Interaction, composition, and optical properties of the MgF 2(MgO)-EuF3 system

Article abstract of DOI:10.1134/S0036023607060186

The interaction between MgO and EuF₃ resulting in the formation of magnesium fluoride and europium(III) oxyfluorides of variable composition are investigated. A substantial difference between the diffuse reflection spectra of the fluoride phase and oxyfluoride phases of europium(III) is found, specifically, an intense band peaked at 260-270 nm appears in the latter. The effect of silicon and high vacuum as reducers on the systems containing the oxide phase is investigated. For these phases, europium(II) compounds are found, which is confirmed by X-ray powder diffraction analysis and diffuse reflection and luminescence spectroscopy. The character of thermogravimetric curves for the MgF₂-EuF₃-Si and MgO-EuF₃-Si systems is fundamentally different, which allows us to propose a new method for estimating the MgO content in MgF₂. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S0036023607060186

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2007, Vol. 52, No. 6, pp. 927–932. © Pleiades Publishing, Inc., 2007.
Original Russian Text © E.V. Timukhin, V.F. Zinchenko, O.G. Eremin, I.P. Kovalevskaya, Z.M. Topilova, 2007, published in Zhurnal Neorganicheskoi Khimii, 2007, Vol. 52, No. 6,
pp. 999–1004.
PHYSICOCHEMICAL ANALYSIS
OF INORGANIC SYSTEMS
Interaction, Composition, and Optical Properties
of the MgF2(MgO)–EuF3 System
E. V. Timukhin, V. F. Zinchenko, O. G. Eremin, I. P. Kovalevskaya, and Z. M. Topilova
Bogatsky Physicochemical Institute, National Academy of Sciences of Ukraine,
Lustdorfskaya doroga 86, Odessa, 650080 Ukraine
Received December 12, 2005
Abstract—The interaction between MgO and EuF3 resulting in the formation of magnesium fluoride and
europium(III) oxyfluorides of variable composition are investigated. A substantial difference between the dif-
fuse reflection spectra of the fluoride phase and oxyfluoride phases of europium(III) is found, specifically, an
intense band peaked at 260–270 nm appears in the latter. The effect of silicon and high vacuum as reducers on
the systems containing the oxide phase is investigated. For these phases, europium(II) compounds are found,
which is confirmed by X-ray powder diffraction analysis and diffuse reflection and luminescence spectroscopy.
The character of thermogravimetric curves for the MgF2–EuF3–Si and MgO–EuF3–Si systems is fundamentally
different, which allows us to propose a new method for estimating the MgO content in MgF2.
DOI: 10.1134/S0036023607060186
EXPERIMENTAL
oxyfluorides of various compositions (EuOF, Eu6O5F8,
Eu4O3F6, Eu(O, F)2.35, etc.).
Magnesium fluoride is obtainable by fluorination of
magnesium carbonate (basic carbonate) with hydroflu-
oric acid (ammonium hydrofluoride) [1]. Due to incom-
plete fluorination after calcination or fusion, MgO is
formed as the major impurity to MgF2, which substan-
tially worsens the performance of both the starting sub-
stance and coatings. Therefore, monitoring of the MgO
content in MgF2-based materials is a very topical prob-
lem. By now, the most widespread monitoring method
is X-ray powder diffraction analysis. However, its sen-
sitivity is insufficient.
Samples of the MgF2–EuF3 system were obtained
by the following two methods: (i) by coprecipitation
(CP) from solution with the subsequent thermal treat-
ment and (ii) by sintering of individual fluorides, or via
solid-phase synthesis (SPS). In the case of coprecipita-
tion from solution, MgO and Eu2O3 of analytical grade
were used. They were dissolved in glassy-carbon ves-
sels in concentrated nitric acid of chemically purity
grade. The solution was evaporated to wet salts, and
water was added. The precipitation was induced by
addition of concentrated hydrofluoric acid of chemi-
cally purity grade, and the mixture was concentrated
We previously [2, 3] used NdF3 and LuF3 as dopants
for magnesium fluoride. It was found that during evap- until the vapor of hydrofluoric acid appeared. Then,
ammonium fluoride of analytical grade was added, and
the mixture was vacuum-dried at 500°ë. Then, the
samples were calcined in alundum crucibles at various
temperatures (750–950°ë) under helium. For sintering,
fused magnesium fluoride (produced by SNPP “Novye
Materialy i Tekhnologii”), and europium fluoride were
used. We synthesized the latter according to the proce-
dure described in [4]. According to X-ray powder dif-
fraction data, both products contained no phase admix-
tures. The starting substances were stirred in an agate
mortar with alcohol, dried in an oven at 100°ë, and cal-
cined under helium at the above-mentioned tempera-
tures. Samples of the systems with magnesium oxide
and europium fluoride were prepared similarly. Some
of the systems were subjected to reduction with ele-
mental silicon of high purity grade under the conditions
oration of the mentioned compositions, oxygen is redis-
tributed between MgO and NdF3 (LuF3) to form oxyflu-
orides with the composition LnOF. However, taking
into account the properties of the starting lanthanide
oxyfluorides, it seemed to be pertinent to use EuF3 as
the dopant: it possesses the best optical and perfor-
mance properties among the LnF3 compounds in the
individual state [4, 5]. In addition, the ability of
europium to vary its valence state can favor the stabili-
zation of the MgF2 stoichiometry. According to calcu-
lations [6], no interaction is observed between MgF2
and EuF3 up to temperatures above 1000°ë, and the
phase diagram of the system is of the eutectic type.
However, by analogy with the above-described sys-
tems, MgO and EuF3 can react to form europium oxy-
fluorides. According to the data [7], europium forms described in [8]. The chemical and phase compositions
927

928
TIMUKHIN et al.
Chemical and phase composition of fluoride--oxide systems of magnesium(II), europium(II), and europium(III)
Sample
no.
System
Synthetic method and parameters
Phase composition of products
(europium fluoride content) (atmosphere, thermal treatment schedule)
1
MgO–EuF3 (50 mol %)
MgO–EuF3 (55 mol%)
SPS (He, 850–900°C)
SPS (He, 850–900°C)
Eu6O5F8 (orthorhomb.)
EuOF (cubic)
MgF2 (tetragon.)
2
Eu6O5F8 (orthorhomb.)
MgF2(tetragon.)
3
4
Eu2O3–EuF3 (50 mol %)
MgF2–EuF3 (30 wt %)
SPS (He, 850–900°C)
Eu6O5F8 (orthorhomb.)
ëé (vacuum, 400–500°C)
MgF2 (tetragon.)
EuF3 (orthorhomb.)
MgO (cubic)
5
6
7
8
9
MgF2–EuF3 (30 wt %)
MgF2–EuF3 (50 mol %)
MgF2–EuF3 (10 wt %)
MgF2–EuF3 (10 wt %)
MgF2–EuF3 (5 wt %)
CO (He, 850–900°C)
CO (He, 850–900°C)
CO (He, 750–800°C)
SPS (He, 850–900°C)
SPS (He, 850–900°C)
MgF2 (tetragon.)
EuF3 (orthorhomb.)
Eu6O5F8 (orthorhomb.)
EuF3 (orthorhomb.)
MgF2 (tetragon.)
Eu(O,F)2.35 (Eu3O2F5) (monoclinic)
MgF2 (tetragon.)
EuOF (cubic)
MgO (traces)
MgF2 (tetragon.)
EuF3 (orthorhomb.)
EuF2 + x (cubic)
MgF2 (tetragon.)
EuF3 (orthorhomb.)
EuF2 + x (cubic)
10
11
MgF2–EuF3 (50 mol %)
MgF2–EuF3 (10 wt %)
ëP (deep vacuum, 1000–1500°C)
ëP (deep vacuum, 1000–1500°ë)
EuF2 + x (cubic)
MgF2 (tetragon.)
MgF2 (tetragon.)
EuOF (tetragon.)
EuF2 + x (cubic) MgO (cubic)
12
13
MgF2–EuF3 (50 mol %)
MgO–EuF3 (50 mol %)
CP, Si (He, 900–950°C)
SPS, Si (He, 900–950°C)
EuF2 + x (cubic)
MgF2 (tetragon.)
EuF2 + x (cubic)
x-phase
of the systems, as well as the synthetic conditions, are curve as an exotherm (curve 1). This exotherm can be
caused by two sequential reactions
MgO + 2/3EuF3 = MgF2 + 1/3Eu2O3,
1/3EuF3 + 1/3Eu2O3 = EuOF.
listed in the table.
(1)
(2)
The interaction mechanism was investigated by dif-
ferential thermal analysis (DTA) and thermogravimet-
ric analysis (TGA). The composition of synthetic prod-
ucts was identified by X-ray powder diffraction, diffuse
reflection spectroscopy, and luminescence analysis.
The DTA curve of the mixture of europium oxide
and europium fluoride (curve 2) shows no specific fea-
tures, excluding the peak height, compared to the DTA
curve for europium fluoride (curve 3). Therefore, the
above-mentioned exotherm should be assigned to reac-
tion (1). This assignment is confirmed by calculations
°
RESULTS AND DISCUSSION
It follows from Fig. 1 that EuF3 and MgO start to
react at 770–800°ë, which manifests itself in the DTA
using the data in [9, 10], which give the value ∆H298, p
≈
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 6 2007

INTERACTION, COMPOSITION, AND OPTICAL PROPERTIES
929
1
770°C
3
3
790°C
765°C
770°C
770°C
2
1
690°C
4
2
875°C
Fig. 2. TGA curves: (1) integrated TGA curve for sample
12, (2) DTA curve for sample 12, (3) integrated TGA curve
for sample 13, and (4) DTA curve for sample 13.
Fig. 1. DTA curves for (1) sample 1, (2) sample 3, and (3)
EuF .
3
–17 kJ/mol. It follows from the X-ray powder diffrac-
tion data (table) that the reaction of EuF3 with MgO or
of EuF3 with Eu2O3 yields oxyfluorides with the com-
position differing from EuOF. This is possibly due to
the incompleteness of the reactions or the easiness of
phase transitions between various oxyfluorides [7].
EuF3 = EuF2 + 1/2 F2↑,
(3)
(4)
EuF2 + x/3EuF3 = EuF2 + x
.
The possibility of similar reactions at high tempera-
tures was pointed out in [11]. Much stronger changes
are observed upon the high-temperature treatment in
deep vacuum. In the sample with a considerable
amount of EuF3 (sample 10), oxygenated phases and
the EuF3 phase are removed virtually completely. This
is apparently caused by the reaction
The formation of oxyfluorides with various compo-
sitions of starting components in the systems listed in
the table (samples 1, 2) and at various temperatures is
confirmed by X-ray powder diffraction data (table). It is
noteworthy that the samples obtained by coprecipita-
tion from aqueous solutions contain a considerable
amount of oxygenated phases. After the low-tempera-
ture treatment (sample 4), this is mainly MgO, and after
the high-temperature treatment (samples 5–7), these
are europium oxyfluorides. In the samples of the
MgF2−EuF3 system (samples 8, 9) prepared by solid-
phase synthesis, no oxygenated phases were found by
X-ray powder diffraction. It seems likely that hydrated
products are formed during the coprecipitation, and
then these products are hydrolyzed under the high-tem-
perature treatment. It is interesting that the samples 8
and 9, in addition to the starting substances, contain a
minor cubic phase EuF2 + x. This minor phase can result
from the reactions
EuF3 + Eu3O2F5 = 4EuF2 + O2↑
(5)
followed by the reaction according to scheme (4). How-
ever, for the sample with a small amount of EuF3 (sam-
ple 12), oxygen-containing phases are not completely
removed.
The reduction of compounds of Eu(III) was mod-
eled by the addition of elemental silicon. The DTA
curves for the MgF2–EuF3 system doped with silicon
show a pattern characteristic of the reduction of
europium(III) fluoride (Fig. 2; curves 1, 3), which is
similar to that given in [8]. The well pronounced endot-
herm in the DTA curve and weight loss in the TG curve
are caused by the formation of a volatile product (SiF4):
4EuF3 + (4MgF2) + Si = 4EuF2 + (4MgF2) + SiF4↑. (6)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 52 No. 6 2007

930
TIMUKHIN et al.
F(R)
(‡)
(b)
(c)
1
0.75
0.03
0.02
0.01
0
0.15
2
2
2
0.25
0
0.05
0
1
1
1600
2000
2400
λ, nm
250 300 350 400
400 500 600 700 800
Fig. 3. Diffuse reflection spectra in (a) UV, (b) visible, and (c) near-IR spectral ranges for (1) starting mixture MgO–EuF
(50 mol %) and (2) the same after sintering at 900°C (sample 1).
3
The distinction lies in a possible formation of the teristic of europium(III) oxide. Certain distinctions in
compound of magnesium fluoride with EuF2, which is
formed due to reduction, by the scheme
intensity and location of the bands are retained.
The comparison of diffuse reflection spectra for the
samples 10, 12, and 13 (Fig. 4) shows the bands char-
acteristic of 4f–5d electron transitions in the
europium(II) ion. Distinctions in the diffuseness of
these bands are apparently associated with various con-
EuF2 + MgF2 = EuMgF4.
(7)
In the system containing magnesium oxide (sample
13), a series of characteristic exotherms appears in the
DTA curve. The interaction in the system MgO–EuF3–Si
starts at somewhat lower temperature than in the above-
described system. This is apparently caused by the exo-
thermic character of the interaction in the system
MgO–EuF3–Si. The absence of weight loss (Fig. 2,
curve 3) and the weakness of the endotherm (curve 4)
indicate that no volatile components are formed, and
the process can be described by the scheme
tributions of the charge transfer Eu(II)
Eu(III) to
the general spectral pattern. The appearance the absorp-
tion peaks of the europium(III) ions in the near-IR
range is caused by incomplete reduction of
europium(III). A noticeable elevation of the spectrum
(“background”) in the IR range for sample 10 is appar-
ently caused by its contamination with the evaporator
material.
4MgO + 4EuF3 + Si = 4MgF2 + 2EuF2 + Eu2SiO4. (8)
The luminescence spectrum of sample 1 in the red
spectral region (Fig. 5a), which is caused by the emis-
The diffuse reflection spectra of the MgO–EuF3 sys-
tem in the starting state and after the thermal treatment sion of trivalent europium, resembles the spectrum of
(sample 1) differ substantially (Fig. 3). In the UV spec- oxyfluoride EuOF obtained in [13]. The distinctions in
tral range of the calcined sample, a broad band appears the character of these spectra are most probably caused
in the region 260–270 nm. This band is most likely by distinctions in the phase compositions of the com-
caused by the charge transfer é2–
europium oxyfluorides. In the visible range, an abrupt
Eu(III) in
pounds. In the blue spectral region (Fig. 5b), which is
characteristic of europium(II) ions (λmax ≈ 440 nm),
increase in intensity is observed for the peaks attributed low-intense, structureless band is observed for sample 1.
to the intracenter electron transitions 4f–4f in the Its appearance, somewhat unexpected, is caused by par-
europium(III) ion. In the near IR range, the spectrum tial reduction of europium(III) to Eu(II) (see Eq. (5)).
characteristic of europium fluoride is noticeably dis- For the reduced sample (sample 13), no luminescence
torted [12]. The spectrum acquires the pattern charac- is virtually observed in the red spectral region, while
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INTERACTION, COMPOSITION, AND OPTICAL PROPERTIES
931
F(R)
(‡)
(b)
1
4
3
2
1
0
0.75
1
3
0.25
2
2
3
250
300
350
400
1600
2000
2400
λ, nm
Fig. 4. Diffuse reflection spectra in (a) UV range and (b) near-IR spectral ranges for (1) sample 10, (2) sample 12, and (3) sample 13.
the characteristic high-intense emission band appears
in the blue region.
I
2
(b)
(‡)
1
In view of the fundamentally different character of
EuF3 reduction in the presence and in the absence of
MgO, we proposed a principal approach to the estima-
tion of the magnesium oxide content in MgF2. We mod-
eled a sample containing 2 wt % MgO and added an
excessive amount of a 4EuF3 + Si mixture relative to the
amount calculated according to scheme (8). The whole
mixture was heat-treated under helium in the alundum
crucible for 1 h. We revealed weight loss that corre-
sponded, according to schemes (6) and (8), to 2.37%
MgO. Some distinction between the introduced and
calculated amounts of MgO is apparently due to the
presence of the oxide phase in starting MgF2. This
approach will further allow us to develop a method for
the quantitative determination of oxygenated impurities
in magnesium fluoride, which is a very important tech-
nological problem.
1
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