Synthesis and studies of magnesium hexafluorozirconates MgZrF6 ? nH2O (n = 5, 2, 0)

Article abstract of DOI:10.1134/S0036023610090020

Crystalline magnesium hexafluorozirconate MgZrF₆ ? 5H ₂O isostructural to MnZrF₆ ? 5H₂O, and having a chain-like structure, was synthesized and studied. According to thermogravimetry, the compound undergoes stepwise dehydration in the temperature range of 50-420°C to give the stable phase MgZrF₆ ? 2H ₂O and the final product MgZrF₆ isostructural to the cubic modification of MZrF₆ (M = Cu, Fe). The vibrational spectra of the initial compound and the dehydration products are analyzed and the structures of the compounds are considered.

Full text of DOI:10.1134/S0036023610090020

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 9, pp. 1339–1346. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © N.A. Didenko, K.A. Gaivoronskaya, E.I. Voit, A.V. Gerasimenko, V.Ya. Kavun, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 9,
pp. 1420–1428.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis and Studies of Magnesium Hexafluorozirconates
MgZrF6 · nH2O (n = 5, 2, 0)
N. A. Didenko, K. A. Gaivoronskaya, E. I. Voit, A. V. Gerasimenko, and V. Ya. Kavun
Institute of Chemistry, Far East Division, Russian Academy of Sciences,
pr. Stoletiya Vladivostoka 159, Vladivostok, 690022 Russia
Received October 8, 2009
Abstract—Crystalline magnesium hexafluorozirconate MgZrF6
having a chainꢀlike structure, was synthesized and studied. According to thermogravimetry, the compound
undergoes stepwise dehydration in the temperature range of 50–420°C to give the stable phase MgZrF6
⋅
5H2O isostructural to MnZrF6
⋅
5H2O, and
⋅
2H2O and the final product MgZrF6 isostructural to the cubic modification of MZrF6 (M = Cu, Fe). The
vibrational spectra of the initial compound and the dehydration products are analyzed and the structures of
the compounds are considered.
DOI: 10.1134/S0036023610090020
Previously, we noted that replacement of one of anhydrous MgZrF6 through its hydrates has not
outerꢀsphere cation
M in hexafluorozirconates been elucidated. Previously [5], it was noted that Mg
M2ZrF6 by another singleꢀcharged cation M' results, and Mn salts that crystallize from solutions contain
most often, in the formation of compounds differing five water molecules and are isostructural to each
in the structure and in the mode and energy characterꢀ other.
istics of ionic motions. Therefore, we synthesized a
number of hexafluorozirconates with heteroatomic
cationic sublattice (isovalent substitution) and studied
their structure, the dynamics of complex ions, and
electrophysical properties [1, 2]. Of obvious interest
was the preparation and study of hexafluorozirconates
with mixed cationic sublattice with heterovalent subꢀ
stitution, for example, substitution of a doubly
charged cation for a singly charged one, which may
bring about the necessity of charge compensation
resulting in vacancies appearing in the structure.
The purpose of this work was to optimize the synꢀ
thesis conditions for individual hydrates and anhyꢀ
drous magnesium hexafluorozirconate, determine the
phase composition, compare the thermal characterisꢀ
tics, and elucidate structurization features in the
higher crystal hydrate–anhydrous compound series on
the basis of IR and Raman spectroscopy data.
EXPERIMENTAL
Special purity grade ZrO2, reagent grade Mg(NO3)2
⋅
6H2O, and special purity grade 40% HF were used.
To date, the structures of the compounds
Magnesium hexafluorozirconate was prepared by slow
ΙΙ
K2M (ZrF6)2
ΙΙ
⋅
6H2O (M = Cu, Zn), in which the
dropwise addition of a solution of Mg(NO3)2 6H2O to
⋅
2−
complex anions are dimers consisting of
ions
ZrF6 ,
a stirred solution of ZrO2 (concentration 1.6 mol/L) in
40% HF heated on a boiling water bath (component
molar ratio 1 : 1). Precipitation took place immediꢀ
ately after mixing the reactants. The resulting salt was
washed with acetone and dried in air. According to the
results of chemical analysis, the precipitated phase was
and the cationic sublattice is composed of
[M(H2O)6]2+ and K+ ions, have been investigated [3,
4]. The difference between the ionic radii in the K+–
Zn2+ and K+–Cu2+ pairs is about 40%. As the doubly
charged cation, we chose Mg2+, whose ionic radius is
close to that of the Li+ ion. Previously, magnesium
hexafluorozirconate was synthesized and studied (as
crystal hydrates and as the anhydrous salt) as the
model object for solving problems concerning the
crystal structure, the character of internal motion and
phase transitions, thermal properties, and so on.
the crystal hydrate MgZrF6 2H2O. According to powꢀ
⋅
der Xꢀray diffraction, this product may contain magꢀ
nesium fluoride as an impurity (precipitated due to
poor solubility as a gelꢀlike product responsible for a
halo in the Xꢀray diffraction patterns). After the subseꢀ
quent dissolution of the contaminated magnesium
hexafluorozirconate precipitate in water, MgF2 was
In the series of hexafluorozirconate salts with douꢀ separated by filtration, while slow evaporation of the
bly charged cations, magnesium hexafluorozirconate filtrate at room temperature or in a refrigerator
has been scarcely studied so far: the structure of its resulted in growth of large transparent colorless crysꢀ
crystal hydrates isolated from solutions has not been tals of MgZrF6 5H2O (I) soluble in water and decayꢀ
⋅
determined, the possibility or conditions of synthesis ing on storage in air (metastable compound) to give
1339

1340
DIDENKO et al.
2 cm–1. Raman spectra were obtained with a 2 cm–1
resolution in the backscattering mode on a Bruker
Δ
m, mg
0
RFS100/S spectrometer (Nd:YAG laser,
50 mW).
λ
=1064 nm,
DTG
In this study, the principal bands in the vibrational
spectra of MgZrF6 5H2O, MgZrF6 2H2O, and
⋅
⋅
25
50
75
MgZrF6 were assigned on the basis of quantumꢀchemꢀ
ical calculations and published data. The quantumꢀ
chemical calculations were carried out using the
Gamess program package [6], by local density funcꢀ
tional theory method in the 321G basis set for all
atoms [7–9] using the B3LYP exchangeꢀcorrelation
potential [10]. All calculations were carried out on a
16ꢀprocessor Linux cluster at the Institute of Chemisꢀ
try (Far East Division, Russian Academy of Sciences).
DTA
TG
RESULTS AND DISCUSSION
According to powder Xꢀray diffraction data, the
60
90
170
synthesized MgZrF6
MnZrF6 5H2O [11, 12] with the parameters
15.5100(4) Å, = 7.4733(2) Å, = 8.8484(2) Å,
124.043(1)°; space group
⋅
5H2O was isostructural to
⋅
a
β
=
=
b
c
0
150
300 450
600 750
T
, °C
С2
. The structure of the latꢀ
ter was composed of chains of edgeꢀsharing ZrF8
dodecahedra reinforced with MnF2(H2O)4 octahedra
and uncoordinated water molecules. Later, monoꢀ
Fig. 1. Curves of heating of MgZrF
⋅
6 2
5H O in air.
clinic crystals of ZnZrF6
analogous manganese salt, were obtained [13].
The isostructural nature of MZrF6 5H2O (M =
⋅
5H2O, also isostructural to
dihydrate. Evaporation of the filtrate at
a finely crystalline precipitate of MgZrF6
T
⋅
> 50°С gave
2H2O whose
⋅
Xꢀray diffraction characteristics coincide with the
characteristics for the same compound that crystalꢀ
lizes from hydrofluoric acid solutions. Unfortunately,
we were unable to grow single crystals of the dihydrate
for Xꢀray diffraction. We studied the starting magneꢀ
sium hexafluorozirconate pentahydrate and the prodꢀ
ucts of its thermal decomposition formed upon partial
Mn, Mg, Zn) implies the same type of thermal behavꢀ
ior. According to published data [14], dehydration of
manganese hexafluorozirconate pentahydrate occurs
stepwise with successive elimination of four water
molecules (in the temperature range of 80–220°С
)
and one water molecule (220–250°C) (recording
under inert atmosphere) and dehydration of similar
zinc salt occurs at 100–112, 123–162, and 186–
194°С with elimination of totally 4.7 H2O molecules.
In the 228–396°С range, the components being
removed are H2O and HF (recording in air) [13].
The results of thermoanalytical studies of comꢀ
(
MgZrF6 2H2O) and complete (MgZrF6) dehydraꢀ
⋅
tion. The products of synthesis were identified by powꢀ
der Xꢀray diffraction, IR and Raman spectroscopy,
and chemical analysis using standard procedures.
For MgZrF6 · 5H2O anal. calcd. (%): Mg, 7.52; Zr,
28.53; F, 35.74; H2O, 28.21.
Found (%): Mg, 7.45; Zr, 28.60; F, 36.35; H2O,
27.6 (TG data).
The Xꢀray diffraction patterns of magnesium
hexafluorozirconate and intermediate and final prodꢀ
ucts of its thermal decomposition were recorded on
pound MgZrF6 5H2O are presented in Fig. 1. As can
⋅
be seen from the derivatogram of magnesium hexafluꢀ
orozirconate pentahydrate, during heating in the temꢀ
perature range of 50–250°С, the DTA curve shows
three endotherms with maxima at 60, 90, and 170°С
reflecting the stepwise dehydration. A comparison of
the position of the maximum at 60°С and the weight
loss curve (in the temperature range of 50–60°С, the
weight loss was 5–6%, no step in the TG curve was
present) leads to the conclusion that the first endotꢀ
herm corresponds to incongruent melting of the initial
salt. The incongruent melting of the crystal hydrates
involves the formation and growth of the nuclei of the
solid phase of the product with inclusion of liquid
solution [15]. We analyzed the products obtained on
Bruker D8 Advance (Cu
K radiation, graphite monoꢀ
α
chromator) and Stoe Stadi P diffractometers (Cu
radiation, Ge monochromator).
K
α
1
The differential thermal analysis of magnesium
hexafluorozirconate hydrates was carried out in
undried air on a Qꢀ1000 MOM derivatograph at samꢀ
ple heating rates of 2.5 and 5 K/min. The sample
weight was 170 mg. Calcined Al2O3 was used as the refꢀ
erence.
IR spectra of the samples at 100–4000 cm–1 were heating of MgZrF6
5H2O in a derivatograph at a rate
⋅
recorded on a Shimadzu FTIR Prestigeꢀ21 and Bruker of 2.5 K/min to 55 and 65°С. Both samples were moist
Vertexꢀ70 FT IR spectrometers with a resolution of thick pulps that dried in air. According to Xꢀray difꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

SYNTHESIS AND STUDIES OF MAGNESIUM HEXAFLUOROZIRCONATES
1341
fraction, the product heated to 55°С consisted of a hexafluorozirconate. The highꢀtemperature hydrolyꢀ
mixture of MgZrF6 5H2O and MgZrF6 2H2O, while sis of MgZrF6 involves the cation and corresponds to
the product heated to 65°С was a single phase, the acidic type according to the equation
MgZrF6 2H2O. The endotherm at 90°С in the therꢀ
mogram of MgZrF6 5H2O is related to evaporation of
the moisture liberated during the incongruent melting
of the pentahydrate. The weight loss ( mexp) at 120–
130°С was 16.7%, which corresponds to the removal
of three water molecules from compound I mcalcd
16.93%) to give MgZrF6 2H2O. After heating to
⋅
⋅
⋅
MgZrF6 + 2H2O
+ 2HF MgF2 + ZrO2 + 4HF.
The evolved HF fluorinates Mg(OH)2 (or MgO) to
Mg(OH)2 + ZrF4
⋅
Δ
Δ
=
give MgF2, and pyrohydrolysis of ZrF4 yields ZrO2
.
⋅
According to powder Xꢀray diffraction, the sample
obtained at 650°C is a mixture of MgZrF6, MgF2, and
ZrO2 phases, while, at 800–900°C, a binary mixture of
MgF2 and monoclinic ZrO2 is formed. The rate of
pyrohydrolysis of MgF2 increases after decomposition
of MgZrF6 has been completed. Isothermal heating of
the product representing a mixture of ZrO2 and MgF2
in a muffle furnace at 950°C for 1–2 h yielded a binary
mixture of zirconium and magnesium oxides.
130°С, the sample was no longer friable but was an
agglutinated material coated with a crust. The Xꢀray
diffraction pattern of this compound was indexed in
the orthorhombic system with parameters of a Cꢀcenꢀ
tered unit cell:
7.391(2) Å; space group Cccm. Proceeding from the
structure of MgZrF6 5H2O, one can assume that, on
a = 7.171(1) Å, b = 11.245(2) Å, c =
⋅
heating this compound, the uncoordinated water molꢀ
ecule should be the first to be eliminated. The disrupꢀ
tion of the hydrogen bond system results in a structural
rearrangement with removal of two more water moleꢀ
cules from the magnesium coordination sphere to give
the dihydrate. The endotherm in the range of 150–
Thus, according to thermal analysis data, singleꢀ
type compounds MZrF6
are more stable (have higher dehydration temperature)
with
transition element cations than with Mg2+
⋅
5H2O (M = Mn, Mg, Zn)
d
,
although the radius of the latter is smaller than the
radius of any of transition metal ions with the charge
+2. Apparently, this is related not only to the cation
radius but also to its polarizing action on the anion and
water molecules. For equal charge and similar radius
(all other factors being the same), the susceptibility for
hydrolysis should be higher for MZrF6 salts of cations
with 18ꢀelectron (Zn2+) and vacant (Mn2+) shells than
for a similar salt of an 8ꢀelectron cation (Mg2+).
250°C corresponds to dehydration of MgZrF6
to give MgZrF6 ( mexp = 27.6%, mcalcd = 28.21%).
Thus, the endothermic processes occurring on heating
MgZrF6 5H2O (to 250°С) can be represented by the
following scheme:
⋅
2H2O
Δ
Δ
⋅
60
MgZrF6
⋅
5H2Osolid
+ liquid phase +
MgZrF6
⋅
2H2Osolid
хH2Ovapor
90
Certain trends in the structure formation in the
higher crystal hydrate MgZrF6 5H2O– anhydrous salt
system can be followed from the IR and Raman specꢀ
troscopy data.
MgZrF6
⋅
2H2Osolid + (3 – x)H2Ovapor,
⋅
170
MgZrF6
⋅
2H2Osolid
MgZrF6 solid + 2H2Ovapor.
It was found that MgZrF6 (obtained at 250°С) conꢀ
tacting with saturated water vapor at room temperaꢀ
ture gradually passes to the dihydrate. Magnesium
hexafluorozirconate is almost stable up to 420–430°С
in a moist air atmosphere. The Xꢀray diffraction patꢀ
terns of the products obtained by heating the pentahyꢀ
drate to 250 and 420°C, respectively, are identical
except for minor differences in the interplanar spacꢀ
ing. According to powder diffraction data, the comꢀ
pound MgZrF6 refers to the ReO3 type and is isostrucꢀ
tural to the cubic modification of MZrF6 (M = Cu,
Vibrational spectra of MgZrF6. Since MgZrF6 is
isostructural to FeZrF6 [17], the main fragments of the
crystal lattice of MgF6 are MgF6 and ZrF6 polyhedra
combined into a threeꢀdimensional framework by linꢀ
ear Mg–F–Zr bridges. The highꢀfrequency position
of the ν1
and the number of bands correspond to the octahedral
structure of the complex anion [ZrF6]2–
(A1 ) line in the Raman spectrum of MgZrF6
g
.
In the quantumꢀchemical calculations, an isolated
[ZrF6]2– anion (Oh point group of symmetry) was taken
as the model fragment of the lattice (Fig. 2a). The calꢀ
Fe) [16, 17]: space group Fm3m with the parameter
2−
ZrF6
culated optimal Zr–F distance (2.00 Å) for the
a
= 7.933(1) Å.
ion is in good agreement with the corresponding disꢀ
tance of 1.991(2) Å for structurally characterized
FeZrF6 (space group Fm3m) [17].
According to thermal analysis data for MgZrF6
⋅
5H2O (Fig. 1), a slight (about 1%) weight loss takes
place in the temperature range of 250–420°C. This
can be related to the removal of residual water (the IR
Table 1 presents the calculated vibration frequenꢀ
spectrum of the sample heated to 250°С exhibits weak cies of the [ZrF6]2– anion and positions of the absorpꢀ
bands for water vibrations) and/or to a minor degree of tion maxima in the experimental vibrational spectra of
pyrohydrolysis of MgZrF6. At higher temperatures magnesium hexafluorozirconate. The [ZrF6]2–
(above 420–430°С), the weight gradually decreases vibrations are responsible for two IR bands: ν3
(
)
(T1
but no clearꢀcut events are present in the DTA curve. (antisymmetric stretching mode νas(Zr–F)) and
This is due to hydrolytic decomposition of magnesium ν4 T1 ꢀZrF4 bending vibration). Three vibrations,
(
)
u
(
γ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

1342
DIDENKO et al.
(а)
(b)
2
F
Zr
1
100 200 300 400 500 600 700
ν, cm
–1
2–
Fig. 2. (a) [ZrF ] anion simulating the lattice of MgZrF ; (b) (
1
) Raman and (
2
) IR spectra of MgZrF .
6
6
6
ν1,
ν2,
and ν5
(A1 ,
g
Eg,
T2 )
g
are active in the Raman are the main fragment of the lattice of MgZrF6 5H2O.
⋅
spectrum [18].
The interanionic space accommodates the
Mg(H2O)24+
The band assignment in the vibrational spectrum of
magnesium hexafluoride is in full agreement with pubꢀ
lished data [19, 20] concerning the assignment for
[ZrF6]2– ions in compounds A2ZrF6 (A = Li, K, Cs).
cations connected to the anion chain
through two terminal fluorine atoms (Ft) (
(Fig. 3a). Thus, short Zr–Ft bonds ( 2.00 Å) and
bridging Zr–Fb bonds ( 2.20 Å) can be distinguished
≈2.00 Å)
≈
≈
The IR spectrum of MgZrF6 exhibits an additional
intense band (369 cm–1), which can be assigned only
to the νas(Mg–F) stretches. No bands for MgF6 vibraꢀ
tions were detected in the Raman spectrum, which can
be attributed to a higher degree of ionicity of Mg–F
bonds compared to the Zr–F bonds.
in the compound structure (Fig. 3a, 3b).
The vibrational spectra were simulated using the
model ion [Zr4F26]10–
(D2h) with fixed Zr atoms at the
chain ends (Fig. 3b). The optimal Zr–Ft and Zr–Fb
distances obtained in calculations (2.01, 2.21 Å) for
two ZrF8 groups connected by two Fb atoms (the cenꢀ
tral region of the fluorozirconate chain) are similar to
the experimental distances found for the isostructural
Vibrational spectra of MgZrF6
⋅
5H2O
.
In view of
5H2O are
the fact that MgZrF6 5H2O and MnZrF6
⋅
⋅
isostructural, we assume that infinite anionic chains
compound MnZrF6
The vibrational spectra of crystal hydrates MgZrF6
H2O ( = 2, 5) can be conventionally divided into freꢀ
⋅
5H2O.
[ZrF6]2nn−
formed from the dodecahedral ZrF8 groups
,
⋅
combined through two bridging fluorine atoms (Fb)
n
n
Table 1. Positions (cm–1) of the absorption maxima in the spectra of MgZrF6 and calculated vibration frequencies and band
assignments for [ZrF6]2– anion (Oh)
Experiment
DFT/321G calculation
assignment
Intensity
ν, cm–1
ν, cm–1
symmetry
IR
raman
IR
raman
661
496
264
597
554
500
νs(ZrF6)
A1
ν1
ν3
ν2
–
0.0
5.3
0.0
–
12.0
0.00
4.7
–
g
543
369
220
νas(ZrF6)
νs(ZrF2–ZrF2)
–
T1
Eg
–
u
250
222
124
δsciss(ZrF4)
T2
T1
T2
ν5
ν4
ν6
0.0
0.6
0.0
2.0
0.0
0.0
g
γ(ZrF4)
u
u
γ(ZrF2–ZrF2)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

SYNTHESIS AND STUDIES OF MAGNESIUM HEXAFLUOROZIRCONATES
1343
(а)
(b)
(c)
2
F
b
Zr
F
F
t
t
Mg
F
Zr
1
t
F
b
100 200 300 400 500 600
ν, cm
(d)
–1
2
1500 1600 1700 1800
3400
3800
–1
, cm
ν
10–
Fig. 3. (a) Fragment of the crystal lattice of MgZrF
⋅
5H O; (b) model [Zr F ]
4 26
anion; (c, d) (1) Raman and (2) IR spectra
6
2
of MgZrF
⋅
5H O (continuous line) and MgZrF
⋅ 2H O (dashed line).
6 2
6
2
quency ranges. The 600–100 cm–1 range covers the quency range covers δlib
, δwag, and δrock modes related to
stretching and bending vibrations of the fluorozirconꢀ the presence of water molecules and the hydrogen
ate chain and Mg2+ꢀformed group (Fig. 3c), and the bonds they form in the structure.
3700–3000 and 1800–1500 cm–1 ranges correspond to
the stretching and bending vibrations of the H2O molꢀ
ecules (Fig. 3d).
The only lines that were not assigned in the experiꢀ
mental spectra are those probably related to the presꢀ
ence of Mg2+ ions, H2O molecules and their interacꢀ
Table 2 shows the band assignment and the calcuꢀ tion with the fluorozirconate lattice: 740, 592, 356,
lated IR and Raman frequencies for the fluorozirconate and 255 cm–1 (IR); 362 cm–1 (Raman). Table 3 lists
≤
anion [Zr4F26]10– (Fig. 3b) and the positions (cm–1) of the vibration frequencies for Mg(H2O)n 6, MgF2, and
the absorption maxima in the experimental vibrational (MgF2)n [22–28], which can be compared with the
spectra of MgZrF6
the IR and Raman frequencies are also presented for and 592 cm–1 can be assigned to νwag,rock,lib vibrations
polycrystalline
ꢀBaZrF6 [21], which has a similar of H2O molecules. While the mode at 362 cm–1 can be
structure of the complex anion. According to calculaꢀ assigned to both the νs(ZrFb) ( 1) vibration of the fluꢀ
⋅
5H2O. For comparison, in Table 2, spectrum of the studied compound. The bands at 740
β
A
tions, the spectral range of 600–480 cm–1 mainly conꢀ orozirconate chain and to the νs(MgOn) and νs(MgF2
)
tains the stretching vibrations of the Zr–Ft bonds, vibrations (Table 3), the intense IR band at 356 cm–1
while the Zr–Fb stretches fall with the range of 450– can be attributed, as for MgZrF6, only to the νas(MgF2)
390 cm–1. The contour of the bands from 600 to vibration, in view of the fact that the Mg–Ft distances
400 cm–1 is considerably complicated since this freꢀ in the lattice are ~2.00 Å; i.e., rather strong bond
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

1344
DIDENKO et al.
Table 2. Experimental IR and Raman spectra of MgZrF6 ⋅ 5H2O and calculated vibration frequencies, intensities, and band asꢀ
signment for the [Zr4F26]10– anion (D2h)
Experiment
β
ꢀBaZrF6 [21]
DFT/321G
[Zr F ]10–
(D2 )
h
MgZrF6
⋅
5H2O
Assignment
4 26
IR
Raman
,
IR
Raman
ν
Raman
,
ν
,
cm–1
ν
cm–1
ν
,
cm–1
,
cm–1
N, cm–1
IR, int.*
int.
740
592
–
–
–
–
–
–
–
–
–
–
–
–
564
485
νs(ZrF4 t)
A1
E
562
562
0.0
276.0
509
νas(ZrF4 t)
500–470
554
532
10.3
11.1
0.0
0.0
νs(ZrF4 t)
B2
480–470
496
481
0.0
2.7
119.0
0.0
455
395
νs(F2 b
νas(F2 b
)
)
A1
B2
429
410
0.0
0.1
74.0
0.2
νas(ZrF2 b – F2 b
)
B2
A1
390
385
8.03
0.0
362
289
sym comb (νs(ZrF4 b))
as comb (νs(ZrF4 b))
380–350
390
366
0.0
0.0
175.0
0.0
356
323
275
–
E
–
–
–
–
–
δ
sciss(ZrF2 b + ZrF2 t)
323
293
280
0.03
0.3
0.0
0.0
0.0
2.0
⊥
δ||sciss(ZrF2 b + ZrF2 t)
δsciss(ZrF4 t + rot ZrF4 b
E
)
B2
γ(ZrF4 t)
B2 280–260
285
–
276
227
0.0
5.8
387.0
0.1
255
210
–
–
–
–
–
γ(ZrF2 t – ZrF2 t)
A1
224
205
0.0
0.8
357.0
0.0
rot(ZrF4 b
)
A2
E
197
0.0
0.4
185
146
γ(ZrF2 t+ ZrF2 b
)
)
188
187
180
0.0
0.1
0.4
0.0
γ(ZrF2 t– ZrF2 b
E
147
210
0.0
0.0
18.0
0.1
δsciss(ZrF4 t – rot ZrF4 b
)
B1
Lattice
sym comb (ZrFn + ZrFn) A1
as comb (ZrFn – ZrFn) B1
204
154
0.0
0.0
944.0
345.0
2
2
4
* The intensity of the calculated IR bands is expressed in D /μA , that of the Raman bands is in A /μ.
between Mg+ and the fluorozirconate lattice is tahydrate (Fig. 3d) changes the shape and intensity on
formed.
The IR spectrum of pentahydrate, apart from the
νas(Mg–F) stretching band (350–370 cm–1), also conꢀ
heating. The presence of several absorption maxima in
this range shifted to lower frequency implies that
hydrogen bonds that differ in energy are present in the
tains an additional band with the maximum at 255 cm–1, structure [29]. The stronger HOH···OH2 hydrogen
probably, corresponding to the
δ(MgF2) bending
bond gives rise to lower OH vibration frequency (3358–
3262 cm–1), whereas the bands at 3547–3505 cm–1 refer
to weaker HOH···FZr– hydrogen bonds. The region of
vibrations of the Мg(H2O)4F2 group (Table 3).
The broad intense H2O stretching band at 3547–
3261 cm–1 recorded in the IR spectrum of the penꢀ 1700–1500 cm–1 exhibits two bending modes
(H2O)
δ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

SYNTHESIS AND STUDIES OF MAGNESIUM HEXAFLUOROZIRCONATES
1345
Table 3. Vibration frequencies and band assignment in the spectra of [Mg(H2O)n
[22–28]
]
2+, MgF2, and (MgF2)n from published data
≤
6
Published data
ν
,
cm–1
Symmetry
Assignment
Objects
IR
Raman
770–720 [22]
620–540 [22]
Tu
Tu
,
,
Tg
Tg
νwag, rock(H2O)
[Mg(H2O)6]2+ + (H2O)12
νwag, lib(H2O)
470–420 [22]
450–410 [27, 28]
487–450 [26]
Tu
Eu
B2
νas(MgOn)
νas(MgF6)
νas(MgF2)
[Mg(H2O)6]2+
Solid MgF2 KMgF3
Mg2F4
,
B3
u
u
399 [27, 28]
νas(MgF2)
Solid MgF2
Solid KMgF3
A2
u
420 [25]
A1
A1
νs(MgF2 b
)
(MgF2)2 (D2 )
h
405 [28]
νs(MgF6)
Solid MgF2
[Mg(H2O)6]2+
g
g
380–350 [22]
360–320 [23]
353 [26]
A1
Ag
Ag
νs(MgO6)
νs(MgOn
2+
)
[Mg(H2O)n
Mg2F4
]
< 6
< 6
νs(MgF2 b
)
)
339 [25]
νs(MgF2 b
(MgF2)2 (D2 )
h
337 [26]
B2
T1
νas(MgF2 b
)
Mg2F4 (D2 )
h
[Mg(H2O)6]2+ + (H2O)12
MgSO4 · 7H2O
330–310 [22, 24]
νas(MgOn + H2Olib)
u
310 [25]
E
νas(MgF2 b
νs(MgOn)
νs(MgF6)
)
(MgF2)2 (D2 )
h
[Mg(H2O)6]2+
315–250 [22]
294 [28]
Eg
E1
Solid MgF2
g
263–240 [22]
250–240 [22]
T
Tg
δ
δ
(MgO6 + H2Olib)
(MgO6)
[Mg(H2O)6]2+ + (H2O)12
[Mg(H2O)6]2+
247 [28]
240 [26]
Eu
B1
δ
δ
(MgF6)
(MgF2)
Solid MgF2
Mg2F4
u
(Fig. 3d), which supports the presence of coordinated bonds in the lattice following the decrease in the numꢀ
and crystal H2O molecules (1668 and 1637 cm–1, ber of water molecules or the decrease in the number
respectively). The δlib and δwag vibrations of the coordiꢀ of hydrogen bonds with the bridging Fb atoms. As
nated water molecules bonded to magnesium occur at
compared with the higher crystal hydrate, the IR specꢀ
trum of the dihydrate no longer exhibits the band at
455 cm–1, possibly due to the decrease in the number
of magnesiumꢀcoordinated H2O molecules (Table 3);
740–750 cm–1.
Vibrational spectra of MgZrF6 2H2O. The intense
⋅
broad absorption bands with pronounced maximum at
509 cm–1 (Fig. 3c, dashed line) are due to the Zr–Ft
simultaneously, the
ν
as(MgOn) mode located in the
ν
s(Fb2) and
ν
as(Fb2) vibration region (455 cm–1) shifts
stretches. The complication of the as(Zr–F) stretchꢀ
ν
to higher frequency. The positions of the bending
modes also change: whereas in the IR spectrum of the
ing band contour in the IR spectra of diꢀ and pentahyꢀ
drates and the shift of the band maximum to lower freꢀ
quency compared with the IR spectrum of MgZrF6 are higher crystal hydrate, the intensity of 255, 275 cm–1
bands is substantially higher, in the spectrum of the
lowꢀwater hydrate, the bands are shifted to higher freꢀ
quency and are less intense. This also confirms the
decrease in the degree of Mg coordination by Н2O
molecules.
the consequences of the increase in the Zr coordinaꢀ
tion number from 6 to 8 and the decrease in the symꢀ
4−
metry of the
polyhedra in the crystal hydrate
⋅
ZrF8
structure on passing from MgZrF6 to MgZrF6 nH2O.
The separation of the second component (419 cm–1)
in the IR spectrum of magnesium hexafluorozirconate
In the structures of compounds in question, the
dihydrate may be due to strengthening of the bridging fraction of the Mg–F bonds increases in the series
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

1346
DIDENKO et al.
6. M. W. Schmidt, K. K. Baldridge, J. A. Boatz, et al.,
higher crystal hydrate–anhydrous compound, which
is reflected in the increased intensity of the relevant bands
νas(Mg–F) (356, 360, 369 cm–1) in the IR spectra.
J. Comput. Chem. 14 (11), 1347 (1993).
7. J. S. Binkley, J. A. Pople, and W. J. Hehre, J. Am.
Chem. Soc. 102 (3), 939 (1980).
In the IR spectrum of MgZrF6 ⋅ 2H2O, two H2O
8. M. S. Gordon, J. S. Binkley, J. A. Pople, et al., J. Am.
absorption bands are observed in the stretching vibraꢀ
tion range (3584, 3626 cm–1) and one intense band in the
Chem. Soc. 104 (10), 2797 (1982).
bending vibration range
(H2O) (1635 cm–1) (Fig. 3d).
δ
9. K. D. Dobbs and W. J. Hehre, J. Comput. Chem.
880 (1987).
8 (6),
Presumably, both water molecules in the structure are
equivalent and involved in symmetrical oneꢀtype Hꢀ
10. A. D. Becke, J. Chem. Phys. 98 (7), 5648 (1993).
4−
bonds with the
anion, and the observed douꢀ
ZrF8 ,
11. L. P. Otroshchenko, R. L. Davidovich, and V. I. Simoꢀ
nov, Koord. Khim. (9), 1416 (1978).
bling of the absorption bands in the stretching region
can be attributed to the presence of Hꢀbonds of water
molecules with fluorine atoms having different length.
4
12. L. P. Otroshchenko, V. I. Simonov, R. L. Davidovich,
et al., Kristallografiya 25 (4), 722 (1980).
Thus, it was established that the transition from the
magnesium hexafluorozirconate pentahydrate to
anhydrous MgZrF6 observed at temperatures of 50–
420°C involves the formation of intermediate MgZrF6
13. M. M. Godneva, D. L. Motov, V. Ya. Kuznetsov, et al.,
Zh. Neorg. Khim. 49 (7), 1198 (2004) [Russ. J. Inorg.
Chem. 49 (7), 1100 (2004)].
14. R. L. Davidovich, T. F. Levchishina, and S. B. Ivanov,
Izv. Akad. Nauk SSSR, Ser. Neorg. Mater. 11 (12),
2180 (1975).
⋅
2H2O phase stable below 150°C. Above 420°C,
MgZrF6 undergoes highꢀtemperature hydrolysis that
occurs according to the acid type and involves the catꢀ
ion.
15. A. P. Chupakhin, A. A. Sidel’nikov, and V. V. Boldyrev,
Izv. Sib. Otd. Akad. Nauk SSSR, No. 12, 12 (1981).
Analysis of the IR and Raman spectra provides
information about the structural features of the comꢀ
pounds. According to IR and Raman spectral data, the
principal vibration bands of the functional groups are
16. V. Propach and F. Steffens, Z. Naturforsch., B: Chem.
Sci. 33, 268 (1978).
17. P. Kohl, D. Reinen, and G. Decher, Z. Kristallogr. 153
(3/4), 211 (1980).
comparable for MgZrF6
⋅
5H2O and MgZrF6 2H2O and
⋅
differ from those for MgZrF6. This is indicative of the
similar features (common structural units) in the latꢀ
tices of the crystal hydrates and a change in the spatial
configuration and Zr coordination number on going
to the anhydrous compound.
18. K. Nakamoto, Infrared and Raman Spectra of Inorganic
and Coordination Compounds (Wiley, New York, 1986;
Mir, Moscow, 1991).
19. P. W. Smith, R. Stoessiger, and A. G. Turnbull,
J. Chem. Soc. A, No. 12, 3013 (1968).
20. V. Dracopoulos, J. Vagelatos, and G. N. Papatheꢀ
odorou, J. Chem. Soc., Dalton Trans., No. 7, 1117
(2001).
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ
tion for Basic Research (project no. 08ꢀ03ꢀ00355).
21. Y. Kawamoto and F. Sakaguchi, Bull. Chem. Soc. Jpn.
56 (7), 2138 (1983).
22. C. C. Pye and W. W. Rudolph, J. Phys. Chem. A 102
(48), 9933 (1998).
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