Studies on the binary system Li2CO3-BaCO3

Article abstract of DOI:10.1016/S0921-4526(01)00502-6

The binary system Li₂CO₃-BaCO₃ was studied by means of heating-cooling curves, differential thermal analysis (DTA), thermogravimetry (TG), X-ray phase analysis and infrared spectroscopy. There is an eutectic at 609 (±4)°C

Full text of DOI:10.1016/S0921-4526(01)00502-6

Physica B 304 (2001) 463–476
Studies on the binary system Li CO –BaCO
3
2
3
a
P.Pasierb , R.Gajerski , M.Rokita , M.Rekas *
a
a
b,
a
Faculty of Materials Science and Ceramics, University of Mining and Metallurgy, 30-059 Cracow, Poland
School of Materials Science and Engineering, The University of New South Wales, UNSW Sydney, New South Wales 2052, Australia
b
Received 18 December 2000
Abstract
The binary system Li
DTA), thermogravimetry (TG), X-ray phase analysis and infrared spectroscopy.There is an eutectic at 609 ( ꢀ 4)8C
and about 55 mol% Li CO .It was shown that lithium carbonate does not form substitutional solid solutions with
2 3 3
CO –BaCO was studied by means of heating–cooling curves, differential thermal analysis
(
2
3
barium carbonate, however, the possible formation of dilute interstitial solid solutions is suggested. # 2001 Elsevier
Science B.V. All rights reserved.
3 2 3
Keywords: Phase diagram; Differential thermal analysis (DTA); Thermogravimetry (TG); Infra red; BaCO –Li CO system
1
. Introduction
The binary systems M CO –M CO (where M
essential in constructing electrochemical sensors
for the determination of CO2.
0
2
3
3
0
and M denote alkaline metal and alkaline-earth
metal, respectively), are of great interest as an
auxiliary phase in electrochemical CO gas sensors
2. Experimental
2
[1–6] and elsewhere as a component of CO2
resistivity gas sensors [7].
2.1. Sample preparation
The studies of phase diagrams of these systems
were the subject of several papers involving the
following binary carbonates: Li–Ca [8,9], Na–Ba
Nine samples covering a whole range of
compositions of the system Li CO –BaCO were
examined in this work.The Li 2CO3 (purity
2
3
3
[
no available data among them about Li CO –
BaCO , the most promising system in the manu-
3
10], Na–Ca [9], K–Ca [9–11].However, there are
99.99%) and BaCO (99+%) from Sigma–Aldrich
3
2
3
(both reagents of A.C.S. reagent grade) used in
this work as starting materials were dried to
constant weight at 2008C (473 K), and stored over
silica gel in desiccators until used.The H 2O
content in Li CO derived from TG measurements
facture of high-sensitive CO gas sensors [2,3,5,6].
2
This work reports the results of our studies for the
Li CO –BaCO system in air atmosphere that is
2
3
3
2
3
before drying was less than 0.5 wt%, and negligible
in the case of BaCO .The binary carbonates were
3
*Corresponding author.Tel :. +61-2-9385-6459; fax: +61-2-
prepared by dry mixing of appropriate amounts of
Li CO and BaCO , one-axis cold pressing in a
9
0
385-6467.
E-mail address: m.rekas@unsw.edu.au (M. Rekas).
2
3
3
921-4526/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 0 5 0 2 - 6

464
P. Pasierb et al. / Physica B 304 (2001) 463–476
pellet die (75 MPa) and subsequent heating (10 K/
3
min) in an artificial air flow (6 dm /h), above the
material.Typical measurements were done with
heating ratio 10 or 3 K/min in the flow of artificial
3
air with flow ratio 6 dm /h.The sample weight was
melting temperature TM, kept for 10 min, and
slowly cooled.The heating conditions ( T ) for
about 50 mg ( ꢀ 5 mg), and time, sample weight,
temperature and temperature differences were
computer recorded simultaneously during the
experiment.
M
binary carbonates were derived from DTA and
TG measurements, as described below.The heat-
ing of pressed samples was done in alumina boats,
no chemical reaction between the alumina and
carbonates was observed.The obtained binary
carbonates were milled in an agate mortar to form
fine powders, and stored over silica gel in a
desiccator until used for further experiments.
Apart from the standard DTA and TG mea-
surements, the heating–cooling measurements
consisting of several heating–cooling cycles were
performed, with the same experimental conditions
as described above (heating–cooling ratio, gas type
and gas flow rate, etc.). In the case of each
heating–cooling measurement, the first cycle dif-
fered from the following ones.In the first heating
cycle, the sample was the appropriate mixture of
simple carbonates and the changes of thermal
properties during the melting of the simple
carbonates were observed.The following cycles
were the observations of thermal properties of
binary carbonate, with different properties than
the carbonate mixture.The sample was not
removed from derivatograph until all cycles were
measured.
2
.2. DTA and TG measurements
DTA and TG measurements were performed in
TA Instruments Derivatograph System, type SDT
960, equipped with the mass spectrometer Balzers
Thermostar GSD 300 and DSC 2010 unit.All
2
3
experiments were done in alumina 60 mm cruci-
bles.It was not possible to use platinum crucibles
because of high reactivity between Pt and carbo-
nates, especially above the carbonate melting
point.High purity a-Al O was used as a reference
2
3
Fig.1. DTA and TG results for BaCO
3
during heating (10 K/min).

P. Pasierb et al. / Physica B 304 (2001) 463–476
465
2
.3. XRD measurements
g–b and b–a phase transition, respectively.As seen
from Table 1, the determined temperatures of the
phase transition well agree with literature data
[12–14].Above 877 8C (1150 K) a drop of the mass
is observed on the TG curve.Firstly, the loss of the
mass is very sluggish (between 877–10008C), then
the sudden weight change is observed (1000–
12408C) and finally establishing of the equilibrium
is observed.The entire drop of the mass within the
temperature range 25 (298 K)–13908C (1663 K) is
equal to 21.4%. This figure is close to the
theoretical change of the mass (22.3%) corre-
sponding to the following decomposition reaction:
XRD measurements were done on SEIFERT
XRD-7 diffractometer with Cu–K filtered radia-
tion.The X-ray diffraction patterns of the
powdered carbonate samples (simple and binary)
were typically measured in 2y range from 108 to
a
8
08, with a step of 0.058, and sampling time of 3 s,
although some more detailed experiments were
also performed, where necessary.All X-ray dif-
fraction measurements were done in air, at room
temperature.
2
.4. IR measurements
BaCO3ðsÞ ! BaOðsÞ þ CO ðgÞ " :
ð1Þ
2
A Fourier spectrometer BIO-RAD, type FTS
0 V was used to collect IR spectra.The resolution
was 4 cm , measuring range 400–4000 cm
MIR).The spectra were obtained using typical
technique of KBr pellets.
The determined decomposition temperature of
BaCO3 can be compared with that determined
from the thermodynamic data.Fig 2. illustrates
the dependence of DG of the reaction (1) as a
function of temperature for several concentrations
6
À1
À1
(
3
. Results and discussion
3
.1. DTA and TG analysis of pure components
Fig.1 illustrates DTA and TG results monitored
for BaCO during heating in air with the rate 10 K/
3
min within the temperature range 258C (298 K)–
1
3908C (1663 K).BaCO 3 exhibits three crystal-
lographic structures}rhombohedral (natural
witherite), b-hexagonal and a-cubic form.The
temperatures of phase transformation in BaCO3
are collected in Table 1.The observed endothermic
peaks on the DTA curve in Fig.1 at the
temperature range 811–8278C (1084–1100 K) and
9
61–9718C (1234–1244 K) may correspond to the
Table 1
Temperatures of phase transformations in BaCO
3
Temperature of phase transformation (K)
References
g–b
b–a
1
1
1
1
083
1253
1241
1255
1234
[12]
[13]
079
084
084
[14]
This work
Fig.2. DG of the decomposition BaCO
3
versus temperature for
several concentrations of CO
2
.

466
P. Pasierb et al. / Physica B 304 (2001) 463–476
(expressed in ppm) of CO2 in the gas phase.
Assuming that p(CO ) in the experiment condi-
tions was equal to 35 Pa [15], the decomposition,
Td temperature corresponding to DG (Td,
35 Pa)=0 is 8938C (1166 K).This value corre-
sponds well with the observed early stage change
corresponds to melting.There is also disagreement
in literature concerning the melting point of
Li CO .According to Barin [21] Li CO melts at
998 K.Other authors report values: 700 8C (973 K)
[16,20], 7128C (985 K) [22], 7238C (996 K) [17]
7258C (998 K) [18], 7288C (1001 K) [19].
2
2
3
2
3
of the mass in the TG curve.
Fig.4 presents differential thermal analysis
curves in the coordinate systems: DT (right axis)
and T (left axis) versus time for Li CO during
The behavior of lithium carbonate during
heating is a subject of controversial reports.Some
authors claim that there are no thermal reactions
before fusions begin at about 6978C (970 K) [16–
2
3
subsequent heating and cooling runs.Two sharp
peaks corresponding to melting and solidification
can be observed.However, temperature ranges of
these peaks differ considerably.First one begins at
6948C (967 K) and achieves extreme value at
7328C (1005 K), the second one begins at 6758C
(948 K) and its extreme value is at 6798C (952 K).
This difference may be explained analyzing a TG
curve.As seen in Figs.3 and 5, the loss of about
3% weight accompanies the melting process.The
more detailed studies of the decomposition process
of Li CO described elsewhere [23] and X-ray
1
8].On the other hand, other authors postulate
phase transition at 405–4158C (678–688 K) [19],
108C (683 K) [20].Barin [21] reports two poly-
morphic phase transitions at 3508C (623 K) and
4
4
108C (683 K) with enthalpies of transformations
.561 and 2.238 kJ/mol, respectively. Fig. 3 shows
0
DTA and TG curves determined during heating of
Li CO in the temperature range 25 (298 K)–7778C
2
3
(
1050 K).As seen in the inset of this figure, no
thermal effects have been observed at the postu-
lated phase transitions temperatures.It may be
due to the rather small heat of phase transition
2
3
analysis showed that the weight loss results mainly
by CO2 liberation during decomposition of
Li CO into lithium oxide and carbon dioxide.
[
20,21].The sharp peak near 727 8C (1000 K)
2
3
Fig.3. DTA and TG results for Li
2
CO
3
during heating (10 K/min).

P. Pasierb et al. / Physica B 304 (2001) 463–476
467
2 3
Fig.4. Dependence DT and T on time during consecutive heating and cooling procedures for Li CO .
2 3
Fig.5. DTA and TG results for Li CO during consecutive heating and cooling procedures.
The Li O formed decreases the melting tempera-
2
Rode [25] reports, that decomposition of LiCO3
takes place at 7288C (1001 K) i.e. soon after the
fusion of solid lithium carbonate (Tmelt=7238C).
These processes are probably responsible for
ture of lithium carbonate, because in the binary
system: Li O–Li CO a eutectic mixture (contain-
2
2
3
ing 13 mol% of Li O, Tmelt=7058C) exists [24].
2

468
P. Pasierb et al. / Physica B 304 (2001) 463–476
2 3 3 2 3
Fig.6. DTA and TG results for Li CO –BaCO (50 mol% Li CO ) during consecutive heating and cooling procedures, first run.
2 3 3 2 3
Fig.7. DTA and TG results for Li CO –BaCO (50 mol% Li CO ) during consecutive heating and cooling procedures, second run.

P. Pasierb et al. / Physica B 304 (2001) 463–476
469
observed shift of the peaks on the DT curve shown
in Fig.5.
corresponding to 2y=40.68 can be observed.On
the other hand, the intensities of reflections change
considerably.For example, low intensity peaks in
the 2y range of 40–508 observed for pure BaCO3
assume higher relative intensities for the samples
containing Li CO .
3.2. DTA and TG analysis of binary
Li CO –BaCO system
2
3
3
2
3
Figs.6 and 7 illustrate by way of example, DTA
and TG curves for the Li CO –BaCO mixture
containing 50 mol% Li CO recorded in two
A more detailed X-ray analysis was performed
using SiO powder as an internal standard.Six
2
3
3
2
samples containing 25, 50 and 75 mol% of Li CO
3
2
3
2
consecutive runs involving heating and cooling.
The shape of the presented curves is similar to that
for Li CO .The DTA for the first run (Fig.6) on
just mixed with BaCO , as well as melted, were
3
ground with the constant mass (20 wt%) of a-
quartz SiO .Such a method allowed us to compare
2
3
2
the heating part of the curve has sharp minimum
beginning at 6228C (895 K) and minimum value at
the reflex intensities coming from the mixed and
melted (binary) carbonates.Figs .1 1–13 illustrate
the results.We observed, that samples after
thermal treatment (melting) exhibit better devel-
oped peaks with higher intensities corresponding
6
538C (926 K).On the other hand, on cooling
there is sharp exothermic peak beginning at 6238C
(896 K) and achieving maximum value at 6158C
(888 K).After this run, the 0 9. 38% loss of weight
(in relation to total mass of the sample) is
observed.Assuming that this loss corresponds
only to the decomposition of Li CO component,
we can conclude that 3.44% of Li CO was
2
3
2
3
decomposed.The second run (Fig 7. ) exhibits
much lower loss of weight 0.258% per total mass
and 0.95% per mass of Li CO .It is interesting to
2
3
mention that the sharp peak on the heating part of
DTA curve observed in Fig.6, now splits in two.
Figs.8 and 9 show DTA curves for all the
studied compositions during heating and cooling,
respectively.Well developed endothermic (Fig .8 )
and exothermic peaks (Fig.9) correspond to
melting and solidification, respectively.However,
their position does not correspond to the same
temperatures.There is about 40 0. ꢀ 6.58C differ-
ence between minimum value on heating curves
and maximum value on cooling curves.
3
.3. XRD analysis
Fig.10 shows X-ray diffraction patterns of the
studied samples after heating in air above the
melting temperature TM and cooling down.Tak-
ing into account the positions of peaks, almost all
peaks observed for Li CO –BaCO compositions
2
3
3
can be identified as either BaCO or Li CO .No
3
2
3
shift in reflex positions was observed.There is only
one exception: for the melted sample a small reflex
for the sample containing 25 mol% Li CO
Fig.8. DTA curves during heating for Li 2CO –BaCO com-
3
3
2 3
positions (12.5–100 mol% Li CO ).
2
3

470
P. Pasierb et al. / Physica B 304 (2001) 463–476
Fig.10. XRD diffraction patterns (Cu K
compositions (0–100 mol% Li CO ).
a 2 3 3
) for Li CO –BaCO
2
3
Fig.9. DTA curves during cooling for Li
sitions (12.5–100 mol% Li CO ).
2 3 3
CO –BaCO compo-
2
3
differences suggest hypothesis about formation of
substituted solid solutions is very unlikely.The
mentioned increase of dhkl for BaCO phase, if it is
real, may indicate the formation of very diluted
interstitial solid solutions of lithium in BaCO3.
to LiCO3 phase, than the mixtures of both
carbonates.
The intensities as well as the dhkl values observed
3
However, the observed increase is close to the
and normalized, with respect to SiO reflections
2
uncertainty of the XRD method, thus the possi-
bility of formation of these solid solutions should
be verified by other methods.We have chosen
MIR spectroscopy as such a method.
were collected in Table 2.As seen, there are no
visible changes in dhkl between the sets of samples
before and after annealing.Only small decrease of
dhkl for the reflection (1 0 2) of BaCO may be
3
noticed.These results suggest, that Li CO and
2
3
BaCO does not form solid solutions within wide
3
3.4. Infrared spectroscopy
ranges of concentrations.Such a conclusion seems
to be justified taking into account the differences
in crystal structures (Li CO }monoclinic,
2
À
The model of CO anions isolated by cations,
was used in this paper for interpretation of IR
3
2
3
+
2À
BaCO }orthorhombic), ionic radii (Li 59 ppm,
3
spectra.Isolated, planar CO 3 anion has symme-
try D3h [26].The absorption bands characteristic
2
+
Ba
136 ppm) and valencies of both ions.Such

P. Pasierb et al. / Physica B 304 (2001) 463–476
471
Fig.11. XRD diffraction patterns (Cu K
a
) for Li
2
CO
3
–BaCO
3
(25 mol% Li
2
CO
3
) and SiO
2
, a}mixture of powders, b}after fusion.
2
À
(iv) bending in-plane vibrations at 720–680 cmÀ1
of intermolecular CO
located within 1800–400 cm
anion vibrations are
À1
3
region [27].Four
(n4)}IR and Raman active.
2
À
vibrations of CO anion are [26–29]:
3
Fig.14 presents MIR spectra of pure Li CO
2
3
(
(
(
i) symmetric stretching vibrations ca.1065 cm À1
n1)}Raman active;
ii) bending out of plane vibrations at 880–
and BaCO and their mixtures before and after
3
(
melting.On the spectrum of BaCO
observe absorption bands, which may be ascribed
we can
3
À1
2À
8
iii) asymmetric stretching vibrations in the region
50 cm (n )}IR active;
to the intermolecular CO3 anion vibrations,
which are only slightly shifted in respect to
2
À1
À1
À1
of 1450–1410 cm (n3)}IR and Raman active;
vibrations of free ion: 1449 cm
(n3), 857 cm

472
P. Pasierb et al. / Physica B 304 (2001) 463–476
Fig.12. XRD diffraction patterns (Cu K
a 2 3 3 2 3 2
) for Li CO –BaCO (50 mol% Li CO ) and SiO , a}mixture of powders, b}after fusion.
À1
(
n ) and 694 cm (n ).There is also a small peak
hand, the band due to n vibrations splits into two
2
4
4
À1
at 1060 cm corresponding to the n1 vibration.
The spectrum of Li CO shows four mentioned
components, and moreover shifts towards lower
wave numbers, which maxima are at 509 and
2
3
À1
bands corresponding to intermolecular vibrations
of isolated anion.The twofold degeneracy of the
normal modes n3 and n4 is removed due to decrease
of symmetry of crystal lattice lithium carbonate in
relation to the symmetry of ideal CO anion.The
double band corresponding to n vibrations is here
422 cm .Also, the n1 vibration becomes active
À1
giving a small band at 1088 cm .The vibration n2
gives a sharp band at 863 cmÀ1.
Both studied carbonates give flat characteristics
2
3
À
À1
within 4000–1800 cm (not shown in Fig.14). It
À
suggests the lack of OH anions and molecules of
water in their crystal structure.
3
À1
observed at 1503 and 1478 cm .On the other

P. Pasierb et al. / Physica B 304 (2001) 463–476
473
Fig.13. XRD diffraction patterns (Cu K
a
) for Li
2
CO –BaCO
3
3
(75 mol% Li
2
CO
3
) and SiO
2
, a}mixture of powders, b}after fusion.
The comparison of spectra Li CO –BaCO
3
during fusion are also suspected by observed
changes of band intensities.It may be noticed that
the increase of this band intensity occurs, which
corresponds to predominant component.This
suggests, that part of ions which are in minority
incorporate into crystal lattice of the predominant
component.Analogously, as it was observed for
2
3
specimens before and after fusion indicates struc-
tural changes during fusion.The shape of the band
corresponding to the mode n3 is affected for
fused samples in relation to not fused mixtures
of the same composition.The band broadens,
and additional bands appear on the part corre-
sponding to higher wavenumbers.This effect is
characteristic of differentiation in bonds C–O
pure components, there is lack of the bands
À
characteristic of OH and H O groups in fused
2
2
3
À
length in the CO anion.The structural changes
mixtures.

474
P. Pasierb et al. / Physica B 304 (2001) 463–476
Table 2
Comparison of XRD data for Li
2
CO –BaCO
3
3
(25 mol%, 50 mol% and 75 mol% of Li
2
3
CO ) before and after annealing
Reflection B : BaCO
quartz)
3
L : Li
2
CO
3
Q : SiO
2
25 mol% Li
2
CO
3
50 mol% Li CO
2
3
75 mol% Li
Mixture
2 3
CO
(
Mixture
Annealed
Mixture
Annealed
Annealed
Observed intensities (arbitrary units)
B (1 1 1)
B (1 0 2)
L (À2 0 2)
L (0 0 2)
Q (1 0 1)
457
197
33
468
355
32
389
165
34
548
322
83
465
197
82
484
259
187
200
258
37
176
41
210
48
168
256
327
125
254
Normalized intensities (arbitrary units)
B (1 1 1)
B (1 0 2)
L (À2 0 2)
L (0 0 2)
2.60
1.12
0.19
0.21
2.23
1.69
0.15
0.20
2.32
0.98
0.20
0.29
1.68
0.98
0.25
0.78
1.83
0.78
0.32
0.49
1.88
1.00
0.72
0.78
Observed dhkl (nm)
B (1 1 1)
B (1 0 2)
0.3714
0.3664
0.2921
0.2812
0.3341
0.3720
0.3657
0.2917
0.2813
0.3341
0.3726
0.3675
0.2925
0.2819
0.3351
0.3720
0.3660
0.2917
0.2812
0.3343
0.3714
0.3657
0.2915
0.2812
0.3338
0.3723
0.3663
0.2919
0.2815
0.3346
L (À2 0 2)
L (0 0 2)
Q (1 0 1)
Corrected dhkl (nm)
B (1 1 1)
B (1 0 2)
0.3716
0.3666
0.2922
0.2813
0.3342
0.3722
0.3658
0.2918
0.2814
0.3342
0.3716
0.3664
0.2918
0.2813
0.3342
0.3719
0.3658
0.2916
0.2811
0.3342
0.3719
0.3661
0.2918
0.2814
0.3342
0.3719
0.3658
0.2916
0.2813
0.3342
L (À2 0 2)
L (0 0 2)
Q (1 0 1)
3
.5. Phase diagram of Li CO : BaCO system
2
and corrosive properties of Li O) such an experi-
2
3
3
ment seems to be very difficult to perform.
Based on the presented results we propose the
schematic phase diagram of Li CO : BaCO sys-
tem, shown in Fig.15. Li CO and BaCO form
2
3
3
2
3
3
4. Conclusions
the eutectic at about 55 mol% of Li CO with the
2
3
melting temperature of 609 ( ꢀ 4)8C.No other
In this work, the results of DTA, TG, XRD and
IR measurements of Li CO : BaCO system were
phases than Li CO and BaCO were detected in
2
3
3
2
3
3
the investigated temperature range.The possible
formation of intersitial solutions is not shown in
this diagram.Based on our results, it is not
possible to determine the phase composition in
the temperature range above solidus line (6098C).
It could be done for example by X-ray diffraction
high temperature measurements.Taking into
account the decomposition of Li CO and possible
presented.Based on XRD results it can be
suggested that Li CO and BaCO do not form
2
3
3
substituted solid solutions over a wide range of
composition.There is a eutectic at 609 ( ꢀ 4)8C
and about 55 mol% of Li CO .
2
3
The observed intensity increase of some X-ray
peaks coming from BaCO , the slight splitting of
3
2
3
DTA peaks, as well as the modification of IR
spectra in the case of binary carbonates compared
evaporation of formed Li O (composition changes
2

P. Pasierb et al. / Physica B 304 (2001) 463–476
475
with simple carbonates, may suggest the formation
of very diluted interstitial solid solutions especially
of lithium in BaCO .This hypothesis has to be
3
verified, because the observed changes in X-ray
diffraction and modification of IR spectra may be
caused by the crystallinity changes, and the
splitting of DTA peaks can be caused by the
Li O–Li CO formation.
2
2
3
Acknowledgements
The financial support of Polish State Committee
for Scientific Research (KBN), Project No.7 T08A
037 15 is acknowledged.
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2 3 3
Fig.15. Phase diagram of Li CO –BaCO system.

476
P. Pasierb et al. / Physica B 304 (2001) 463–476
[
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[
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