Characterization studies of physicochemical modifications conceded by equimolar-mixed chromia and barium carbonate powders as a function of temperature

Article abstract of DOI:10.1016/j.tca.2008.10.014

Occurrence and products of solid/solid interactions in equimolar-mixed BaCO₃ and Cr₂O₃ powders were examined isothermally (700-1000 °C) and non-isothermally (25-1200 °C) under different gas atmospheres, employing thermogra

Full text of DOI:10.1016/j.tca.2008.10.014

Thermochimica Acta 483 (2009) 8–14
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Characterization studies of physicochemical modifications conceded
by equimolar-mixed chromia and barium carbonate powders as
a function of temperature
L.A. Al-Hajjia, M.A. Hasana, M.I. Zakib,∗
a Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
b Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2008
Received in revised form 17 October 2008
Accepted 21 October 2008
Available online 30 October 2008
Occurrence and products of solid/solid interactions in equimolar-mixed BaCO3 and Cr2O3 powders were
examined isothermally (700–1000 ◦C) and non-isothermally (25–1200 ◦C) under different gas atmo-
spheres, employing thermogravimetry, X-ray diffractometry, infrared and Raman spectroscopies, scanning
electron microscopy, and energy dispersive X-ray spectroscopy. Irrespective of the gas atmosphere, a non-
catalytic decomposition of the barium carbonate commenced at 570 ◦C, a temperature that is much less
than the decomposition temperature (≥970 ◦C) of separate BaCO3. Characteristics and distribution of the
yielding products were found to be quite sensitive to the surrounding gas atmosphere. Under N2 atmo-
sphere, the interactions were thermochemical in nature, leading eventually to the formation of barium
chromite (BaCr2O4) spinel as the sole detectable product. Under O2 atmosphere, however, the oxygen
molecules contributed oxidatively to the chromite thermal genesis course, thus imposing a topochemi-
cal nature to the reaction, leading to formation of BaCrO4, as the major product, and barium chromites
(BaCr2O4 and Ba3Cr2O6), as minor products. Proposed reaction pathways and characteristics of products
have been presented and discussed.
Keywords:
BaCO3 and Cr2O4 powder mixture
Thermochemical interactions
Topochemical interactions
Influence of the surrounding gas
atmosphere
Product analysis
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
chemical, but also the physical changes conceded by the reaction
mixture throughout the solid-state synthesis course. Topochemical
Solid-state synthesis is a frequently used method of production
of industrially and technologically important materials [1]. Despite
the fact that it is more time and energy demanding than other
methods, such as mechanochemical [2], sol–gel [3], hydrothermal
[4], electrochemical [5] and self-assembly method [6], it is rela-
tively much less sophisticated and, thus, more practical. Moreover,
the necessity for high temperatures to trigger the chemical reac-
tivity in reaction mixtures of solids [7] warrants obtaining yields of
high thermal and mechanical stabilities; i.e. suitable for heavy-duty
applications.
Characterization of thermochemical events involved in solid-
state syntheses is a prime demand if a fine control over the product
particle properties (size, crystallinity and morphology) would ever
be achieved [8]. As a matter of fact, the synthesis of solids of tailored
particle properties is largely sought for specific performances [9].
On the other hand, designing of necessary means and ways of con-
trol over particle properties requires a full awareness of not only the
impacts of the surrounding gas atmosphere, i.e. chemical changes
influenced by the latter molecules, also worth consideration [8].
Bearing in mind the above considerations, the present inves-
tigation was undertaken to characterize physicochemical changes
conceded by an equimolar powder mixture of chromia (Cr2O3)
and barium carbonate (BaCO3) as a function of temperature. The
effective temperature range was determined by thermogravime-
try, and influence of the surrounding gas atmosphere (oxygen or
nitrogen) was probed. Structural modifications of crystalline and
non-crystalline domains of the mixture were probed by X-ray
diffractometry, and infrared and Raman spectroscopies. Particle
morphological modifications were visualized by scanning electron
microscopy, whereas the chemical composition was microprobed
by energy dispersive X-ray spectroscopy. The principle objective of
the investigation was to characterize chemical and physical mod-
ifications to be conceded by the reaction mixture to the onset of
formation product(s), and to elucidate relationships that may exist
between these modifications and nature of the product(s). Proba-
bly the most prominent finding of this investigation has been the
proven critical dependency of nature of the product(s) on the kind
of the surrounding gas atmosphere.
∗
Corresponding author. Tel.: +20 86 2360833; fax: +20 86 2360833.
E-mail address: mizaki@link.net (M.I. Zaki).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.10.014

L.A. Al-Hajji et al. / Thermochimica Acta 483 (2009) 8–14
9
2. Experimental
DIFFRACT AT software (Siemens Corp.) for automatic JCPDS [10]
library search and match. Crystallite sizing was carried out imple-
2.1. The powder mixture
menting the XRD line broadening technique and Scherrer’s formula
ꢀ
[11]: D = 0.9ꢀ/ (BM2 − BS2) cos ꢁ, where D is the crystallite size (in
Powders of chromia (Cr2O3) and barium carbonate (BaCO3),
99.5% pure products of Aldrich (USA), were ground to ≤120 mesh,
mixed in equimolar quantities by first tumbling the two powders
(using a few drops of diethyl ether), and then blending with agate
mortar and pestle for 30 min. The resulting powder mixture was
found by stereomicroscopy to be at least 75% homogenously mixed,
and by preliminary physicochemical characterization (vide infra) to
have not suffered any detectable mechanochemical activation. It is
worth noting that a single patch of the mixture was prepared in a
sufficient amount for all tests and analyses performed in the present
investigation.
Based on non-isothermal thermogravimetric results (vide infra),
5-gm portions of the separate and mixed barium carbonate and
chromia powders, placed in pre-cured porcelain crucibles, were
calcined at various temperatures (for 3 h in a static atmosphere
of air) using electronically controlled Thermolyne 6000 muffle fur-
nace (actual temperature = set temperature 2 ◦C). The calcination
products were naturally cooled to room temperature (RT) prior to
storing over silica gel till further use. For clarity, the uncalcined
mixture is denoted below BaCrOx(RT), whereas the calcination
temperature applied is used to distinguish calcination products
of the mixture. Thus, for example, BaCrOx(800) means the 800 ◦C
calcination product of BaCrOx(RT).
Å), BM and BS are the width in radians of the diffraction peaks (at
half maximum height) of the test sample and a highly crystalline
standard sample, respectively, and ꢀ (in Å) is the wavelength of the
X-ray beam.
Infrared (IR) and Laser Raman (Ra) spectroscopy were used to
characterize crystalline and non-crystalline domains of test sam-
ples. IR spectra were taken of KBr-supported test samples (<1
mass%) in the frequency range 4000–400 cm−1 (at the resolution
of 5.2 cm−1), using a Perkin-Elmer System 2000 FT-IR spectrom-
eter (Germany). Ra spectra were taken of lightly compacted test
samples, at 3600–200 cm−1 and the resolution of 0.2 cm−1, using
a Perkin-Elmer System 2000 FT-Ra spectrometer equipped with
a near infrared diode pumped Nd:YAG laser (ꢀ = 1.064 ␮m and
50–100 mW).
For morphological and elemental microprobing of test samples,
a model JSM-6300 Jeol scanning electron microscope (SEM, Japan)
equipped with a LINK’s exl II Oxford energy dispersive X-ray spec-
trometer (EDX, U.K.) was employed. The microscope was operated
at 20 kV and 100 ␮A. Test samples, spread in a thin layer over a dou-
ble adhesive tape on a 10-mm aluminum stub, were sputter-coated
with gold prior to examination. EDX data were acquired over 100 s
with 2000–3000 X-ray counts per second. The sample preparation
was similar to that for SEM examination, except for the gold coating.
2.2. Thermogravimetry
3. Results and discussion
Thermogravimetry (TG) was carried out, using a model TGA-50H
Shimadzu (Japan) automatically recording thermobalance. A small
portion (15–20 mg) of test sample was placed in a platinum cell
mounted inside the thermobalance, in a dynamic atmosphere of air,
oxygen or nitrogen (50 cm3/min). The mass change was recorded,
without correction for possible buoyancy force contributions, on
heating up to 1200 ◦C at 10 ◦C/min. Data acquisition and handling
were facilitated by an on-line workstation (TA-50WS, Shimadzu),
and evolved gas analysis by an on-line mass analyzer (a model
Thermostar Balzars quadrupole mass analyzer, Switzerland).
3.1. The powder mixture at room temperature
3.1.1. Characteristics of the constituents
X-ray powder diffractograms obtained for separate, uncalcined
BaCO3 and Cr2O3 powders displayed a number of sharp peaks of
high and medium intensities at 2ꢁ = 20–80◦. The three strongest
peaks are compiled for each material in Table 1. Moreover, Table 1
sets out the average crystallite size derived for each material, using
the X-ray line broadening technique [11]. It also shows characteris-
tics of the best matching standard XRD data found for BaCO3 (JCPDS
05-0378) and Cr2O3 (JCPDS 38-1479). These results indicate that
both of the test powders consist of microcrystallites. Whereas the
former powder was found to consist of large crystallites (273 5 Å)
of Witherite-like orthorhombic structured BaCO3, the latter was
found to contain smaller crystallites (164 5 Å) of Eskolaite-like
rhombohedral structured ␣-Cr2O3.
2.3. Spectroscopy and microsopy
X-ray powder diffractometry (XRD) was carried out at room
and higher temperatures, using a Siemens D5000 diffractometer
´
˚
(Germany) equipped with Ni-filtered Cu K␣ radiation (ꢀ = 1.5418 A,
40 kV and 30 mA), and a high-temperature attachment (Bühler’s
HDK S1, Germany). The data were acquired stepwise (0.02◦/s) in
the 2ꢁ range 10–80◦ with a divergence slit of 1◦, and handled
with an on-line micro-computer installing standard SEARCH and
Observed and reported IR and Ra analysis results of the sep-
arate BaCO3 and Cr2O3 powders are compared in Table 2. The
obvious closeness of the observed band structure and frequencies
to those reported for IR and Ra-active vibrations of pure, crys-
Table 1
XRD-derived phase composition and average crystallite size of separate constituents of the test mixture at room temperature.
Constituent
Observeda
Reportedb
Average crystallite size/ 5 Åc
273
´
´
d (A)
I/I◦
d (A)
I/I◦
JCPDS
Phase
˚
˚
Barium carbonate
3.72
3.67
2.06
100
43
33
3.72
3.66
2.15
100
50
30
05–0378
Witherite
BaCO3
Chromium oxide
2.68
2.49
1.67
97
90
100
2.67
2.48
1.67
100
90
90
38–1479
Eskolaite
␣-Cr2O3
164
a
The three strongest diffraction peaks.
Found in Ref. [10].
Determined by means of the X-ray line broadening technique [11].
b
c

10
L.A. Al-Hajji et al. / Thermochimica Acta 483 (2009) 8–14
Table 2
Observed and reported IR and Ra results for separate constituents of the test mixture at room temperature.
Constituent
IR
Ra
Observed
Reported
ꢂ (cm−1
Observed
ꢃꢂ (cm−1
Reported
ꢂ (cm−1
)
Profilea
)
[Ref.]b
[12,13]
)
Profilea
ꢃꢂ (cm−1
)
[Ref.]b
[13]
BaCO3
2451
1750
1448
1059
856
w, sp
m, sp
vs, b
w, sp
s, sp
2499–50
1770–40
1455–35
1065–40
864–34
695–85
1057
688
221
vs, sp
w, sp
w, sp
1088–60
699–80
233–20
693
m, sp
␣-Cr2O3
649
586
444
416
vs, sp
s, sp
vw, sp
w, sp
655–49
580–70
440–30
420–10
[14]
610
551
s, sp
vs, sp
620–00
560–50
[15]
a
vw, very weak; w, weak; m, medium; s, strong; vs, very strong; sp, sharp; b, broad.
Source references.
b
talline BaCO3 [12,13] and Cr2O3 [14,15] is quite consistent with the
above described XRD results. Apart from the weak IR absorption (at
2451 cm−1) due to ꢂOH vibrations of minority associated surface-
OH groups [12,13], all of the observed IR bands (at 1750, 1448, 1059,
856 and 693 cm−1) for BaCO3 are assignable to ꢂ1–ꢂ4 C–O vibrations
of CO32− groups assuming aragonite-type site symmetry [12]. Sim-
ilarly all of the IR bands given rise by the chromia powder (Table 2)
are due, solely, to various lattice vibrations of Cr(III)–O bonds orga-
nized in ␣-Cr2O3 structure [14]. Consistently, the corresponding Ra
bands (Table 2) are assignable solely to Ra-active lattice vibrations
of CO32− groups (in case of BaCO3) [13] and Cr(III)–O bonds (in case
of Cr2O3) [15,16].
A further support to the high chemical purity of the separate
Cr2O3 and BaCO3 powders was provided by EDX microprobing of
the particles visualized for these materials in the SEM images dis-
played in Fig. 1A and B, respectively. The longitudinal, flake-like
particles of BaCO3 (Fig. 1B) have given rise to an EDX-determined
molecular formula (Ba1.00 C1.01 O3.02) very close to the expected
formula (Ba1.0 C1.0 O3.0) for the pure material. Similarly, an EDX-
determined molecular formula (Cr1.0 O1.51 ) for the smaller and
irregularly shaped particles of chromia (Fig. 1A) is very close to
that expected (Cr1O1.5 ) for pure Cr2O3. The fact that SEM images
shown in Fig. 1A and B reveal larger particles for BaCO3 than Cr2O3
is in line with the XRD findings in this regard (see average crystallite
sizes given in Table 1).
3.2. The reaction mixture at high temperatures
3.2.1. Non-isothermal heating
TG curves obtained on heating at 10 ◦C/min, in a dynamic atmo-
sphere of air (50 cm3/min), of the separate powders of BaCO3
(curve a) and Cr2O3 (curve b), and their equimolar physical mixture
(BaCrOx(RT); curve c) are compared in Fig. 2. It is obvious from the
results that BaCO3 is thermally stable to heating up to ca. 970 ◦C,
where it commences suffering detectable mass loss due to ther-
mal decomposition. An earlier differential thermal analysis [18] of
BaCO3 revealed occurrence of two mass-invariant, reversible poly-
morphic transformations at 830 and 940 ◦C, that is, at the vicinity
of its thermal decomposition. In contrast, Cr2O3 is shown (Fig. 2,
curve b) to remain considerably mass-invariant on heating up to
1200 ◦C.
The TG curve (c, Fig. 2) obtained for BaCrOx(RT), i.e. the equimo-
lar powder mixture of BaCO3 and Cr2O3 indicates mass invariance
on heating up to 570 ◦C, followed, on further heating, by (i) a grad-
ual mass loss (ca. 5.8%) completed near 970 ◦C and (ii) a subsequent
slight, but distinct, mass gain (0.4%) subsiding near 1150 ◦C. More-
over, the mass loss course is shown to be initially relatively steeper
than towards the end.
It is worth noting, that an on-line gas analysis of volatile decom-
position products of BaCO3 and BaCrOx(RT) detected nothing but
CO2 molecules, whereas XRD analysis of the solid residues detected
BaO (and minority BaO2) for the separate BaCO3 only. Thus, it is evi-
dent that the mass loss conceded by BaCrOx(RT) at 570–970 ◦C is
due to an enhanced decomposition of BaCO3, influenced by Cr2O3.
The fact that in neither of the calcination products of BaCrOx(RT) at
>570 ◦C (vide infra) BaO (or BaO2) was detected may indicate that
the influence of Cr2O3 is chemical and not catalytic. Accordingly,
the mass loss conceded by BaCrOx(RT) at 570–970 ◦C is concur-
rent to a solid-state reaction between components of the powder
mixture.
Thermodynamic data determined (at RT) for pure, crystalline
BaCO3 and ␣-Cr2O3 are, respectively, the following: (i) ꢃGf, −1137.7
and −1058.1 kJ/mol; (ii) ꢃSf, 112.1 and 81.2 J/K mol; and (iii) Cp, 85.3
and 120.4 J/K mol [17]. The ꢃGf values of the carbonate and the
oxide compounds are rather close, however the lower ꢃSf value of
chromia may account for its higher tolerance to mobilization than
the barium carbonate.
3.1.2. Influence of mixing
In order to have a deeper insight into the eventual, slight mass
gain observed for BaCrOx(RT) at >970 ◦C (curve c; Fig. 2), two addi-
tional TG curves were measured for the physical mixture: a curve
was measured in O2 atmosphere, whereas the other in N2 atmo-
sphere. The TG curves thus obtained are compared in Fig. 3. The
curve (a) obtained in O2 atmosphere is largely similar to that previ-
ously obtained in air (curve c, Fig. 2) in showing the mass loss step to
commence also at 570 ◦C, maximize at 720 ◦C, slow down detectably
as of 780 ◦C, be followed by the slight mass gain at >970 ◦C, and
result in a net mass loss of 5.4%. In contrast, the curve (b; Fig. 3)
obtained in N2 atmosphere is markedly different. In N2 atmosphere,
the mass loss is shown to commence at the higher temperature of
IR spectrum taken of the physical mixture of the carbonate
and chromia powders, i.e. BaCrOx(RT), was merely a summation
of the absorption bands cited for the separate materials in Table 2.
The same is shown (Fig. 1C) to apply to the corresponding SEM
micrograph, which helps discerning the much larger particles of
the carbonate from the smaller ones of chromia. It also reveals a
satisfactory uniform mixing of the reactants’ particles. The XRD
and Ra results obtained for the physical mixture also served as a
kind of summation of results obtained for the separate materials
(Tables 1 and 2). These observations confirm that the manual mix-
ing undertaken did not result in sufficient local heating to trigger
chemical changes to the mixture composition.

L.A. Al-Hajji et al. / Thermochimica Acta 483 (2009) 8–14
11
Fig. 2. TG curves obtained on heating (at 10 ◦C/min and 50 cm3 air/min) of separate
(a and b) and equimolar-mixed (c) powders of BaCO3 and Cr2O3.
(or air) than in N2 atmosphere is due, most likely, to compensation
by a considerable, simultaneously occurring mass gain.
Hence, the results communicated in Fig. 3 reveal that the
solid/solid interactions at BaCO3/Cr2O3 particle interfaces are quite
sensitive to whether the surrounding gas atmosphere is oxidizing
(air or O2) or non-oxidizing (N2). In N2 atmosphere, the determined
net mass loss (11.6%) is slightly less than that (12.6%) expected for
the formation of barium chromite (BaCr2O4) spinel (the chromium
content of which is in the trivalent state (Cr(III)). The slight differ-
ence (1%) occurring between these two values may be attributed to
the insignificant mass gain observed at ≥1000 ◦C (curve b, Fig. 3),
which might be due to the containment of the nitrogen gas used
of trace amounts of O2, or to the lack of mass measurement cor-
rection for possible buoyancy force contributions. Accordingly,
solid/solid interactions in N2 atmosphere are mostly thermochem-
ical in nature.
In O2 (or air) atmosphere, however, the significant mass gain
observed (curve a, Fig. 3) may account for a considerable O2 uptake
and a consequent oxidative contribution to the reaction course; for
instance, to the oxidation of Cr(III) into higher-valent chromium,
and, hence, formation of chromate species. An example of a sim-
ilar behaviour is what has been reported in the literature [19] for
formation of magnesium chromite (MgCr2O4) via magnesium chro-
mate (MgCrO4) intermediate upon heating of a powder mixture of
MgO and Cr2O3 in air. In contrast, formation of cadmium chromate
intermediate in the reaction course towards formation of cadmium
chromite (CdCr2O4) in a powder mixture of CdO and Cr2O3 (in air)
went unnoticed [19]. This fact has been ascribed [19] to the basic
character of MgO, which is a requirement not only for stabilization
of chromate species, but also for stabilization of metalate species
in general [20]. Thus, the known strong basicity of BaO, which has
been exploited [20] to justify formation of barium peroxide (BaO2),
can also justify the ability of Ba2+ to stabilize high-valent metalate
species including chromates.
Fig. 1. SEM micrographs obtained for uncalcined, separate (A and B) and equimolar-
mixed (C) powders of BaCO3 and Cr2O3.
700 ◦C, maximize at 880 ◦C, slow down near 940 ◦C, be followed
by an insignificant mass gain (<0.1%), and lead to almost twice as
much as the net mass loss observed in air or O2, viz. 11.6% versus
5.4%. It is evident from these results that (i) the solid/solid inter-
action between BaCO3 and Cr2O3 particles, which are responsible
for the mass loss observed, is markedly accelerated in the oxidizing
atmosphere of O2 (or air), and (ii) the slow eventual mass loss pro-
cess is confined to a narrower temperature regime (940–1000 ◦C)
in N2 atmosphere than that (780–1000 ◦C) observed in O2 atmo-
sphere. Furthermore, the much less net mass loss observed in O2
Fig. 3. TG curves obtained on heating (at 10 ◦C/min and 50 cm3/min) of BaCrOx(RT)
in O2 (curve a) and N2 (curve b) atmosphere.

12
L.A. Al-Hajji et al. / Thermochimica Acta 483 (2009) 8–14
Fig. 4. XRD diffractogram exhibited by the equimolar powder mixture of BaCO3 and
Cr2O3 following its calcination at 800 ◦C for 3 h. The diffraction peaks are indicated
by the corresponding d-spacing values (in Å), whereas those discerned by arrows
are due to the Pt/Rh sample holder.
The fact that the separate Cr2O3 powder was mass-invariant
to heating in the oxidizing atmosphere of air (TG curve b; Fig. 2)
may assign the oxygen uptake to freshly genesized species (most
likely of BaO) during the very initial reaction steps occurring at the
BaCO3/Cr2O3 particle interface. In support of such suggestion are
several literature reports [e.g., 21–23] confirming that the oxida-
tion of Cr(III)–O species is facilitated on the surface rather than in
the bulk, and as they crystallize into the compact ␣-chromia phase
the high-valent Cr–O species, thus obtained, are destabilized and
deoxygenated back to the initial Cr(III)–O species.
Fig. 5. IR and Ra spectra taken of the 800 ◦C calcination product of the equimolar
powder mixture of BaCO3 and Cr2O3.
being due to enhancement of its oxidative transformation to the
chromate compound.
It is worth noting, that 800 ◦C calcination products of sep-
arate BaCO3 and Cr2O3 powders gave rise to almost the same
XRD, IR and Ra results as before calcination (Tables 1 and 2). This
observation confirms persistence of the separate reactants to the
high-temperature heating in air, and, hence, attributes the forma-
tion of the XRD-detected BaCrO4, as a major product, and Ba3Cr2O6
and BaCr2O4, as minor products, to solid-state interactions involv-
ing the reactant particles and oxygen gas molecules. Moreover, the
fact that the chemical composition and mass of the separate Cr2O3
phase remain invariant to the high-temperature treatment in air
can, indeed, exclude the ␣-chromia particles from being the pri-
mary target of the oxidative action of the oxygen molecules.
IR and Ra spectra taken from BaCrOx(800), Fig. 5, are quite
consistent with the corresponding XRD results, Fig. 4. This is in
the sense that both spectra display peaks assignable, solely, to
bond vibrations of Cr(VI)–O species at >1000 cm−1, and Cr(III)–O
at <1000 cm−1 [12,14,16,21]. These results may, moreover, confirm
the absence of crystalline products other than the XRD-detected
ones; i.e. BaCrO4, Ba3Cr2O6 and BaCr2O4.
3.2.2. Isothermal heating
HT-XRD powder diffractograms were obtained upon in-situ
heating up to 1000 ◦C (in air) of BaCrOx(RT), allowing for a 10-min
isothermal heating at each set temperature. These diffractograms
were sought to determine the on-set temperature of the chemi-
cal reactivity, within the XRD error margin, leading to a crystalline
product(s) in the reaction mixture. Results obtained revealed that
it was not until the temperature reached 700 ◦C that detectable
diffraction peaks other than those diagnostic of the reactants
emerged. On further increase of the temperature, those emerging
peaks intensified, and additional peaks appeared. Due to the short
duration of heating (10 min) these emerging diffraction peaks were
generally weaker than those of the reactants.
Accordingly, calcination of BaCrOx(RT) was carried out ex-situ at
various temperatures in the range from 700 to 1000 ◦C for a longer
period of time (3 h). A typical example of the X-ray powder diffrac-
tograms of the calcination products is that shown for the product at
800 ◦C (BaCrOx(800)) in Fig. 4. As indicated in the figure, most of the
major diffraction peaks displayed are assignable to a Hashemite-like
BaCrO4 (JCPDS 15-0376). Rest of the peaks may account for minor-
ity phases of Ba3Cr2O6 (JCPDS 21-0063) and BaCr2O4 spinel (JCPDS
16-0322) products, as well as a small amount of unreacted Cr2O3.
None of the peaks displayed could be unequivocally assigned to
unreacted BaCO3. Similar XRD results were obtained for the powder
mixture following calcination at the lower temperature of 700 ◦C,
except for the detection of trace amounts of a compound assuming
the composition of Ba3(CrO4)2 that is filed in JCPDS 24–159. The
fact that this latter product is a Cr(V)-compound may be consid-
ered indicative of its being as an oxidation product and, probably, a
precursor for further oxidation to the Cr(VI)-compound (BaCrO4).
Its absence to XRD of BaCrOx(800) (Fig. 4) may be thought of as
SEM images obtained for the 800 ◦C calcination products of
separate and equimolar-mixed powders of CaCO3 and Cr2O3 are
compared in Fig. 6. Accordingly, particle morphologies of the sepa-
rate BaCO3 and Cr2O3 powders (Fig. 6A and B) are shown to remain
similar to those observed before calcination (Fig. 1A and B). In
contrast, the SEM of BaCrOx(800), Fig. 6C, visualizes a departure
from the initial particle morphologies of the uncalcined mixture
(Fig. 1C). After calcination, the longitudinal particles of BaCO3
(Fig. 1B) are hardly discernable in the product’s image (Fig. 6C).
Instead, two different particle morphologies are exhibited: (i) con-
siderably large particles of well defined crystal habit and facets
(particles (a); Fig. 6C and the inset magnified image), and (ii)
agglomerates of much smaller particles of irregular contours (par-

L.A. Al-Hajji et al. / Thermochimica Acta 483 (2009) 8–14
13
are shown (Fig. 3) to go straight on to the formation of BaCr2O4
spinel compound:
BaCO3(s) + Cr2O3(s) → BaCr2O4(s) + CO2(g)↑;
(1)
where, s = solid and g = gas.
The absence of detectable, separate phases of BaO or BaO2 at any
stage throughout the reaction course might be considered sugges-
tive of a unidirectional migration of Cr(III)–O species towards the
reaction front. The thermodynamic data presented and discussed
in Section 3.1 may lend a support to such a suggestion. In the oxi-
dizing atmosphere of O2 gas molecules, the formation of BaCr2O4
spinel seems to have been considerably hampered by a competitive
oxidation reaction of Cr(III) into higher valencies by activated oxy-
gen molecules. The oxygen activation is most likely facilitated by
the known tolerance of freshly genesized barium oxide species to
the following redox cycle: 2BaO(s) + O2(g) = 2BaO2(s) [22]. Peroxide
species are reactive oxidants [22]. Hence, formation of the XRD-
detected oxidation product Ba3(CrO4)2, which declares oxidation
of Cr(III) into Cr(V) species, may be represented by the following
conceptual equation:
4BaCO3(s) + Cr2O3(s) + 1.5O2(g)
→ Ba3(CrO4)2(s) + BaO2(s) + 4CO2(g)↑ .
(2)
In fact, the Cr(V)-compound is detectable, in minor proportions,
at ≤700 ◦C only, whereas BaO2 escapes detection through out the
reaction course. Hence, both compounds are, most likely, interme-
diates to subsequent products, but BaO2 seems to be used up at a
faster rate than Ba3(CrO4)2.
The barium monochromate (BaCrO4) and chromite (BaCr2O4),
which are XRD-detected as major and minor products, respectively,
at ≥700 ◦C may be thought to form concurrently via the following
further reaction of the BaCr(V)Ox and BaO2 intermediate species
with chromia species:
Fig. 6. SEM micrographs obtained for the separate (A and B) and equimolar-mixed
(C) powders of BaCO3 and Cr2O3 following their calcination at 800 ◦C for 3 h. The
EDX microprobing results of the (a)- and (b)-labeled locations of the micrograph (C)
are set out in Table 3.
Ba3(CrO4)2(s) + 2Cr2O3(s) + BaO2(s) → 2BaCrO4(s) + 2BaCr2O4(s)
.
(3)
ticles (b); Fig. 6C). Molecular stoichiometry calculations based on
EDX analysis results of these particles (a and b, Fig. 6C) resulted
in the data compiled in Table 3. Accordingly, the chemical compo-
sition derived for the large particles (a), Ba1.0 Cr1.0 O4.1 (Table 3), is
very close to that expected for the BaCrO4 product. On the other
hand, the composition derived for the small-particle agglomer-
ates (b), Ba0.01Cr1.0 O7.5 , may account for a chromium and oxygen
rich composite of mixed Cr(III)–Cr(VI)–oxygen compounds, or of
highly polymeric chromate. Probably, particles (b) are of precur-
sor/intermediate compounds to the major BaCrO4 product.
The product distribution characterized in thermally treated
equimolar-mixed powders of BaCO3 and Cr2O3 may help envisag-
ing different scenarios of interaction events involving the reactant
particles, depending on the surrounding gas atmosphere. In the
non-oxidizing atmosphere of N2 gas molecules, these interactions
Summation of Eqs. (2) and (3) results in the following overall Eq. (4),
which depicts the topochemical nature of the solid-state reaction
involving particles of BaCO3 and Cr2O3, and oxygen gas molecules:
4BaCO3(s) + 3Cr2O3(s) + 1.5O2(g) → 2BaCrO4(s) + 2BaCr2O4(s)
+ 4CO2(g)↑ .
(4)
The fact that XRD-detected relatively larger proportions of BaCrO4
than BaCr2O4 in the high-temperature (800–1000 ◦C) calcination
products of BaCrOx(RT) may be ascribed to differences in the crys-
tallization rates of the two compounds, or to occurrence of the
following alternative reaction course:
2BaCrO4(s) + Cr2O3(s) → BaCr2O7(s) + BaCr2O4(s)
3BaCr2O7(s) + Cr2O3(s) → 2BaCr3O10(s) + BaCr2O4(s)
(5)
(6)
Table 3
In this alternative reaction course, the chromite spinel is
thought to form as a decomposition product of the monochro-
mate compound. However due to the high thermal stability of the
monochromate, its decomposition, often, occurs via a slow deoxy-
genating polymerization into polychromates [23,24]. Ammonium
polychromates as high as (NH4)2Cr3O10 and (NH4)2Cr4O13 have
been successfully synthesized and characterized [24].
EDX-analysis results for the locations marked “a” and “b” in Fig. 6C for BaCrOx(800).
Location
“a”
Element
Line
Proportiona
(%)
Derived composition
(atomic)
O
K-ser
25.79
20.40
53.81
1.611
0.392
0.392
Ba1.0 Cr1.0 O4.1
Cr
Ba
K-alpha
K-ser
As a matter of fact, the eventual mass loss determined for
the reaction in oxygen atmosphere (5.4%, Fig. 3) cannot be met
neither exactly nor approximately by a postulated overall reac-
tion unless it is leading to formation of a polychromate; for
“b”
O
Cr
Ba
K-ser
K-alpha
K-ser
69.24
29.85
0.920
4.328
0.574
0.007
Ba0.01Cr1.0 O7.5
a
Normalized data.

14
L.A. Al-Hajji et al. / Thermochimica Acta 483 (2009) 8–14
instance,
lytical Facility (SAF) and Electron Microscopy Unit (EMU) of the
faculty of Science of Kuwait University are highly appreciated.
2BaCO3(s) + 5Cr2O3(s) + 7.5O2(g)
→ 2BaCr5O16(s) + 2CO2(g). . . (ML = 6.3%)
References
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The above presented and discussed results may help drawing the
following conclusions as to the solid/solid interactions occurring at
particle interfaces in powder mixtures of BaCO3 and Cr2O3:
1. A non-catalytic, thermochemical decomposition of BaCO3 par-
ticles is triggered by interactions at solid/solid interfaces with
Cr2O3 particles, at a much lower temperature (570 ◦C) than the
decomposition temperature (≥ 970 ◦C) of a separate barium car-
bonate in the atmosphere of air.
2. In the non-oxidizing atmosphere of N2 gas molecules,
the reaction goes straight on to the formation of barium
chromite, BaCr2O4, as the sole detectable crystalline product at
700–1000 ◦C.
3. In the oxidizing atmosphere of O2 gas molecules, the oxygen
is uptaken presumably by chemisorption at freshly genesized
interfaces of BaO/Cr(III)–O. Consequently, topochemical interac-
tions at BaO2/Cr(III)–O thus generated may lead to oxidation of
the Cr(III)–O species into higher-valent chromium and forma-
tion of Ba3(CrO4)2 intermediate compound at ≤700 ◦C, whose
further reaction with Cr2O3 at higher temperatures results in
the formation of barium monochromate, BaCrO4.
4. Formation of barium chromite, BaCr2O4, in the oxygen atmo-
sphere most probably follows further interaction of the barium
monochromate and Cr2O3 to the onset of formation of barium
polychromate and chromite.
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Acknowledgements
[24] T.V. Rode, in: J.P. Redfern (Ed.), Thermal Analysis, Macmillan, London, 1965, pp.
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