Observation of chemical reactions between alkaline-earth oxides and tungsten at high pressure and high temperature

Article abstract of DOI:10.1016/j.jpcs.2009.06.009

The potential chemical reactions of alkaline-earth oxides (AeO with Ae: Mg, Ca, Sr, and Ba) and tungsten are studied at high pressure and high temperature. At pressures ranging from 5 to 10 GPa and temperatures of 2000 K, a noticeable reaction between AeO

Full text of DOI:10.1016/j.jpcs.2009.06.009

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Journal of Physics and Chemistry of Solids 70 (2009) 1117–1120
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Observation of chemical reactions between alkaline-earth oxides and
tungsten at high pressure and high temperature
Daniel Errandonea
MALTA Consolider Team, Departamento de F ´ı sica Aplicada-ICMUV, Fundaci o´ n General de la Universidad de Valencia, C/Dr. Moliner 50, 46100 Burjassot, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The potential chemical reactions of alkaline-earth oxides (AeO with Ae: Mg, Ca, Sr, and Ba) and tungsten
are studied at high pressure and high temperature. At pressures ranging from 5 to 10 GPa and
temperatures of 2000 K, a noticeable reaction between AeO and powder tungsten (W) was detected.
Received 19 January 2009
Received in revised form
3
June 2009
4
As a product of the reaction, scheelite-structured orthotungstates (AeWO ) were formed. The reactivity
Accepted 16 June 2009
of alkaline-earth oxides with tungsten increases in the order CaoSroBa, being the reaction not
detected for MgO. Possible chemical reactions leading to the formation of alkaline-earth orthotung-
states have been considered. Our results support the conclusion that the most probable reaction
PACS:
6
8
8
2.50.ꢀp
1.05.Je
2.33.Pt
occurring under high-pressure and high-temperature conditions is AeO+W+3/2 O
2 4
-AeWO .
&
2009 Elsevier Ltd. All rights reserved.
Keywords:
A. Ceramics
A. Inorganic compounds
C. High pressure
1
. Introduction
sical and geochemical studies. Further, CaO and MgO have been
frequently used as pressure-transmitting media in HP–HT experi-
ments [11,12] and it is therefore very important to know whether
they can chemically react. Finally, the presented results will also
help to improve the understanding of the role of pressure in
The study of alkaline-earth oxides (AeO) is of great interest for
many reasons. These oxides have long been considered a typical
case for understanding bonding in ionic oxides [1] and are also
one of the most fundamental refractory ceramics for industrial
applications [2]. Furthermore, they are major constituents of the
Earth’s lower mantle [3]. Because of these motives, the high-
pressure–high-temperature (HP–HT) phase diagram of AeO has
been extensively studied [4–10]. In particular, the melting curve of
MgO has been measured up to 40 GPa and 4000 K [4] and
the pressure effects on bonding have also been studied [6].
In addition, phase transitions have been detected in BaO, SrO, and
CaO at pressures ranging from 10 to 60 GPa [8–10]. The ambient-
pressure NaCl-type structure of these oxides transforms upon
compression in a CsCl-type structure. On the other hand, MgO is
predicted to remain in NaCl-type phase beyond 200 GPa [7].
In spite of all these studies, the possible existence of chemical
reactions between AeO and metals at HP–HT has not been
explored yet. In the present contribution, our attention is focused
on the chemical reactions involving AeO and transition metals
synthesizing some functional materials like scintillating CaWO
4
.
2. Experimental details
Micron-sized magnesium oxide (MgO), calcium oxide (CaO),
strontium oxide (SrO), and barium oxide (BaO) powders (Alfa
Aesar, purity 499.5%) were loaded in a diamond-anvil cell (DAC)
together with a 10-
tungsten (W) of about 50
placed in the approximate center of a 150
W gasket that was pre-indented to a thickness of 30
m
m-thick pre-compressed disk of powder
m in diameter. The W disks were
m diameter hole of a
m. They
m
m
m
were surrounded in the top, bottom, and the sides by AeO powder.
The W:AeO molar ratio in the experiments was approximately
1
:10. The loading of the samples was done in air and prior to
pressurizing the DAC was not treated to eliminate possible H O,
, or N absorbed by AeO powders [12,13]. Once loaded the DAC,
pressure was increased up to 5 (BaO), 7 (CaO and SrO), and 10 GPa
MgO). No pressure medium was used in order to reduce the
2
O
2
2
(in particular tungsten). The triggering of reactions by the
combination of HP–HT conditions that we observed can be
relevant for both the industrial applications of AeO and geophy-
(
potential chemical reactions that could take place under HP–HT
conditions. The pressure was determined by the ruby fluorescence
technique [14]. Under the above described pressure conditions,
E-mail address: daniel.errandonea@uv.es
0
022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2009.06.009

ARTICLE IN PRESS
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D. Errandonea / Journal of Physics and Chemistry of Solids 70 (2009) 1117–1120
the W disk was laser heated [12,15,16] for approximately 10 min
using a Nd:YLF laser (Quantronix, 25 W, TEM00 mode,
¼ 1.053
m) up to 20007200 K, a temperature well below the melting
point of W [17,18]. The temperature was determined by fitting a
Planck radiation function to the emission spectrum of W,
assuming that the emissivity is independent of the wavelength
3. Results and discussion
l
m
Within the pressure range of our experiments, the AeO has a
NaCl-type crystalline structure [7–10] and W has a bcc structure
[20]. Both materials are therefore Raman inactive. After the
compression of the sample, observations under an optical
microscope showed that the AeO had a semi-transparent whitish
color and the W foil has the typical dark-grey color. The W disk
was then heated for about 10 min to 20007200 K. During the first
minutes of the heating treatment fluctuations in the thermal
emission of the W disk were observed. Subsequent heating leaded
to a runaway heating effect. These facts suggest the possible
occurrence of a chemical reaction between AeO and W. After
temperature quenching, examination of the sample under an
optical microscope showed that the color of the tungsten surface
had become white. A darkening of the AeO was also observed.
Raman spectra taken from the recovered samples at ambient
pressure and room temperature revealed up to eleven Raman
[
12,15,16]. The starting material and the product of the HP–HT
treatment were examined at ambient conditions by Raman
spectroscopy and X-ray diffraction. Raman measurements were
performed in the backscattering geometry using the 488 nm line
+
of an Ar ion laser with a power of less than 100 mW before the
DAC using a set-up similar to the one described in Ref. [19].
Powder X-ray diffraction experiments were carried out at the I15
beamline of the Diamond Light Source using an incident
˚
monochromatic wavelength of 0.6149 A and a MAR345 image
plate detector located at 423 mm from the sample. The X-ray
beam was focused down to 30 ꢁ 30
mm using Kickpatrick–Baez
mirrors. A pinhole placed before the sample position was used as a
clean-up aperture for filtering out the tail of the focused X-ray
beam. Therefore we do exclude a possible contribution from the
W gasket in the measured diffraction pattern.
peaks. Since the AeO and
W are Raman inactive in the
pressure–temperature range covered in our experiments
[7–10,20] (no structural modifications occur in them), the
appearance of the Raman peaks can be only a consequence of a
chemical reaction triggered by the laser-heating treatment. The
rapid temperature increase observed a few minutes after the start
of the laser-heating treatment indicates that it might trigger an
exothermic chemical reaction. An interesting fact we observed
in our experiments is that the time it takes for the observed
reaction to be started decreases in the order Ca4Sr4Ba. In
addition, no chemical reaction was detected when W was heated
together with MgO during 10 min. This observation suggests that
the reactivity of the AeO and tungsten increases in the order
MgoCaoSroBa, which is in agreement with previous observa-
tions of Chekulaeva et al. [21].
BaO + W
SrO + W
CaO + W
Representative Raman spectra of the products obtained after
the heating treatment of the samples are shown in Fig. 1. After a
deep examination of the Raman spectra, we concluded that all the
peaks observed can be assigned to Raman-active phonons of
scheelite-structured alkaline-earth orthotungstates (e.g. CaWO
4
,
SrWO , and BaWO ). In order to better illustrate this fact, we
4
4
compare in Table 1 the frequencies reported in the literature for
these compounds [22] with the peaks we detected in our
0
200
400
600
800
1000
1200
-1
Raman Shift (cm )
measurements. Scheelite-type CaWO
thirteen Raman-active zone-center modes: 3A
these modes, one of them is very difficult to detect due to the
4
, SrWO
4
, and BaWO
4
have
g
+5B +5E . Among
g
g
Fig. 1. Raman spectra of the products obtained after the laser-heating treatment of
CaO+W, SrO+W, and BaO+W. The spectra were collected in a DAC after pressure
release at room temperature. The ticks indicate the peaks identified as Raman
ꢀ
1
overlap of Ag and Bg modes located near 330 cm [22]. The rest
of the modes, with the exception of one, coincide reasonably well
4 4 4
modes of CaWO , SrWO , and BaWO .
Table 1
Raman frequencies (
o) of alkaline-earth orthotungstates [22] compared with the peaks detected from the products of our experiments after the heating treatment.
Mode
BaWO
4
ꢀ
BaO+W
SrWO
4
ꢀ1
SrO+W
CaWO
4
ꢀ1
CaO+W
1
o
(cm
)
After heating
o
(cm
)
After heating
o
(cm
)
After heating
ꢀ
1
ꢀ1
ꢀ1
o
(cm
)
o
(cm
)
o
(cm
)
B
g
63
62
75
74
84
86
E
E
g
g
74
101
133
150
191
331
332
344
352
795
831
926
75
102
135
171
190
238
337
337
371
380
799
837
921
100
131
168
187
239
333
116
196
227
212
276
334
334
402
406
797
838
911
114
195
228
210
276
337
103
B
g
A
g
152
191
E
g
A
g
B
B
g
g
332
345
355
796
831
925
370
402
E
E
g
g
798
837
921
794
839
910
B
g
A
g

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D. Errandonea / Journal of Physics and Chemistry of Solids 70 (2009) 1117–1120
1119
with the positions of the peaks observed in our measurements.
The other non-observed mode is expected to be weak and it is also
very close in frequency to another Raman mode, which usually
makes difficult its detection [22]. We think that the agreement
conditions [27]. However, they usually stabilize at higher
pressures than those covered by our experiments. In addition,
all these phases revert to the scheelite structure upon pressure
release. Therefore, only the scheelite-phase should be present in
our diffraction and Raman experiments (performed at ambient
conditions), as we indeed observed. Unfortunately, there are two
between the measured Raman spectra with those of CaWO
4
,
SrWO , and BaWO suggests that these compounds are good
4
4
candidates for the products observed after the chemical reactions.
Therefore, we discarded the formation of other potential
4
reflections of SrWO , (0 0 4) and (2 2 2), that overlap with the
(111) and (2 2 0) peaks of SrO and little information (on top of
the unit-cell parameters of different phases) can be extracted from
the analysis of the SrO peaks. However, the stoichiometry of the
reaction can be deduced from the intensities of W(110) and
SrO(2 0 0) reflections. Both peaks exhibit a marked decrease in
intensity after the transformation, but their relative intensities
before and after the heating treatment remains almost identical
(see Table 2). This can be identified as a (1:1) reaction between
6 9 3 6
candidates like Ca WO [23] and Ca WO [24]. This conclusion
is supported by the symmetry analysis of their crystalline
structures, which predicts a larger number of Raman-active
modes than observed in our experiments.
In order to additionally support the formation of alkaline-earth
orthotungstates, we shall use our X-ray diffraction results on the
SrO+W system. An X-ray diffraction pattern collected in a
quenched sample obtained after the heating treatment of SrO+W
is shown in Fig. 2. An X-ray pattern collected before heating is also
shown for comparison. The expected positions of the peaks of the
element Sr (fcc structure [25]) are shown to illustrate that this
element is not present after the heating treatment. The lower
trace shows the diffraction pattern of the starting material.
On it the peaks of SrO and W are easily identified. In the upper
trace we show the diffraction pattern of the reaction product.
There are many new peaks, all of them can be assigned to the
SrO and W, suggesting that the formation of SrWO
through reaction:
4
takes place
AeO þ W þ 3=2O
2
-AeWO
4
ð1Þ
Since this reaction involves an important excess of oxygen in
the reaction chamber, we suspect that most of the oxygen
involved in the reaction comes from the air occluded within the
AeO powder, since the sample preparation was performed at
ambient conditions. Other possibility is the oxygen coming from
the dissociation of small amounts of alkaline-earth peroxides
present in the AeO powders, since they were the main impurities
of the used oxides.
In addition, reaction (1) involves an exothermic reaction, which
will be consistent with the runaway heating effect we observed in
our experiments. We also think that probably the reaction does
not take place directly, but more likely through the following
intermediates:
scheelite-phase of SrWO
4
[26] (see Fig. 2). The unit-cell
˚
˚
parameters obtained for it are a ¼ 5.403 A and c ¼ 11.902 A.
These values are in good agreement with those reported in
the literature [26]. Other phases different than scheelite
(
e.g. fergusonite) exist in AeWO
4
compounds at HP–HT
AeO þ W þ 3=2O
2
-AeO þ WO
3
-AeWO
4
ð2Þ
Indeed in the course of the heating the following reaction can
take place [28]:
W þ 3=2O
2
-WO
3
ð3Þ
which might probably occur in the early stages of the thermal
treatment of the sample when fluctuations of the thermal
emission of the W disk were observed. Subsequently, the second
step of reaction (2) might develop, leading to the observed
After heating
Sr
formation of AeWO
temperature of 1200 K for CaWO
4
compounds. This reaction has a forming
and a standard Gibbs free
4
energy of ꢀ149 kJ/mol [29], which is coherent with our observa-
tions. There is another fact that also supports the occurrence of
this reaction. The enthalpies of formation of alkaline-earth
orthotungstates from constituent oxides (e.g. CaWO
4
from CaO
and WO ) decreases following the order Mg4Ca4Sr4Ba [30].
3
This is in full agreement with our observation that the reactivity of
alkaline-earth oxides with tungsten increase in the order
MgoCaoSroBa. However, it should be noted here, that in situ
Before heating
5
10
15
θ (degrees)
20
2
Table 2
Fig. 2. X-ray diffraction patterns of the SrO+W mixture (before heating) and the
product obtained after its laser heating treatment (after heating).
¼ 0.6149 A˚ .
Both spectra were collected in a DAC at ambient pressure and room temperature.
The peaks corresponding to SrO, W, and SrWO are indexed. The product spectrum
Relative intensities of W(110) and SrO(2 0 0) reflections before and after the high-
pressure laser-assisted reaction.
l
4
Before
0.41
After
0.38
has been multiplied by a factor 3 to facilitate the intensity comparison of W and
SrO peaks of the upper trace with those of the lower trace. The expected positions
for diffraction peaks of Sr are indicated with vertical marks. The most intense
peaks of Sr – (111) at 10.11, (2 0 0) at 11.61, and (2 2 0) at 16.51 – are clearly not
present in the diffraction pattern collected after heating.
IW(110)/ISrO(200)
The intensity of W peaks is originated only by the W disk because of the size of the
gasket hole (150 m) and the X-ray beam (30 m).
m
m

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D. Errandonea / Journal of Physics and Chemistry of Solids 70 (2009) 1117–1120
experiments of the formation of AeWO
4
compounds are needed to
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This work was made possible through financial support of the
MICINN of Spain and the Generalitat Valenciana (Grants:
MAT2007-65990-C03-01, CSD-2007-00045, and GV2008-112).
The author is greatly indebted to V. Garcia-Baonza, A. Vegas,
F.J. Manjon, E. Martinez, and F. Rey for many useful discussions.
He also acknowledges support in collecting diffraction data at
beamline I15 by A. Kleppe, A. Segura, J. Ruiz-Fuertes, and
R. Lacomba-Perales. X-ray diffraction experiments were carried
out with the support of the Diamond Light Source.
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