Synthesis of zirconium (hafnium) fluoride compounds and their X-ray luminescence properties

Article abstract of DOI:10.1134/S0036023607050026

Twenty-five zirconium (hafnium) fluoride compounds have been synthesized at room temperature in the systems MO₂-H₂SO ₄-M′_nA(HF)-H₂O (M = Zr (Hf); M′ = Na, K, Rb, Cs, NH₄; A = F, SO₄) and their X-ray luminescence spectra (luminescence wavelengths and relative intensities) have been measured. The X-ray luminescence of the compounds has been considered as a function of the composition (cations, anions, water content) and different structural factors (CNs, polyhedra, H-bonds). Ammonium compounds do not luminesce, and sodium fluorozirconates and heptafluorozirconates are weakly luminescing. Hexafluorozirconates M₂Zr(Hf)F₆ (M= K, Rb, Cs) and M₅Zr₄F₂₁ ? 3H₂O (M = Rb, Cs), as well as oxofluorozirconate Rb₂Zr₃OF ₁₂, are strongly luminescing compounds. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607050026

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 5, pp. 661–666. © Pleiades Publishing, Inc., 2007.
Original Russian Text © M.M. Godneva, D.L. Motov, N.N. Boroznovskaya, V.M. Klimkin, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 5,
pp. 725−730.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis of Zirconium (Hafnium) Fluoride Compounds
and Their X-ray Luminescence Properties
M. M. Godnevaa, D. L. Motova, N. N. Boroznovskayab, and V. M. Klimkinc
a Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Research Center,
Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk oblast, 184209 Russia
b Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia
c Institute of Atmosphere Optics, Siberian Division, Russian Academy of Sciences,
Akademicheskii pr. 1, Tomsk, 634055 Russia
Received April 18, 2006
Abstract—-Twenty-five zirconium (hafnium) fluoride compounds have been synthesized at room temperature
'
in the systems MO2–H2SO4–Mn A(HF)–H2O (M = Zr (Hf); M' = Na, K, Rb, Cs, NH4; A = F, SO4) and their
X-ray luminescence spectra (luminescence wavelengths and relative intensities) have been measured. The
X-ray luminescence of the compounds has been considered as a function of the composition (cations, anions,
water content) and different structural factors (CNs, polyhedra, H-bonds). Ammonium compounds do not lumi-
nesce, and sodium fluorozirconates and heptafluorozirconates are weakly luminescing. Hexafluorozirconates
M2Zr(Hf)F6 (M= K, Rb, Cs) and M5Zr4F21 · 3H2O (M = Rb, Cs), as well as oxofluorozirconate Rb2Zr3OF12,
are strongly luminescing compounds.
DOI: 10.1134/S0036023607050026
The luminescence properties of compounds are tered off, washed with water and alcohol, and identified
among their most important characteristics, which by the chemical analysis and X-ray powder diffraction
reflect their composition and structure [1]. X-ray lumi- methods.
nescence of compounds is widely used for the creation
The X-ray luminescence spectra were recorded on a
of phosphors [2–4]. In particular, halides, such as
homemade setup containing a spectrometer with a
BaFCl, NaI, KI, and CsCl [2, 3], are used for X-ray
DMR-4 double-prism monochromator. Luminescence
visualization. Many salts containing d-metal oxo
was excited by γ quanta from a BSV-2 dc X-ray tube (a
anions, such as tungstates, vanadates, tantalates, etc.,
URS-55 apparatus) with a molybdenum anticathode
exhibit strong X-ray-excited luminescence [4]. In some
operating at 25 kV and 10 mA. The luminescence inten-
silicates and oxides containing titanium subgroup ele-
sity and spectral composition were measured in the visi-
ments, X-ray luminescence of intrinsic defects has been
ble and UV ranges. The spectral range 200–700 nm was
documented [1]. In the present work, we synthesized
covered by using FEU-79 and FEU-39 photomultipli-
zirconium (hafnium) fluoride compounds and studied
ers. Spectral scanning and calibration of wavelength
their X-ray luminescence spectra with the aim of
scale were performed automatically. The luminescence
revealing the relationship between their X-ray lumines-
intensities were measured with the use of internal refer-
cence properties and composition and structure.
ences, natural and synthetic minerals. In addition, the
X-ray luminescence spectrum of the standard phosphor
ZnS was used as a reference. The luminescence yield
was measured in arbitrary units (1 arb. unit is approxi-
mately equal to 1 nit).
EXPERIMENTAL
Compounds were synthesized at room temperature
by introducing alkali-metal or ammonium fluoride into
zirconium or hafnium sulfate solutions, the composi-
tion of mixtures being previously determined for each
compound [5]. The following reagents were used for
synthesis: Zr(SO4)2 · 4H2O (chemically pure), Hf(SO4)2
· 4H2O prepared by prolong treatment of HfO2 with sul-
furic acid followed by salting-out with sulfuric acid,
H2SO4 (chemically pure), KF · 2H2O (pure for analy-
sis), NH4F (pure for analysis), RbF (chemically pure),
and CsF (chemically pure). The reaction mixtures were
allowed to stand, and the resulting solid phases were fil-
RESULTS AND DISCUSSION
The following zirconium (hafnium) fluoride com-
1
pounds were isolated: Na5Zr2F13
,
β-Na2ZrF61,
Na7Zr6F31 , K3ZrF71, K2ZrF61, δ-KZrF51, γ-KZrF5, KZrF5 ·
H2O1, (NH4)3ZrF71, α-NH4ZrF51, Rb3ZrF71, Rb2ZrF61,
1
1
The composition was determined based on X-ray powder diffrac-
tion data.
661

662
GODNEVA et al.
Table 1. Chemical composition of compounds*
caused by different environments of zirconium and fluo-
rine. Previously [6], we assigned similar centers in flu-
oro- and fluorosulfatozirconates to the excited states F*
and O*, which agrees with the data on other halide com-
pounds [1].
Found, wt %
Calculated, wt %
MI ZrIV F–
Compound
γ-KZrF5
MI ZrIV F–
17.9 41.0 41.3 17.35 40.49 42.16
25.0 31.8 32.4 29.50 31.48 32.79
37.9 27.6 27.5 39.42 27.05 28.17
The variety of compounds synthesized makes it pos-
sible to establish some correlations between their lumi-
nescence properties, on the one hand, and their compo-
sition, structure, or crystal-chemical features, on the
other hand.
RbZrF5 · H2O
β-CsZrF5 · H2O
Cs2Zr3F14 · 1.5H2O 31.5 32.6 30.8 31.92 32.87 30.95
Rb2Zr3OF12
22.2 39.0 31.7 24.82 39.74 33.11
41.9 29.0 41.22 34.34
The fluorides NaF and CaF2 are characterized by F*
luminescence at λ = 290 nm [1]. The coordination of
MF to ZrF4 leads to a bathochromic shift of the lumi-
nescence maximum (310–390 nm) in the X-ray lumi-
nescence spectrum. The substitution of a heavier alkali
metal for a lighter one leads to the shift of the maximum
in the same direction (Fig. 1). Compounds with ZrF8
and HfF8 polyhedra (CN = 8) were found to luminesce
at 310–320 nm. Compounds with ZrF7 polyhedra
(CN = 7) either have no luminescence or have weak
luminescence at 390 nm. Compounds with ZrF6 poly-
hedra (CN = 6) luminesce at 350 nm.
β-ZrF4 · 3H2O
–
–
* The Fe and SO24– contents were no more than 0.04 and 2 wt %,
respectively.
RbZrF5 · H2O, Rb5Zr4F21 · 3H2O1, Cs2ZrF61, Cs5Zr4F21 ·
3H2O1, α-CsZrF5 · H2O1, β-CsZrF5 · H2O, Cs2Zr3F14
·
1
1.5H2O; oxofluorozirconates K2Zr3OF12 , Rb2Zr3OF12,
and β-CsZrOF3 · H2O; fluorohafnate K2HfF61 (Table 1).
The oxygen-containing compounds under consider-
ation exhibit X-ray luminescence in a wide spectral
range, 290–600 nm (Table 3, Figs. 2 and 3). All com-
pounds containing ZrF7O and ZrF7 coordination poly-
hedra with bridging bonds give rise to luminescence
The compounds under consideration are character-
ized by luminescence in the range 310–440 nm
(Tables 2 and 3). The X-ray luminescence spectra show
complicated bands, which is evidence either of differ-
ent electronic transitions for a given luminescence cen-
ter or of the presence of several luminescence centers maxima in a wide range of wavelengths, mainly with a
Table 2. Structural and X-ray luminescence characteristics of zirconium (hafnium) fluoride compounds
Types
of bridging
bonds
Luminescence maximum
Polyhe-
dron*
Average M–F distances,
Å**
Compound
K3ZrF7 [7]
CNZr
λ, nm
intensity, arb. units
ZrF7 (7)
7
7
7
8
6
6
8
–
–
None
0.006
0.017
0.510
0.600
0.460
–
Na5Zr2F13 [8] ZrF7 (6)
A
2.35
2.49
2.90
3.04
3.18
–
–
β-Na2ZrF6 [9] ZrF7 (5)
B
390
320
350
350
–
K2ZrF6 [10]
ZrF8 (4)
B
Rb2ZrF6 [11] ZrF6 (6)
Cs2ZrF6 [11] ZrF6 (6)
–
–
A, B
KZrF5 [12]
γ-KZrF5
δ-KZrF5
ZrF8 (2)
HfF8 (4)
Data are absent
Data are absent
B
330
360
310
0.275
0.065
0.330
K2HfF6 [13]
8
2.85
F
Notes: Here and in Table 3, the following designations are used for the types of bridging bonds: A, Zr–F–Zr; B,
F
,
Zr
Zr
F
C,
, D,
.
Zr
Zr
Zr
O
Zr
O
Zr
* The number of terminal fluorine atoms is parenthesized.
** M is an alkali metal.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 5 2007

SYNTHESIS OF ZIRCONIUM (HAFNIUM) FLUORIDE COMPOUNDS
663
Table 3. Structural and X-ray luminescence characteristics of zirconium (hafnium) compounds with different ligands
Luminescence maximum
Types
Compound
Polyhedron
CNZr
7, 8
of bridging bonds
λ, nm
intensity, arb. units
Rb5Zr4F21 · 3H2O [14, 15] ZrF7 (5),
B
430
0.030
0.007
ZrF7O (3),
ZrF6O (4)
Cs5Zr4F21 · 3H2O* [15]
ZrF7 (5),
ZrF7O (3),
ZrF6O (4)
7, 8
8
B
420
KZrF5 · H2O [16]
ZrF7O (3)
A, B
B
310
380
410
–
0.028
0.090
0.046
0.003
0.015
0.013
0.650
0.040
0.007
RbZrF5 · H2O
Data are absent
α-CsZrF5 · H2O [17]
β-CsZrF5 · H2O*
ZrF7O (3)
8
Data are absent
Cs2Zr3F14 · 1.5H2O
K2Zr3OF12* [18, 19]
Rb2Zr3OF12 [18, 19]
α-ZrF4 · 3H2O [20, 21]
β-ZrF4 · 3H2O* [21, 22]
Data are absent
440
–
ZrF7O (1)
ZrF7O (1)
Zr2F8O6 (6)
ZrF6O2 (2)
8
8
8
8
A, B, C, D
A, B, C, D
330
280
400
B
B
* Very weak broad luminescence bands without pronounced peaks.
shift to a long-wavelength region. The simultaneous fact that the compound has four different bridges,
presence of oxygen and fluorine atoms in the lattice which leads to easier excitation of F* and O*. In addi-
increases the luminescence range; however, the lumi- tion, the longer Rb–F bond can also favor luminescence
nescence intensity of hydrated fluoride compounds not excitation. This oxofluorozirconate is isostructural with
subjected to heat treatment is considerably lower than the analogous thallium compound, with channels in the
that of anhydrous compounds, evidently due to the fact structure. The existence of such channels implies the
that water is coordinated to zirconium and fluorine is possibility of isomorphous substitution, which allows
involved in a system of hydrogen bonds preventing the one to improve the luminescence properties of the com-
formation of F* luminescence centers.
pound by doping.
The fact that it exhibits strong X-ray luminescence
makes anhydrous Rb2Zr3OF12 an exception among oxy-
gen-containing compounds. This is likely due to the luminescence properties is observed: the ammonium
For compounds with the same composition of the
anion or polyhedron, the effect of the cation on the
Luminescence intensity, arb. units
0.70
Luminescence intensity, arb. units
0.10
2
0.35
0
3
0.05
0
1
4
200
300
400
500
600
λ, nm
200
300
400
λ, nm
Fig. 1. UV range of the X-ray luminescence spectra of flu-
orometallates: (1) K ZrF , (2) Rb ZrF , (3) Cs ZrF , and
2
6
2
6
2
6
Fig. 2. X-ray luminescence spectrum of Rb Zr F · 3H O.
(4) K HfF .
5
4
21
2
2
6
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 5 2007

664
GODNEVA et al.
Luminescence intensity, arb. units.
0.70
Luminescence intensity, arb. units
0.10
0.05
0
3
0.35
1
4
2
0
200
300
400
500
600
λ, nm
200
300
400
500
600
λ, nm
Fig. 3. Luminescence as a function of the composition of the cationic sublattice in fluorides with the same anions: (1) Rb Zr OF ,
2
3
12
(2) K Zr OF , (3) RbZrF · H O, and (4) KZrF · H O.
2
3
12
5
2
5
2
derivatives (NH+4 ((NH4)3ZrF7, α-NH4ZrF5)) do not
We found that the polymorphs of compounds differ
in both the luminescence range and intensity (Fig. 4).
Significant differences in X-ray luminescence between
α- and β-ZrF4 · 3H2O can be due to their structural fea-
tures, exemplified by HfF4 · 3H2O [21], isostructural
with β-ZrF4 · 3H2O: α modification is composed of dis-
crete dimers Zr2F8O6, each of which contributes to the
X-ray luminescence, whereas β-ZrF4 · 3H2O is com-
posed of infinite chains linked with one another by
H-bonds of water molecules, which are not incorpo-
rated in ZrF6O2 units and quench the X-ray lumines-
cence of a sample. It is likely that similar differences in
X-ray luminescence between hydrates α- and β-CsZrF5 ·
H2O are due to the role of water molecules. For the
luminesce, the sodium compounds (Na5Zr2F13,
Na2ZrF6, Na7Zr6F31) exhibit weak luminescence,
whereas the potassium, rubidium, and cesium deriva-
tives have better luminescence properties. The follow-
ing trends are observed for the luminescence intensity:
å2ZrF6: Na < K(Rb) > Cs
åZrF5: H2O K < Rb > Cs
M2Zr3OF12: K < Rb,
The last two compounds are isostructural; thus, they
obey the rule of higher luminescence intensity for
heavier cations.
Luminescence intensity, arb. units
0.30
0.050
0.050
0.15
0.025
0.025
1
3
5
2
4
6
0
200
0
0
300
400
500
600
λ, nm
200
300
400
500
300
400
500
600
Fig. 4. X-ray luminescence spectra of polymorphs of zirconium fluoride compounds: (1) γ-KZrF , (2) δ-KZrF , (3) α-ësZrF ·
5
5
5
H O, (4) β-CsZrF · H O, (5) α-ZrF · 3H O, and (6) β- ZrF · 3H O.
2
5
2
4
2
4
2
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SYNTHESIS OF ZIRCONIUM (HAFNIUM) FLUORIDE COMPOUNDS
665
Table 4. X-ray luminescence characteristics of anhydrous
Our findings show that, for the same anion, the lumi-
nescence intensity increases in the series Na
Cs Rb. For the same cation, the luminescence
intensity builds up with an increase in the number of
MF molecules coordinated to ZrF4: it passes through a
maximum in hexafluorozirconates and vanishes in hep-
tafluorozirconates. The presence of coordinated and
crystal water decreases the luminescence intensity and
shifts its maximum toward longer wavelengths.
No dependence of luminescence on the CN is
observed. For the same polyhedron (ZrF7), the lumines-
cence intensity increases with a decrease in the number
of terminal fluorine atoms or with an increase in the
number of bridging bonds. Hydrogen bonds suppress
the luminescence intensity. The absence of hydrogen
bonds and the presence of bridging fluorine atoms and
only one terminal fluorine atom in the zirconium poly-
hedron in Rb2Zr3OF12 are responsible for its strong
luminescence.
zirconium (hafnium) compounds*
K
Luminescence maximum
Initial compound
λ, nm
intensity, arb. units
Rb5Zr4F21 · 3H2O
Cs5Zr4F21 · 3H2O
KZrF5 · H2O
340
360
0.230
0.265
0.425
0.175
0.008
0.030
0.018
1.100
0.138
0.022
0.115
340
RbZrF5 · H2O
α-CsZrF5 · H2O
β-CsZrF5 · H2O
K2Zr3OF12
350
370–400
360
350–360
340–350
365
Rb2Zr3OF12
β-CsZrOF3 · H2O**
α-ZrF4 · 3H2O
β-ZrF4 · 3H2O*
200–250
370
ACKNOWLEDGMENTS
Notes: * After dehydration of the initial compound at 280°C.
** The compound was heated to 600°C.
This work was supported by the Presidium of the
RAS in the framework of the project “Fundamental Sci-
ences—Their Contribution to Medicine.”
anhydrous modifications δ- and γ-KZrF5, the difference
in X-ray luminescence is presumably caused by other
factors, for example, by the CN or the mode of connec-
tion of polyhedra.
Analogous hafnium and zirconium compounds
exhibit virtually the same luminescence intensity, the
luminescence band for hafnium being shifted toward
shorter wavelengths.
Analysis of the X-ray luminescence of zirconium
(hafnium) fluoride compounds allows us to conclude
that it considerably depends on the fluorine–alkali
metal bond length: the longer the bond, the higher the
probability of F* centers forming. For the same anion,
this distance decreases with the radius of the cation. It
is evident that, for smaller cations, this bond is stronger
and the formation of luminescence centers is more
impeded. Among Na, K, Rb, and Cs fluorozirconates
with the same complex anion, the strongest lumines-
cence is expected for the Rb and Cs compounds. As for
the O* excited states, the strongest X-ray luminescence
due to these centers should be observed for oxofluo-
rozirconates. For the hydrated fluoride compounds in
which water is coordinated to zirconium and fluorine is
involved in hydrogen bonding, luminescence intensi-
ties comparable with those of anhydrous fluorides are
unattainable without breaking of these bonds.
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