Partial phase diagram of MoO3 rich section of the ternary Bi-Mo-O system

Article abstract of DOI:10.1016/j.jallcom.2018.02.147

Partial phase diagrams of MoO₃ rich section of Bi-Mo-O system have been established at 773, 873 and 1023 K based on phase equilibration studies. Electrical conductivity measurements along with equilibration experiments were used to determine th

Full text of DOI:10.1016/j.jallcom.2018.02.147

Accepted Manuscript
Partial phase diagram of MoO rich section of the ternary Bi-Mo-O system
3
P.M. Aiswarya, R. Ganesan, R. Rajamadhavan, T. Gnanasekaran
PII:
S0925-8388(18)30592-9
10.1016/j.jallcom.2018.02.147
JALCOM 45023
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 11 September 2017
Revised Date: 8 February 2018
Accepted Date: 12 February 2018
Please cite this article as: P.M. Aiswarya, R. Ganesan, R. Rajamadhavan, T. Gnanasekaran, Partial
phase diagram of MoO rich section of the ternary Bi-Mo-O system, Journal of Alloys and Compounds
3
(2018), doi: 10.1016/j.jallcom.2018.02.147.
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ACCEPTED MANUSCRIPT
Partial phase diagram of MoO3 rich section of the ternary Bi-Mo-O system
P.M. Aiswarya, R. Ganesan, R. Rajamadhavan, T. Gnanasekaran*
Materials Chemistry Division
Indira Gandhi Centre for Atomic Research, HBNI
Kalpakkam 603 102, INDIA
Corresponding Author: Dr. T. Gnanasekaran
Former Raja Ramanna Fellow
Indira Gandhi Centre for Atomic Research
Kalpakkam 603 102, India
Email: gnani@igcar.gov.in
Abstract
Partial phase diagrams of MoO3 rich section of Bi-Mo-O system have been
established at 773, 873 and 1023 K based on phase equilibration studies. Electrical
conductivity measurements along with equilibration experiments were used to determine
the decomposition temperature of Bi2Mo2O9 and the polymorphic transformations of
Bi2MoO6.
Keywords: Lead-Bismuth Eutectic (LBE), bismuth molybdates, equilibration, phase
diagram
1. Introduction
Liquid lead and Lead-Bismuth Eutectic alloys (LBE) are currently used as spallation
target and as coolant in accelerator driven systems. The favorable chemical and
physical properties of LBE such as high boiling and low melting temperatures, low
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chemical reactivity towards air and moisture, high thermal conductivity, low neutron
absorption cross section, etc make it to be considered as an alternative to liquid sodium
coolant in fast breeder reactors [1]. However, the use of LBE as coolant is limited by
the high solubility of structural steel components in LBE [2], which could lead to
heavy corrosion [3]. Among the different steel components, nickel has very high
solubility in LBE [1]. Hence, nickel free ferritic steels with optimum concentrations of
chromium, molybdenum, etc are considered for this application. However, the
solubility of other alloying components of steels such as Fe, Cr, Mo etc would still be
significant. For example, the solubility of Fe in LBE at 823 K (operating temperature
of reactor) is 4.87 wppm [1]. As solubility is temperature dependent, dissolution of
steel components would occur at high temperature sections followed by deposition at
low temperature sections which would lead to corrosion. This corrosion can be
minimized by the insitu formation of a protective oxide layer over the steel surface by
appropriately controlling the oxygen concentration in the coolant [3, 4]. For a better
understanding of the formation and stability of the passive oxide film, it is necessary to
have knowledge on the chemical interaction of the components of structural steel with
LBE containing dissolved oxygen. In this context, studies on phase diagrams of Bi-M-
O and Pb-M-O (M = Fe, Cr, Mo) systems are important and a systematic study on these
systems is carried out in the authors’ laboratory [5-10]. This paper reports the studies
carried out to establish the partial phase diagram of MoO3 rich section of the Bi-Mo-O
ternary system.
2. Literature data
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Pseudo binary phase diagram of the Bi2O3-MoO3 system has been investigated in
the past [11-16]. The MoO3 rich section of the system, i.e. in the composition range of 0
to 50 mol% Bi2O3 is well established. Buttrey et al [17] in 2008 proposed a phase
diagram of the Bi2O3-MoO3 system by taking into account of all the data available in
literature and the part of that phase diagram valid in the composition range of 0 to 50
mol % Bi2O3 is shown in Fig. 1.
The presence of three ternary compounds, namely, Bi2Mo3O12, Bi2Mo2O9 and
Bi2MoO6 has been reported in this section. These three compounds exhibit significant
oxide ion conductivity and are known to catalyze selective oxidation of several organic
compounds and ammoxidation of olefins [18-20]. The data in literature which deal with
thermodynamic stability of these three ternary compounds and phase diagrams of the
pseudo binary Bi2O3-MoO3 system in the composition range of 0 to 50 mol% Bi2O3 are
summarized in Table 1.
The literature data on the existence and stability of the ternary compounds in this
composition range are discussed in the following section.
a. Bi2Mo3O12
Zambonini et al [21] reported the presence of this compound and structurally
characterized it. Later, several authors [11-15, 22-25] have studied this compound and
showed it to be congruently melting at ~930 K. Erman et al [22] reported an eutectic to
be formed between Bi2Mo3O12 and MoO3 and this has been supported by the later works
of Bleijenberg et al [12], Egashira et al [15] and Chen [24].
b. Bi2Mo2O9
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Erman et al [22] reported the existence of Bi2Mo2O9 which was formed by the
peritectic reaction between MoO3 rich liquid and Bi2MoO6 at 923 K. Structure of this
compound was established as fcc in their later study [26]. They showed that Bi2Mo2O9
formed an eutectic with Bi2Mo3O12 at 909 K with a eutectic composition of 72.5 mol%
MoO3. Erman et al [13] pointed out that the authors of references [11], [12] and [16]
erroneously considered the peritectic decomposition of Bi2Mo2O9 as the eutectic between
Bi2Mo3O12 and Bi2MoO6 in their studies. Hence, these authors [11, 12 and 16] did not
include the presence of Bi2Mo2O9 in their reported phase diagrams. Later studies by Chen
and Smith [14] confirmed the results of Erman et al [13]. Results of the studies made by
Egashira et al [15] were also similar except for small differences in the eutectic and
peritectic temperatures. Investigations by Batist et al [27] and Grzybowska et al [28] on
thermal stability of Bi2Mo2O9 showed it to be decomposing to Bi2Mo3O12 and Bi2MoO6
below 823 K. Investigations by Egashira et al [15] showed Bi2Mo2O9 to be stable in the
temperature range of 813 to 938 K and it disproportionated to form Bi2Mo3O12 and
Bi2MoO6 at 813 K.
c. Bi2MoO6
Zemann [29] reported this compound belonging to orthorhombic system. Although,
subsequent investigators reported it to be congruently melting [11, 12 and 16], studies
by Erman et al [13] and Chen and Smith [14] showed it to be peritectically
decomposing to liquid rich in MoO3 and the solid solution with composition 1.26:1
Bi2O3:MoO3 at ~1220 K. The latest phase diagram on Bi2O3-MoO3 system reported by
Buttrey [17] also indicated the peritectic decomposition of Bi2MoO6. A high
temperature modification of this compound was first reported by Blasse [30]. Later
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work by Erman et al [31] showed three polymorphic forms of this compound to be
present: i) γ- the low temperature phase (orthorhombic structure), ii) γ’’- the
intermediate phase (tetragonal structure) and iii) γ’- the high temperature phase
(monoclinic structure). Several studies have been carried out on the polymorphic
transformations of Bi2MoO6 [13, 15, 31-35], but the reported transition temperatures
are in disagreement. Erman et al [31] reported a reversible transition of γ to γ’’ at
temperatures between 593 K and 793 K and an irreversible transition of γ’’ to γ’ at
temperatures between 823 K and 913 K based on the high temperature X-ray
diffraction studies whereas Lim et al [34] reported 533 and 918 K as the corresponding
transition temperatures. Polymorphism of the compound was not observed in the
studies of Bleijenberg et al [12] and Chen and Smith [14]. They observed only the γ
phase of Bi2MoO6. Egashira et al [15] observed only the γ and γ’ phases of Bi2MoO6
with a transition temperature ~873 K. Studies by Watanabe and Kodama [32, 33]
showed the γ to γ’’ transition (877 K) to be reversible while γ’’ to γ’ transition (913 K)
to be irreversible and reported the corresponding enthalpies of transitions as 0.42 kJ
mol-1 and 13.4 kJ mol-1 respectively. Buttrey et al [35] also reported the γ to γ’’
,
transition to be reversible and occurring at ~ 840 K and the γ’’ to γ’ transition to be
irreversible and occurring at ~ 880 K. The three polymorphic forms of Bi2MoO6
significantly differ in their structures. Both the low temperature γ phase and the
2+
intermediate γ’’ phase are Aurivillius phases consisting of alternating layers of Bi2O2
2-
and MoO4 stacked perpendicular to the b-axis. The Mo atom has highly distorted
octahedral coordination in the γ phase, while, Mo in the γ’’ phase is tetrahedrally
coordinated to oxygen atoms. The structure of the high temperature polymorph γ’ is
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defined as a fluorite related supercell with infinite channels of bismuth polyhedra
surrounded by molybdenum tetrahedra. Although Bi2MoO6 is an oxide ion conductor,
extensive structural reorganization of metal ions and oxide ions is required for the
transition to occur and hence activation energy for transition would be high. Further the
relatively low temperature (~873 K) hinders the occurrence of these transitions. This
sluggish nature of the transition would lead to contradictory transition temperatures
depending upon the heating and cooling rates as well as the experimental conditions
employed by the investigators.
Based on the literature data, existence of three ternary compounds have been
reported in the MoO3 rich section of the Bi2O3-MoO3 system. Although the pseudo
binary phase diagram of Bi2O3-MoO3 system has been investigated by several authors,
no data exists on the ternary Bi-Mo-O system and the thermochemical properties of the
ternary oxides of the system are not available.
3. Experimental
3.1 Materials and preparation of ternary compounds
Bi2O3 powder (99.99% purity on metal basis, M/s Alfa Aesar, USA), Bi powder
(99.9% purity on metal basis, M/s Alfa Aesar, USA) and Mo powder (99.9% purity on
metal basis, M/s Alfa Aesar, USA) were used. The Bi2O3 powder was calcined at 1023 K
in air for about 6 h to remove any carbonate impurity and adsorbed moisture present in it.
The calcined powder was stored in a desiccator. The MoO3 powder was prepared by
heating ammonium heptamolybdate tetrahydrate (99.96% purity on metal basis, M/s
Sigma-Aldrich, USA) at 723 K in air for 24 h. MoO2 was prepared by the reduction of
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MoO3 at 823 K for 16 h under flowing hydrogen gas (100 %) which was presaturated
with water vapor. All the compounds were characterized by an X-ray diffractometer (M/s
Inel, France) with Cu Kα radiation and graphite monochromator and the XRD patterns
were matched with the corresponding PCPDF patterns [36].
The three ternary compounds, namely, Bi2Mo3O12, Bi2Mo2O9 and Bi2MoO6, were
prepared by the solid state reaction between the oxides. For this, stoichiometric amounts
of Bi2O3 and MoO3 were mixed well and pelletized. The pellets were placed in alumina
crucibles and heated in air at different chosen temperatures. The molar ratios of Bi2O3
and MoO3 taken for the preparation of each compound along with the temperature and
duration of heating are given in Table 2. At the end of 50% of the duration of heating, the
pellets were crushed and uniformly mixed. The phases were characterized by XRD. The
powders were repelletized and heated again. The resulting products were quenched to
room temperature and characterized by XRD. Although the mass loss of samples while
heating in open crucibles was negligible (~0.01 to 0.03%), the ternary compounds were
also prepared under vacuum by placing the alumina crucibles containing the sample
pellets in small volume (~20 ml) evacuated and sealed quartz ampoules. The ampoules
were kept inside a constant temperature zone of the furnace and heated at the chosen
temperatures. There was no observation of interaction of sample and crucible in all the
above experiments.
3.2 Equilibrations of samples with compositions on the Bi2O3 – MoO3 pseudo binary
line and the line joining Bi2O3 and MoO2
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To examine the possible existence of any new compound other than those
reported in the MoO3 rich section of the Bi2O3-MoO3 pseudo binary system, samples with
different molar ratios of Bi2O3 and MoO3 were prepared and equilibrated. The overall
composition of each sample was chosen such that it would correspond to a composition
between those of two reported adjacent ternary compounds. The initial mixture of Bi2O3
and MoO3 was thoroughly ground and made into pellets. The pellets were then kept
inside alumina crucibles and enclosed in quartz ampoules under vacuum. The samples
were equilibrated for prolonged periods at 773 K and 873 K. After the equilibration, the
samples were quenched to room temperature and the products formed were identified by
XRD. The details of these experiments are given in Table 3 (samples 1 to 3) and the
compositions chosen are shown in Fig. 2.
Samples containing different molar ratios of Bi2O3 and MoO2 were also
equilibrated for prolonged periods to examine the existence of any ternary compound
involving Bi2O3 and MoO2. For this, mixtures of Bi2O3 and MoO2 were compacted into
pellets and taken in alumina crucibles. The crucibles were then kept enclosed in
evacuated quartz ampoules and subjected to long term equilibrations at 773 K and 1023
K. After the equilibrations, samples were quenched to room temperature and the products
were identified by XRD. The compositions chosen are shown in Fig. 2 and the results
obtained from these equilibration experiments are given in Table 3 (samples 4 and 5).
3.3 Partial reduction of ternary compounds followed by long term equilibrations
To obtain preliminary information on the coexisting phases in the Bi-Mo-O
system, a known mass of Bi2Mo3O12 was subjected to partial reduction under flowing
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hydrogen gas (100 %) at 773 K for about 15 min. The overall composition of the
products obtained after this reduction was then determined based on the final mass of the
product by assuming that the mass loss was solely due to removal of oxygen (i.e. Bi to
Mo ratio remained constant during reduction). The product obtained was ground well and
made into pellets, which were then kept in alumina crucibles and sealed in low volume
quartz ampoules under vacuum. Different pellets of the samples were equilibrated at 773
K, 873 K and 1023 K for prolonged periods. After equilibration, they were quenched to
room temperature and the phases in the products were analyzed by XRD. Similar
experiments were carried out with samples of Bi2Mo2O9 and Bi2MoO6 also. The details of
the samples are given in Table 4 (samples 6 to 8) and the compositions after the reduction
are indicated in Fig. 2.
3.4 Long term equilibrations of phase mixtures
To confirm the phase fields identified from the results of the preliminary
experiments described above, experiments involving long term equilibrations were
carried out. For these experiments, samples with different overall compositions falling
within the region bound by Bi, Mo and Bi2MoO6 were prepared using different starting
materials. The samples were thoroughly ground and compacted into pellets. Each of these
sample pellets were then kept inside alumina crucibles and enclosed in evacuated quartz
ampoules. They were equilibrated for prolonged duration at 773, 873 and 1023 K. Details
of the samples and equilibration parameters are given in Table 5 (samples 9 to 21). The
overall compositions of the samples are shown in Fig. 2.
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To confirm the phases which coexist with liquid bismuth, additional equilibrations
were carried out in liquid bismuth. For this, two samples were prepared by mixing
powders of molybdenum and bismuth metals and molybdenum trioxide and made into
porous pellets. Excess of bismuth metal was taken in an alumina crucible. The crucible
was then taken inside an argon atmosphere glove box and the metal was melted to
avoid its interaction with air and formation of bismuth oxide. Each sample pellet was
kept immersed in the liquid bismuth using alumina strips which were finally tied to the
crucible by means of a binding wire. The crucibles containing the samples were then
sealed in quartz ampoules under vacuum and subjected to long term equilibration at
773 K and 1023 K. Details of these samples and equilibration parameters are given in
Table 5 (samples 22 and 23). The compositions of the samples are shown in Fig. 2.
While preparing the ternary compounds and during all the long term equilibration
experiments described in sections 3.2 to 3.5 (except the experiments which involve the
immersion of sample pellets in excess liquid bismuth), the samples were quenched to
room temperature at the end of about half of the total period of equilibration, ground well
and analyzed by XRD. The powdered samples were then repelletized and equilibration
was continued in an identical manner.
3.5 Electrical conductivity measurements
Electrical conductivity measurements were carried out by complex impedance
spectroscopy. The conductivity cell used for the measurement consisted of two platinum
electrical leads which were brazed to platinum pellets of identical dimensions (10 mm
diameter and 2 mm thickness). The sample pellet was mounted between the platinum
pellets using a spring loading arrangement. A K- type thermocouple was kept very close
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to the sample pellet so that the sample temperature could be accurately measured. Entire
assembly was kept inside a quartz chamber which in turn was kept in a furnace for
heating. The electrical conductivity was measured using a frequency response analyzer
(Model SI 1225, Solartron, M/s Schlumberger, UK) coupled with an electrochemical
interface (Model 1286, Solartron, M/s Schlumberger, UK). The impedance measurements
were made in the frequency range 10 Hz to 1 MHz with an applied perturbation potential
of 500 mV. At the end of the conductivity measurements, the sample pellets were
retrieved, powdered and characterized by XRD to establish any structural changes that
had occurred during the measurement.
During impedance measurements, the system is perturbed by applying a small
amplitude sinusoidal alternating voltage as a function of frequency and the response of
the system is measured. The impedance (Z), which is the resistance to the flow of current
can be obtained as the ratio of applied voltage to the resultant current and can be
expressed as a complex number as below:
Z = Z0 cos
φ
− jZ0 sin
φ
= Z'− jZ''
(1)
where Z'= Z0 cos
φ
, Z''= Z0 sin
φ
and
. Z’ and Z’’ are the real and imaginary
j = −1
parts of the impedance and ϕ is the phase difference. The data was plotted with real part
of impedance along the x-axis and imaginary part along the y-axis (Nyquist plot). The
impedance data obtained at each temperature was fitted using the software Zview® [37].
The bulk resistance (R) of the material was obtained by fitting a semicircle to the
experimentally obtained Nyquist plot. The conductivity σ (which is the ease with which
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electric charges pass through the material) was calculated from the bulk resistance R
using the following equation:
l
σ
=
(2)
AR
where σ is conductivity,
l
is the thickness of the sample pellet,
A
is the area of the pellet,
and R is resistance. The activation energy Ea for conduction was estimated using
Arrhenius equation:
−Ea
kT
σ
=
σ 0
e
(3)
(4)
Ea
kT
or ln
σ = lnσ 0 −
where σ0 is a constant, T is temperature and k is Boltzmann’s constant. The Arrhenius
plot for conductivity was obtained by plotting log as a function of 1/T. A change in the
σ
slope indicates the difference in the conduction mechanism and / or a phase change.
As mentioned in section 2, Bi2Mo2O9 is unstable at low temperatures and
decomposes to Bi2MoO6 and Bi2Mo3O12. The decomposition temperature has been
reported to be 813 K [15] as well as 823 K [27, 28]. In the present work, electrical
conductivity of Bi2Mo2O9 was measured to determine its decomposition temperature.
The Bi2Mo2O9 sample was prepared by the solid state reaction between
stoichiometric amounts of Bi2O3 and MoO3 at 873 K in air for 48 h. A sintered pellet of
Bi2Mo2O9 having 8 mm diameter and 3 mm thickness was made and used for
conductivity measurements. The pellet was sintered in air at 903 K for about 6 h. The
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measurements were carried out in air in the temperature range of 905 K to 754 K in the
frequency range of 10 Hz to 1 MHz with an applied perturbation potential of 500 mV.
First measurement of conductivity was made at 905 K after maintaining the sample for 24
h at that temperature. This was followed by decreasing the sample temperature by 10 K
and holding at that temperature for about 2 h. The conductivity was then measured at
each temperature. This procedure was continued till the final temperature of 754 K was
reached.
Electrical conductivity of Bi2MoO6 was also measured to determine the
temperatures at which it undergoes phase transitions. For these measurements, sintered
pellets of both the orthorhombic and monoclinic phases of Bi2MoO6 were used. The
monoclinic phase of Bi2MoO6 was prepared by the solid state reaction between
stoichiometric amounts of Bi2O3 and MoO3 at 873 K in air for 48 h whereas the reaction
at 773 K for 48 h yielded the orthorhombic phase of Bi2MoO6. With the orthorhombic
phase as sample pellet, measurements were carried out on heating it first from 700 to 963
K in steps of 10 K. The sample was maintained at the initial temperature of 700 K for
about 24 h and the conductivity was measured at that temperature. This was followed by
raising the sample temperature by 10 K and maintaining at that temperature for about 2 h.
The conductivity was then measured. When the starting phase was the high temperature
monoclinic phase, conductivity was measured while cooling the sample in steps from 973
to 723 K.
In order to follow the structural changes that could occur during conductivity
measurements, equilibrations were performed separately at different selected
temperatures falling within the temperature ranges of conductivity measurements. For
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these experiments, pellets of Bi2Mo2O9 and Bi2MoO6 were individually kept inside
alumina crucibles and equilibrated in air at different temperatures by following the
temperature program used for conductivity measurements. In the case of Bi2Mo2O9,
equilibrations were carried out at 873, 823, 798 and 773 K and for the monoclinic phase
of Bi2MoO6, equilibrations were performed at 973, 873, 823 and 773 K. In each case, the
first pellet of the sample was equilibrated at the highest temperature for 24 h and then
quenched to room temperature. The second pellet of each sample was first equilibrated at
the highest temperature for 24 h which was followed by successively reducing the
temperature by 10 K at a time and holding at each temperature for 2 h until the desired
temperature of equilibration was reached. This was followed by quenching it to room
temperature. Similar equilibrations were carried out at the other temperatures also. In the
case of orthorhombic phase of Bi2MoO6, equilibrations were carried out at 773, 823, 843,
853, 873, 923 and 973 K in air by following the heating cycle used during conductivity
measurements. The first pellet was equilibrated at the lowest temperature for 24 h
followed by quenching it to room temperature. Equilibration of the next pellet started at
the lowest temperature for 24 h followed by raising its temperature by 10 K and
maintaining at that temperature for 2 h. This raise of temperature and dwelling at that
temperature was continued till the desired temperature of equilibration was reached and
after which the sample was quenched to room temperature. All the samples quenched to
room temperature were powdered and analyzed by XRD. Details of these experiments are
given in Table 6.
4. Results and Discussion
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The XRD patterns of Bi2O3, MoO3 and MoO2 matched with the PCPDF file nos.
00-041-1449, 00-005-0508 and 00-032-0671, respectively. Similarly the XRD patterns of
Bi2Mo3O12 and Bi2Mo2O9 matched with the PCPDF file Nos. 00-021-0103 and 00-033-
0209, respectively. The preparation of Bi2MoO6 at 773 K yielded the orthorhombic phase
while the preparation at 873 K yielded the monoclinic phase. The XRD patterns of the
above two phases matched with the PCPDF files 00-084-0787 and 04-009-3413,
respectively.
The XRD patterns of the products obtained after the long term equilibrations of
different molar ratios of Bi2O3 and MoO3 (Table 3) have shown the presence of either a
mixture of two ternary compounds reported in literature or that of a ternary compound
and the metal oxide. It is again to be noted that the orthorhombic phase of Bi2MoO6 was
obtained when equilibration was carried out at 773 K while the monoclinic phase was
obtained when the equilibration was carried out at 873 K and 1023 K. The presence of
the intermediate phase (γ’’) reported in literature was not observed. In our equilibration
experiments, we did not observe the compound Bi2Mo2O9 when equilibration was carried
out at 773 K (samples 2 and 3) whereas it was observed when the equilibrations were
carried out at 873 K. This is in agreement with the data reported in the literature. The
above results also confirm the presence of only three ternary compounds, namely,
Bi2Mo3O12, Bi2Mo2O9, and Bi2MoO6 in the Bi2O3-MoO3 pseudo binary line when
MoO3:Bi2O3 ≥1.
The long term equilibrations of samples with different molar ratios of Bi2O3 and
MoO2 as starting materials have shown that there are no ternary compounds involving
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Bi2O3 and MoO2 (Table 3). However, the results revealed the existence of the phase field,
Bi-MoO2-Bi2MoO6 at 773 K and at 1023 K.
The equilibrium phases obtained after the long term equilibration of the partially
reduced ternary compounds are given in Table 4. It can be seen that upon partial
reduction, the overall composition of the reduced samples have moved towards lower
oxygen content (see Fig. 2). The XRD analysis of these samples after long term
equilibration confirmed the existence of Bi-MoO2-Bi2MoO6 phase field at 773 K as well
as at 1023 K.
Further evidence for the presence of this phase field was obtained from the
results of several long term equilibrations including equilibrations in liquid bismuth
which are summarized in Table 5. The data in the table shows the existence of Bi2MoO6-
MoO2-Bi2Mo3O12 phase field at 773 K in addition to Bi-MoO2-Bi2MoO6 and Bi-Mo-
MoO2 phase fields. These results also indicate the existence of Bi2MoO6-MoO2-
Bi2Mo2O9 and Bi2Mo2O9-MoO2-Bi2Mo3O12 phase fields at 873 K and the presence of Bi-
Mo-MoO2 and Bi-MoO2-Bi2MoO6 phase fields at 1023 K.
Thus based on the long term equilibration experiments, the existence of the
following phase fields is confirmed at 773 K: (1) Bi-Mo-MoO2, (2) Bi-MoO2-Bi2MoO6
and (3) MoO2-Bi2MoO6-Bi2Mo3O12. The phase fields identified at 873 K are: (1) Bi-Mo-
MoO2, (2) Bi-MoO2-Bi2MoO6, (3) MoO2-Bi2MoO6-Bi2Mo2O9 and (4) MoO2-Bi2Mo2O9-
Bi2Mo3O12. The phase fields identified at 1023 K are: (1) Bi-Mo-MoO2 and (2) Bi-MoO2-
Bi2MoO6. Using these results, the partial phase diagrams of the Bi-Mo-O system valid at
773 K, 873 K and 1023 K have been constructed and are shown in Figs. 3 to 5
.
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To identify the temperature above which Bi2Mo2O9 is stable, its electrical
conductivity as a function of temperature was measured. The XRD pattern of the
Bi2Mo2O9 sample used for electrical conductivity measurement matched with the PCPDF
pattern no. 00-033-0209. Fig. 6 shows the typical Nyquist plots showing the variation of
the imaginary part of the complex impedance (- Z'') versus its real part ( Z') at
representative temperatures (773 K, 800 K, 813 K, 833 K and 863 K). The variation of
conductivity with temperature (as log σ Vs 1/T) is shown in Fig. 7. As seen from the
figure, a slope change is observed at 817 ± 5 K. The XRD analysis of the sample
retrieved after the conductivity measurements (sample quenched to room temperature
from 754 K) has shown the presence of Bi2MoO6 (orthorhombic phase) and Bi2Mo3O12
phases. The results of simultaneous equilibration of Bi2Mo2O9 at different temperatures in
air (Table 6) also show that Bi2Mo2O9 is stable at 823 and 873 K but had decomposed
during equilibration at 773 and 798 K. Consideration of conductivity data in conjunction
with these XRD data indicates that Bi2Mo2O9 would decompose to form Bi2MoO6 and
Bi2Mo3O12 at 817 ± 5 K. The activation energies determined from the Arrhenius plot are
found to be 1.19 eV and 1.47 eV, respectively for the temperature ranges of 753 to 817 K
and 817 to 913 K. Hartmanova et al [38] have also measured electrical conductivity of
Bi2Mo2O9 in the temperature range of 454 to 713 K and reported the activation energy as
1.15 eV which is in agreement with the present value of 1.19 eV obtained for conduction
in the low temperature regime.
The XRD patterns of the orthorhombic and monoclinic phases of the Bi2MoO6
samples used for electrical conductivity measurements matched with the corresponding
PCPDF patterns. The variation of conductivity of the orthorhombic phase of Bi2MoO6
17

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with temperature is shown in Fig. 8. Conductivity was measured in a heating cycle from
700 to 963 K. It can be seen that at temperatures between 868 and 871 K a sharp drop in
conductivity occurred. Further increase in the temperature resulted in the steady increase
in conductivity. The XRD analysis of the samples equilibrated at 773, 823, 843 and 853
K (Table 6) had shown the presence of orthorhombic phase of Bi2MoO6 while the
equilibrations at 873, 923 and 973 K had shown the presence of the monoclinic phase
only. Hence, the drop in conductivity is due to the transformation of orthorhombic phase
to monoclinic phase at ~870 K. The presence of an intermediate phase (γ’’) has been
reported in literature in the temperature range of ~840-880 K [35]. However, in samples
after equilibration at 843 and 853 K, we could observe only the orthorhombic phase of
Bi2MoO6. Intermediate γ’’ phase could not be observed in samples from both
conductivity and equilibration experiments. The activation energies for conductivity
deduced from the plot are 1.85 eV and 1.97 eV, respectively for the temperature ranges of
763 to 868 K and 872 to 963 K corresponding to the orthorhombic and monoclinic phases
of Bi2MoO6. The Arrhenius plot obtained for the electrical conductivity measurements
carried out while cooling the monoclinic phase of Bi2MoO6 is also shown in Fig. 8. No
change in slope is observed in the conductivity plot suggesting that the compound had not
undergone any phase transition at low temperatures and this is in agreement with the
irreversible nature of the orthorhombic to monoclinic phase transition reported in
literature. This is supported by the results of the equilibration experiments with the
monoclinic phase of Bi2MoO6 given in Table 6. XRD analysis of the samples obtained
after the equilibration at all temperatures (773, 823, 873 and 973 K) showed the presence
of only monoclinic phase of Bi2MoO6.
18

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5. Conclusions
The experimental studies carried out have confirmed the existence of only three
ternary compounds, namely, Bi2Mo3O12, Bi2Mo2O9 and Bi2MoO6 in the MoO3 rich
section of the Bi2O3-MoO3 pseudo binary system. Based on long term equilibration
studies, the partial phase diagrams of Bi-Mo-O system valid at 773, 873 and 1023 K
have been deduced. Using electrical conductivity measurements along with
simultaneous equilibration experiments, the decomposition temperature of Bi2Mo2O9
was determined to be 817 ± 5 K. The polymorphic transformations of Bi2MoO6 were
studied by electrical conductivity measurements. The transition of the orthorhombic to
monoclinic phase has been found occur at ~870 K and the irreversible nature of this
transition has also been confirmed. The partial phase diagrams of Bi-Mo-O system
show that upon increasing the oxygen concentration in liquid bismuth which coexists
with Mo metal, MoO2 would precipitate first followed by the formation of Bi2MoO6
with simultaneous oxidation of Mo metal solubilized in liquid Bi
Acknowledgement
The authors are thankful to Mr. K. Haridas, for help in constructing the quartz
ampoules. One of the authors (PMA) acknowledges Department of Atomic Energy, India,
for providing the financial support.
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Figure captions
Fig. 1 MoO3 rich part of the Bi2O3-MoO3 pseudo binary phase diagram adapted from ref [17]
α: Bi2Mo3O12
β: Bi2Mo2O9
γ: Bi2MoO6
Fig. 2 Compositions chosen for equilibrations
a-c: ternary compounds present in the Bi2O3-MoO3 line; a = Bi2Mo3O12, b = Bi2Mo2O9 and c = Bi2MoO6
1-3( ): compositions chosen for equilibration using Bi2O3 and MoO3 (Table 3)
4-5( ): compositions chosen for equilibration using Bi2O3 and MoO2 (Table 3)
6-8( ): compositions correspond to the products obtained after partial reduction of ternary compounds (Table 4)
9-21( ): compositions chosen for equilibration using different starting components (Table 5)
22-23( ): compositions chosen for equilibration in liquid Bi (Table 5)
Note: Many compounds are present between Bi2O3 and c
Fig. 3 Partial phase diagram of Bi-Mo-O system at 773 K
Fig. 4 Partial phase diagram of Bi-Mo-O system at 873 K
Fig. 5 Partial phase diagram of Bi-Mo-O system at 1023 K
Fig. 6 Complex impedance spectra of Bi2Mo2O9 at various temperatures ( Z'and Z''are the real and
imaginary parts of the impedance)
Fig. 7 Variation of electrical conductivity of Bi2Mo2O9 as a function of temperature
(Below 817 K, the phases present are Bi2Mo3O12 and Bi2MoO6)
Fig. 8 Variation of electrical conductivity of orthorhombic and monoclinic phases of Bi2MoO6 as a
function of temperature

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Table 1
Literature survey on Bi2Mo3O12, Bi2Mo2O9 and Bi2MoO6
Compound
Author
Preparation route
Experimental
Observations
techniques
employed
Bi2Mo3O12
Zambonini [21]
Fusion of oxide mixtures
followed by
recrystallization [11, 13,
22]
Solid state reaction at 873
K [12, 15]
Fusion of oxide mixtures
followed by annealing at
873-943 K [14]
Visual polythermal ꢀ Reported the existence [21]
method [11]
ꢀ Congruent melting point : 921 K [11], 949 K
Belyaev and Smolyaninov [11]
Erman et al [13, 22]
Chen and Smith [14]
Egashira et al [15]
Miyazawa et al [25]
XRD [12-15, 22]
DTA [12-14]
[12], 935 K [14], 943 K [25]
ꢀ Eutectic with MoO3 : 909 K [13, 22], 891 K
[11, 14, 22]
ꢀ Eutectic with Bi2Mo2O9: 913 K [15]
ꢀ Eutectic with Bi2MoO6: 909 K [11, 22]
ꢀ Peritectic decomposition: 913 K [13]
Coprecipitation [22]
Single crystal growth by
Czochralski method [25]
Bi2Mo2O9
Erman et al [13, 22, 26]
Chen and Smith [14]
Egashira et al [15]
Fusion of oxide mixtures
followed by crystallization 26]
and by coprecipitation
[13, 22, 26]
XRD [13-15, 22,
ꢀ Structural characterization and determination
of lattice parameters [26]
ꢀ Peritectic decomposition: 938 K [13], 955 K
[14], 923 K [13, 22]
DTA [13-15]
Fusion of oxide mixtures
followed by annealing at
873-943 K [14]
ꢀ Eutectic with Bi2MoO6: 909 K [22]
ꢀ Eutectic with Bi2Mo3O12: 913 K [15], 909 K
[22]
Solid state reaction at 873
K [15]
ꢀ Stable range: 813 to 938 K[15]
Bi2MoO6
Belyaev and Smolyaninov [11]
Erman et al [13, 31]
Chen and Smith [14]
Fusion of oxide mixtures
followed by crystallization method [11]
[11]
Fusion of oxide mixtures
followed by quenching
and subsequent heating at 30, 32]
Visual polythermal ꢀ Congruent melting point :1243 K [11], 1211
K [12], 1238 [26]
DTA [12-15, 32,
34]
ꢀ Eutectic with Bi2Mo3O12 : 909 K [11]
XRD [12, 14-16,
ꢀ Eutectic with Bi2Mo2O9: 909 K [13, 14]
ꢀ Peritectic decomposition: 1193 K [11], 1173

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Bleijenberg et al [12]
Egashira et al [15]
Blasse [30]
Watanabe and Kodama [32]
Lim et al [34]
873 K [13, 31]
High temperature
XRD [31]
Dilatometry [32]
DSC [26, 32]
K [12], 1220 K [14]
ꢀ Observed only the low temperature
orthorhombic (γ) phase [12]
Fusion of oxide mixtures
followed by annealing at
873-943 K [14]
Solid state reaction at 873
K [12, 15], 1023 K [30],
800 K [32]
ꢀ Observed three polymorphic forms [13, 31]
ꢀ Observed the high temperature modification
(γ’ phase) [30]
Single crystal growth [34]
ꢀ Observed only γ and γ’ modifications;
γ to γ’ transition at 873 K and irreversible
[15]
ꢀ Transition of the low temperature phase (γ) to
the intermediate phase (γ’’ ) at : 593-793 K
[31], 877 K [32], 533 K [34] and is reversible
ꢀ Transition of the intermediate phase ( γ’’ ) to
the high temperature phase (γ’) at 823-913 K
[31], 913 to 943 K [32], 918 K [34] and is
irreversible
ꢀ
∆Htransition for γ to γ’’ transition : 0.1
kcal/mol [32]
ꢀ ∆Htransition for γ’’ to γ’ transition: 3.2
kcal/mol [32]

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Table 2
Preparation of ternary compounds in the Bi2O3-MoO3 system
Compound Molar ratios of
Bi2O3 and MoO3
Temperature Duration of Product formed*
heating
Bi2Mo3O12 1Bi2O3 : 3MoO3
873 K
773 K
48 h
Bi2Mo3O12
Bi2Mo2O9
1Bi2O3 : 2MoO3
48 h
Bi2Mo3O12,
γ-Bi2MoO6 (orthorhombic phase)
Bi2Mo2O9
873 K
773 K
873 K
48 h
48 h
48 h
Bi2MoO6
1Bi2O3 : 1MoO3
γ-Bi2MoO6 (orthorhombic phase)
γ'-Bi2MoO6 (monoclinic phase)
*
Same phases were obtained at the end of 24 h of heating

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Table 3
Equilibration of samples with mixtures of a) Bi2O3 and MoO3 and b) Bi2O3 and MoO2
Sample
No
Composition chosen
Products obtained after equilibration*
a) Samples with composition on the Bi2O3 -MoO3 pseudo binary line
At 773 K
(960 h)
At 873 K
(720 h)
1
2
1Bi2O3: 7MoO3 (composition between MoO3 and
Bi2Mo3O12)
1Bi2O3: 2.5MoO3 (composition between Bi2Mo3O12
and Bi2Mo2O9)
MoO3, Bi2Mo3O12
MoO3, Bi2Mo3O12
Bi2Mo3O12, Bi2MoO6
(orthorhombic phase)
Bi2Mo3O12, Bi2Mo2O9
3
1Bi2O3: 1.4MoO3 (composition between Bi2Mo2O9
and Bi2MoO6)
Bi2Mo3O12, Bi2MoO6
(orthorhombic phase)
Bi2Mo2O9, Bi2MoO6
(monoclinic phase)
b) Samples with compositions on the line joining Bi2O3 and MoO2
1Bi2O3 : 3MoO2
Bi, MoO2, Bi2MoO6
(orthorhombic phase)
Bi, MoO2,
Bi2MoO6(monoclinic phase)
4
5
1Bi2O3 : 1MoO2
Bi, MoO2, Bi2MoO6
(orthorhombic phase)
Bi, MoO2, Bi2MoO6
(monoclinic phase)
*Same phases were obtained at the end of 50% of the total duration of heating