ray dif-
fraction analysis of thermolysis products.
The thermal curves for an (NH4)4Li2Ge3F18 sample
(Fig. 1a) show a considerable endotherm (390–435°ë),
which is accompanied by a substantial weight loss and
associated with removal of four NH4F molecules
and, in part, GeF4. X-ray powder diffraction shows
β-Li2GeF6 in the sample calcined to 435°ë. As temper-
ature rises, β-Li2GeF6 degrades to LiF and GeF4. GeF4
partially hydrolyzes to GeO2. The (NH4)4Li2Ge3F18 ther-
molysis can be represented by the scheme
(NH4)4Li2Ge3F18
Li2GeF6
GeF4
4NH4F↑ + 2GeF4↑ + Li2GeF6,
2LiF + GeF4,
The phase transition in NH4NaTiF6 was studied by
DSC in more detail. Figures 2 and 3 display the results
of two sets of calorimetric measurements on NH4NaTiF6
in the range 60–210°ë. As a result of the first heating,
GeO2 + 4HF↑.
According to this scheme, a weight loss of 60.5 wt %
corresponds to removal of 4NH4F and 2GeF4. The
weight loss derived from the TG curve is 56.4 wt %,
which is slightly lower than the calculated value. This
disagreement can arise from small hydrolysis of GeF4 to
GeO2, which precipitates onto the walls of the crucible.
two anomalies with peaks at T1 = 143
2°C and
T2 = 124 2°C are found on the DSM signal versus
temperature curve (Fig. 2a). The thermal anomaly
observed in DTA–TG measurements corresponds to an
intermediate temperature; most likely, averaging
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

THERMAL BEHAVIOR OF AMMONIUM-CONTAINING TITANIUM
585
(‡)
occurred because of the lower temperature resolution of
1500
T
2
DTA compared to DSC. A temperature hysteresis is
found upon cooling, corresponding to the DSM signal
peaks (Fig. 2b): δT1 = 17, and δT2 = 13°C. Repeated
thermocycling shows considerably lower temperatures
of thermal anomalies: T1 = 126 2°C, and T2 = 111
2°C (Fig. 3). These temperatures remain unchanged in
further experiments: for the third heating, T1 = 126
T
1
1000
500
2°C and T2 = 110
2°C. Most likely, annealing
1000
occurred during the first heating.
(b)
(c)
The reproducibility of the thermal anomalies in
thermocycling implies that NH4NaTiF6 consecutively
experiences two enantiotropic transitions. The great
values of T1 and T2 hysteresis argue in favor of first-
order transitions. The peak heights of the DSM signal
at T1 and T2 are redistributed as a result of annealing,
which is made clear by the comparison of the panels of
Fig. 2: the heat capacity anomaly at T1 considerably
increases, whereas that at T2 decreases. In order to gain
quantitative data on thermal features, we converted the
DSM signal to the excess (anomalous) heat capacity
∆Cp, which is associated with the enthalpy and entropy
changes, by the procedure used in [7]. At the first stage,
the systematic contribution was subtracted (the dashed
line in Fig. 2). These data were used to calculate the
enthalpy and entropy changes in phase transitions.
However, we failed to resolve the excess heat capacity
contributions from individual phase transitions; we
only calculated the overall enthalpy change Σ∆Hi =
500
0
T
2
T
1
1500
1000
500
0
T
T
2
1
60
80
100 120 140 160
200
180
T, °C
Fig. 2. DSM signal vs. temperature for the compound
NH NaTiF recorded during (a) first heating, (b) cooling,
and (c) second heating. The dashed line shows a systematic
contribution.
4
6
∆Cpdt. This quantity is 3.3 0.45 kJ/mol for the first
∫
heating, 3.1 0.45 kJ/mol for the second heating, and
2.6 0.45 kJ/mol for the third one. The average entropy
change determined by integrating the (∆Cp/T)(T) func-
tion is Σ∆Si = 7.2 J/(mol K). A weight loss of 0.7% was
found when the sample was weighed after the experi-
ment was over. Thus, the decrease in the enthalpy as a
result of three heating cannot be associated with weight
loss. Quite likely, thermocycling can cause blurring of
the anomalies associated with phase transitions, which
made it difficult to determine the integration boundaries
and changed ∆H.
200
(‡)
100
0
200
Next (Fig. 1d), NH4NaTiF6 thermolysis in the range
230–450°ë is accompanied by a set of endotherms of
complex shapes with more considerable weight loss
than resulted from removal of an NH4F molecule. This
is because the test fluorotitanates below 450°ë under an
undried air atmosphere thermally dissociate to NH4F
and TiF4; the latter at these temperatures in the presence
of atmospheric moisture partially hydrolyzes to íiO2
(anatase) and partially leaves, because the íiF4 subli-
mation temperature is 285.5°ë [8]. Above 450°ë, TiF4
hydrolyzes further to yield oxytitanate and oxyfluoroti-
tanate products as shown by its TG curve. X-ray pow-
der diffraction shows these products in the residues
obtained at 450–950°ë. In an NH4NaTiF6 sample cal-
cined to 450°ë, we observed reflections of TiOF2 and
(b)
100
0
60
80
100 120 140 160 180 200
T, °C
Fig. 3. Excess heat capacity vs. temperature for the com-
pound NH NaTiF recorded during (a) first heating and
4
6
(b) second heating.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008

586
ANTOKHINA et al.
X-ray diffraction data for α-NH4NaTiF6 and β-NH4NaTiF6
d, Å
d, Å
I/I0
hkl
I/I0
hkl
obs.
calc.
obs.
calc.
β-NH4NaTiF6 (Orthorhombic; a = 16.085(5), b = 12.951(1), c = 10.825(3) Å)
1
13
7
6.46
6.45
201
021
002
102
301
221
202
320
122
103
420
421
402
412
033
611
531
151
333
621
251
612
134
252
343
005
702
205
153
044
15
17
28
15
14
8
2.046
2.040
2.015
2.010
2.005
2.001
1.947
1.896
1.888
1.875
1.851
1.846
1.825
1.811
1.806
1.798
1.794
1.763
1.736
1.712
1.697
1.682
1.677
1.642
1.631
1.619
1.572
1.549
1.523
2.046
2.043
2.018
2.011
2.005
2.003
1.946
1.900
1.886
0.874
1.850
1.847
1.824
1.813
1.805
1.798
1.794
1.763
1.736
1.711
1.697
1.682
1.678
1.642
1.630
1.619
1.572
1.549
1.523
514
623
550
244
062
360
262
732
415
740
070
741
071
171
263
831
462
901
714
272
902
463
164
922
625
080
851
941
282
5.54
5.39
5.56
5.41
1
5.14
5.13
3
4.83
4.81
16
3
4.57
4.48
4.57
4.49
6
>1
100
2
4.14
4.02
3.52
4.13
4.02
3.52
4
2
2
>1
3
3.42
3.26
3.42
3.26
12
6
25
22
2
3.23
3.14
2.77
3.23
3.13
2.77
3
7
6
4
2
13
5
2
2
5
3
2
2.55
2.51
2.55
2.51
71
38
3
1
2
7
9
5
2
1
3
3
2
2.487
2.461
2.414
2.404
2.363
2.271
2.242
2.198
2.163
2.115
2.090
2.087
2.075
2.489
2.460
2.414
2.404
2.362
2.270
2.244
2.198
2.165
2.115
2.091
2.087
2.077
6
10
10
32
21
6
3
NH4NaTiF6 (Orthorhombic; a = 8.034(2), b = 12.953(3), c = 10.755(3) Å) [3]
9
47
36
100
27
73
6
45
40
9
13
10
5
8
4
6.51
5.54
5.37
4.56
4.46
4.00
3.41
3.24
6.47
5.54
5.38
4.56
4.47
4.00
3.41
3.24
200
210
020
211
021
310
202
400
230
113
240
501
511
123
042
341
242
250
004
214
6
5
8
5
8
4
4
5
9
5
9
4
4
5
4
4
4
5
3
1.881
1.868
1.851
1.817
1.794
1.778
1.749
1.707
1.692
1.680
1.637
1.590
1.566
1.523
1.484
1.459
1.432
1.362
1.339
1.882
1.865
1.850
1.817
1.793
1.778
1.749
1.707
1.692
1.680
1.637
1.590
1.568
1.523
1.487
1.459
1.432
1.362
1.339
024
540
700
541
060
711
720
404
542
603
452
015
353
650
812
154
254
372
006
3.13
2.55
3.14
2.55
2.484
2.460
2.401
2.357
2.235
2.193
2.110
2.040
2.008
1.888
2.483
2.464
2.402
2.358
2.235
2.195
2.113
2.041
2.009
1.889
6
8
11
31
5
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THERMAL BEHAVIOR OF AMMONIUM-CONTAINING TITANIUM
587
Na2TiF6 along with those of titania; at 575°ë, the
REFERENCES
amount of Na2TiF6 decreases and a sodium salt of ter-
valent titanium (Na5Ti3F14) appears. We failed to iden-
tify several reflections, but at 650°ë the unknown phase
disappears and reflections from TiO2 (anatase),
Na2TiF6 (minor amounts), and Na5Ti3F14 exist. Above
750°ë, TiO2 (anatase), TiO2 (rutile), and Na5Ti3F14 are
produced. Tervalent titanium appears in the thermolysis
products because NH4F manifests the reducing proper-
ties of ammonia: at 167°ë, it decomposes to HF and
NH3 [9], and the latter above 400°ë dissociates to N2
and H2, creating a reducing atmosphere. Because laby-
rinth crucibles were used, the processes studied
occurred under the own vapor atmosphere, i.e., in a
reducing atmosphere.
1. T. F. Antokhina, N. N. Savchenko, L. N. Ignat’eva, and
T. A. Kaidalova, Zh. Neorg. Khim. 50 (6), 935 (2005)
[Russ. J. Inorg. Chem. 50 (6), 856 (2005)].
2. D. Yu. Popov, V. Ya. Kavun, A. V. Gerasimenko, et al.,
Koord. Khim. 25 (10), 750 (1999).
3. D. Yu. Popov, V. Ya. Kavun, T. F. Antokhina, et al.,
Koord. Khim. 27 (12), 883 (2001).
4. A. V. Gerasimenko, S. B. Ivanov, T. F. Antokhina, and
V. I. Sergienko, Koord. Khim. 18 (10–11), 1139 (1992).
5. R. L. Davidovich, T. A. Kaidalova, T. F. Levchishina,
and V. I. Sergienko, The Atlas of IR Absorption Spectra
and X-ray Diffraction Data for Group IV and V Metal
Fluoride Complexes (Nauka, Moscow, 1972) [in Rus-
sian].
6. K. Nakamoto, Infrared Spectra of Inorganic and Coor-
dination Compounds (Wiley, New York, 1963; Mir,
Moscow, 1966).
The X-ray diffraction pattern of the (NH4)4Li2Ti3F18
calcined to 450°ë shows reflections belonging to
Li2TiF6, TiO2 (anatase), and TiOF2. At 850°ë, TiO2
(rutile) and LiF are produced.
7. S. V. Mel’nikova, V. D. Fokina, and N. M. Laptash, Fiz.
Tverd. Tela 48 (1), 110 (2006).
Having compared the thermal stabilities of ammo-
nium fluorotitanate complexes and their fluoroger-
manate analogues, we inferred that the fluoroger-
manates are more thermally stable.
8. E. G. Rakov and E. I. Mel’nichenko, Usp. Khim. 53 (9),
1463 (1984).
9. E. G. Rakov, Itogi Nauki Tekh., Ser.: Neorg. Khim. 15
(1988).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 4 2008