Studies on the thermal stability of BiFeO3 and the phase diagram of Bi-Fe-O system

Article abstract of DOI:10.1016/j.jallcom.2019.03.205

The thermal stability of BiFeO₃ was examined by different experimental techniques and found that it is metastable at low temperatures and it attains thermodynamic stability at around 940 K. Long term equilibration studies at 773 and 1023 K, and characterization of coexisting phases by XRD revealed the phase fields. Based on this, the isothermal section of the ternary phase diagram of Bi-Fe-O system was constructed at 773 and 1023 K.

Full text of DOI:10.1016/j.jallcom.2019.03.205

Accepted Manuscript
Studies on the thermal stability of BiFeO and the phase diagram of Bi-Fe-O system
3
A.V. Meera, Rajesh Ganesan, T. Gnanasekaran
PII:
S0925-8388(19)31021-7
DOI:
https://doi.org/10.1016/j.jallcom.2019.03.205
JALCOM 49969
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 4 January 2019
Revised Date: 9 March 2019
Accepted Date: 13 March 2019
Please cite this article as: A.V. Meera, R. Ganesan, T. Gnanasekaran, Studies on the thermal stability
of BiFeO and the phase diagram of Bi-Fe-O system, Journal of Alloys and Compounds (2019), doi:
3
https://doi.org/10.1016/j.jallcom.2019.03.205.
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ACCEPTED MANUSCRIPT
Studies on the thermal stability of BiFeO and the phase diagram of Bi-Fe-O system
3
A.V. Meera, Rajesh Ganesan* and T.Gnanasekaran
Materials Chemistry Division, Materials Chemistry and Metal Fuel Cycle Group
Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, India
*
Corresponding Author:
Dr. Rajesh Ganesan
Indira Gandhi Centre for Atomic Research
Kalpakkam 603 102, India
Email: rajesh@igcar.gov.in
Abstract
The thermal stability of BiFeO was examined by different experimental techniques
3
and found that it is metastable at low temperatures and it attains thermodynamic stability at
around 940 K. Long term equilibration studies at 773 and 1023 K, and characterization of
coexisting phases by XRD revealed the phase fields. Based on this, the isothermal section of
the ternary phase diagram of Bi-Fe-O system was constructed at 773 and 1023 K.
Key words: Coolant, Bi-Fe-O system, Equilibration, Phase diagram, Thermal stability, Emf
measurement.
1
. Introduction
Liquid lead and lead-bismuth eutectic alloy are considered as candidate coolant and
spallation target in Accelerator Driven Systems (ADS). They are also considered as
candidate coolants in advanced nuclear reactors. However, they are highly corrosive towards
1

ACCEPTED MANUSCRIPT
the structural steels, and this is because of the high solubility of the major alloying elements
in them [1]. This corrosion could be minimized by forming a passive oxide layer on the
steels by maintaining controlled oxygen levels in the liquid metal coolant [2]. To understand
the composition and thermodynamic stability of the passive oxide film, information on the
phase diagrams of Pb-M-O and Bi-M-O (M = alloying elements in steels) systems as well as
the thermodynamic data of the relevant ternary compounds existing in these systems are
required. Our studies on Pb-Fe-O and Pb-Cr-O systems were earlier reported [3-8]. A partial
phase diagram of Bi-Fe-O system and the standard Gibbs energy of formation of one of the
ternary compounds of this system viz., Bi Fe O have been reported by the present authors
2
4
9
recently [9]. This paper presents the results of the studies carried out to establish the thermal
stability of BiFeO and the investigations to deduce the complete phase diagram of Bi-Fe-O
3
system at 773 and 1023 K.
2
. Literature Survey
Pseudo-binary Bi O -Fe O system was investigated by several authors in the past
2
3
2
3
[
10-19]. Salient features of these investigations were in reference 9. In Bi-Fe-O system, only
three ternary compounds viz., Bi FeO , Bi Fe O and BiFeO are present. Phapale et al.
25
39
2
4
9
3
[
20] measured the thermochemical properties of these compounds by calorimetry. They
measured enthalpy of formation (ꢀH°f,298 K) of these compounds by acid solution
calorimetry and molar heat capacity by differential scanning calorimetry. Using the data,
they deduced their Gibbs energy of formation using estimated values for the entropy of
formation. BiFeO is one of the most widely studied multiferroic material and a candidate
3
for new generation of lead-free electroceramics [21-23]. Zhang et al. [21] reviewed on
chemical-route derived nanostructured BiFeO3 such as thin films, nanowires, and
nanoparticles. Wu et al.[22] discussed about the BiFeO based ceramic bulk, thin films, and
3
2

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nanostructures of BiFeO3 and their multifunctional applications. Synthesis and
characterization of electrical features of BiFeO and the effect of doping were reviewed by
3
Molak et al. [23]. Experimentally determined temperature range of stability of BiFeO3
reported by different authors is not in agreement. Attempts by Achenbach et al. [24] to
prepare phase pure BiFeO by solid-state reaction between Bi O and Fe O at temperatures
3
2
3
2
3
below 973 K were unsuccessful, and they observed the reaction to be incomplete. They also
reported BiFeO to be unstable above 1023 K. According to them, the formation of Bi Fe O
9
3
2
4
was energetically competitive to the formation of BiFeO in the temperature range of 973 to
3
1
023 K. Preparation of BiFeO always yielded a small amount of Bi Fe O and the
3 2 4 9
corresponding amount of residual Bi O . They could prepare phase pure BiFeO only by
2
3
3
taking an excess of Bi O (Bi O :Fe O = 2:1) and heating in the air at 1023 K followed by
2
3
2
3
2
3
air quenching and removal of the excess Bi O by leaching with concentrated HNO .
2
3
3
Experiments by De Sitter et al. [25] confirmed these observations. Morozov et al. [26]
studied the characteristics of formation of BiFeO and concluded that BiFeO prepared by
3
3
heating a mixture of Bi O and Fe O always contained impurity phases such as the phase
2
3
2
3
with γ-Bi O structure and Bi Fe O , presumably due to slow kinetics. Valant et al. [27]
2
3
2
4
9
reported that phase pure BiFeO could be prepared by heating together high purity (99.9995
3
%
pure) iron and bismuth oxides. They showed that the presence of impurities of the order
of 0.1 mol% in the reactants could produce ~3 vol% Bi FeO and 7.6 vol% of Bi Fe O .
2
5
39
2
4
9
Carvalho and Tavares [28] reported that temperature and time were critical in the synthesis
of phase pure BiFeO . According to them, higher BiFeO content could be obtained by a
3
3
thermal treatment at 873 K for 1 h. Preparations at temperatures below and above this
temperature always yielded secondary phases, namely, Bi Fe O and Bi FeO . Also, heat
2
4
9
25
39
treatment for a longer time at 873 K resulted in the decomposition of BiFeO to the
3
secondary phases. So they concluded that BiFeO to be metastable at 873 K [28]. Selbach et
3
3

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al. [29] studied the thermodynamic stability of BiFeO . High-temperature XRD (HTXRD)
3
experiments performed on BiFeO prepared by solid-state route as well as by wet chemical
3
route and the analysis of the isothermally heat-treated Bi O -Fe O mixtures, showed that
2
3
2
3
BiFeO was metastable in the temperature range of 720 to 1040 K and decomposed to give
3
Bi Fe O and Bi FeO . At higher temperatures, these secondary phases reacted back to
2
4
9
25
39
form BiFeO . Using thermodynamic data reported by Phapale et al. [20], they deduced the
3
standard Gibbs energy change of the following reaction to be positive in the temperature
range of 720 to 1040 K and would be negative above 1040 K indicating BiFeO to be stable
3
only above 1040 K:
Bi FeO +12 Bi Fe O → 49 BiFeO
3
2
5
39
2
4
9
Lu et al. [19], based on their studies using 99.999 wt% Bi O and 99.99 wt% Fe O ,
2
3
2
3
determined the peritectic decomposition temperature of BiFeO as 1125 K and identified a
3
ferroelectric transition for this compound at 1098 K. Maurya et al. [30] studied the reactions
in equimolar mixtures of Bi O and Fe O by heating them at different temperatures for 1 h
2
3
2
3
in the air and recording the XRD patterns of the products after cooling them to room
temperature. The authors found that Bi FeO (the authors represented it as Bi FeO ) was
2
5
39
25
40
formed at temperatures above 823 K followed by the appearance of BiFeO phase at 973 K.
3
The impurity phase viz., Bi FeO was present at all those temperatures. Activation of the
2
5
39
reactants by ball milling reduced the temperatures by about 100 K but the results obtained
were similar. Egorysheva et al. [31] investigated the extent of formation of BiFeO when
3
equimolar mixtures of Bi O and Fe O were heated in the air for 20 minutes at 873 K, 973
2
3
2
3
K, 1023 K, 1073 K, and 1123 K. Their experiments showed that BiFeO did not form at 873
3
K while Bi FeO was formed as major phase. From 973 K onwards BiFeO was one of the
2
5
39
3
phases formed, and its fraction gradually increased with increase in temperature. At 1123 K,
BiFeO was the major phase while Bi FeO was present as a minor phase. The authors
3
25
39
4

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reasoned that Bi FeO was formed (during the initial phase of the reaction) in large
2
5
39
3
+
fraction because of its wide homogeneity range and high diffusion coefficient of Bi in it, in
comparison with the corresponding diffusion coefficients in BiFeO , Bi O , and Fe O . The
3
2
3
2
3
3
+
-8
2 -1
diffusion coefficient of Bi in Bi FeO at 1013 K was reported as 1.5 × 10 m s while in
25
39
-16
2
-1
BiFeO it was reported as 3.2 × 10 m s . The corresponding reported values in Bi O and
3
2
3
-17
2 -1
Fe O were found to be 1.3 × 10 m s . Lomanova and Gusarov [32] studied the reactions
2
3
between coprecipitated bismuth and iron hydroxides while heating them in a stepwise
manner with isothermal equilibrations at 753, 773, 793, 813, 833, 873 and 903 K for 5 to 7
minutes followed by high-temperature XRD measurements. Their results showed BiFeO3
and Bi FeO form at 753 K. At 773 and 793 K, Bi Fe O appeared along with BiFeO and
2
5
39
2
4
9
3
Bi FeO . At 813 K and above, the fraction of Bi Fe O increased while that of Bi FeO
39
2
5
39
2
4
9
25
decreased substantially.
To summarize, the thermodynamic stability of BiFeO has been under disagreement.
3
Even preparation of phase pure BiFeO was difficult which was attributed to the purity of
3
chemicals used. Even though a partial phase diagram of Bi-Fe-O system was established in
the author’s laboratory, a complete phase diagram of Bi-Fe-O ternary system was not
deduced.
3
. Experimental
Bi O powder (99.99 wt% purity on metals basis, M/s Alfa Aesar, UK), Fe O
3
2
3
2
powder (99.99 wt% purity on metals basis, M/s Alfa Aesar, UK) were used as the starting
materials for the preparation of the ternary compounds. For phase equilibration studies, Fe
powder (99.9+ wt% purity on metals basis, M/s Alfa Aesar, USA), FeO (99.9 wt%, M/s
Cerac, USA) and Bi powder (99.5 wt% on metals basis, M/s Alfa Aesar, USA) were also
used as the starting materials. Fe O was prepared by reducing Fe O at 800 K for 6 h under
3
4
2
3
5

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Ar-1% hydrogen gas mixture saturated with water vapor. Bi O was calcined in the air at
2
3
9
73 K for 5 h to remove the carbonate impurity and moisture while Fe O was calcined at
2 3
5
23 K for 2 h to remove the moisture. These oxides were stored in a desiccator before using
them in experiments.
3
.1 Preparation of ternary oxides:
The ternary compounds were prepared by solid-state reaction between Bi O and
2
3
Fe O taken in appropriate molar ratios. The starting materials were ground thoroughly,
2
3
made in the form of pellets and heated in platinum crucibles for 144 h in the air at 973 K for
preparing Bi FeO and Bi Fe O . For preparing BiFeO , the temperature chosen was 1073
2
5
39
2
4
9
3
K. All these samples were ground at the end of 50% of the total time of heating. They were
repelletized followed by heating at the same temperature. The resulting products were
characterized by XRD. BiFeO and Bi Fe O were also prepared by a method involving co-
3
2
4
9
precipitation of the hydroxides. For this preparation, stoichiometric amounts of Bi O and
2
3
-1
Fe O were first dissolved in a minimum amount of hot nitric acid (35.2 mol kg ), and the
2
3
resultant nitrate solution was added to an excess ammonium hydroxide solution. The
precipitate formed was then filtered, washed, dried, and heated in the air at 773 K for 24 h
while preparing BiFeO . While preparing Bi Fe O , the temperature of heating was 823 K in
3
2
4
9
the air. The products obtained were characterized by XRD. Phase purity and compositional
homogeneity were checked by SEM/EDS. The prepared compounds assayed for the metallic
impurities present in them by using inductively coupled plasma atomic emission
spectroscopy (ICP-AES). Bi to Fe ratio in the prepared samples was also determined by ICP-
AES.
3
.2. Investigations on thermal stability of BiFeO3
6

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To determine the temperature range of stability of BiFeO3, studies involving
thermogravimetry/differential thermal analysis (TG/DTA), differential scanning calorimetry
(DSC), long term equilibrations and oxygen potential measurements were carried out.
3
.2.1. TG/DTA and DSC studies with BiFeO3
For the TG/DTA experiment, ~50 mg of BiFeO taken in Pt crucible was heated up
3
to 973 K in argon atmosphere at a heating rate of 10 K/min followed by cooling at a rate of
0 K/min up to room temperature, and the thermal events were analyzed using TG/DTA
1
system, SETSYS Evolution 16/18 model, M/s Setaram, France. After the experiment, the
sample was characterized by XRD for identifying any change it might have undergone
during TG/DTA experiments. Thermal behavior of BiFeO was also studied by DSC.
3
Approximately 150 mg of sample was used, and DSC runs were carried out up to 823 K at a
heating rate of 10 K/min in an argon atmosphere and maintained at that temperature for 10
minutes. The sample was then cooled to room temperature. A heat-flux type differential
scanning calorimeter, of M/s. Mettler Toledo GmbH (model number DSC821e/700),
Switzerland was used in this study. Before and after the experiment, the sample was
characterized by XRD.
3
.2.2. Equilibration of BiFeO in the vacuum and the air at different temperatures
3
As discussed in section 2, BiFeO has been reported to be metastable at low
3
temperatures (720 to 1040 K) and attains thermodynamic stability at relatively high-
temperature (above 1040 K). Hence, experiments involving equilibrating BiFeO in the air
3
and vacuum for prolonged periods were carried out to determine the temperature range of
stability of this compound. The schematic of the experimental setup used for this study is
shown in Fig. 1. For this experiment, BiFeO prepared by co-precipitation method described
3
in section 3.1 was used. Samples made in the form of pellets were taken in zirconia
7

ACCEPTED MANUSCRIPT
crucibles. Crucibles containing the pellets were placed in two one-end closed quartz tubes
(45 cm long and 1.5 cm dia) at different heights using quartz spacers. In each quartz tube,
-6
nine samples were loaded. One of the quartz tubes was sealed under vacuum (10 torr)
while the other tube was kept open to air. A K-type thermocouple which can move vertically
at a low speed (0.38 cm/min) with the help of a motor was inserted inside another quartz
tube of 8 mm diameter and was used to measure the temperature of the samples as a function
of its height. The three quartz tubes were tied together and placed inside a temperature
gradient furnace assembly. The assembly consisted of two furnaces which were stacked one
over the other. The temperature of each furnace was maintained at different temperatures
and controlled within ± 1 K. This arrangement was useful to maintain a required temperature
gradient in the furnace. The temperature data was logged using a data acquisition system.
From the actual position of the sample, the temperatures of the samples were determined.
The temperature of the samples varied between 716 to 1121 K. Samples were equilibrated
for 480 h, quenched in ice-cold water and the products were characterized by XRD. The
details of the equilibrations are given in Tables 1 and 2.
3
.2.3. Equilibration of samples containing BiFeO with liquid Bi
3
Thermal stability of BiFeO was also examined by equilibrating phase mixtures
3
containing BiFeO with liquid Bi. Equilibrations were carried out at 873, and 1023 K.
3
Starting materials were made in the form of porous pellets. They were kept forcibly
immersed in liquid Bi (taken in alumina crucibles) using alumina strips tied to the crucible
as the density of pellets are less than that of liquid Bi. The sample preparation was carried
out in an inert atmosphere glove box. Then the samples were taken out from the glove box,
vacuum sealed in quartz ampoules and equilibrated at the chosen temperatures for 480 h.
8

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After equilibration, the ampoules were quenched in ice-cold water; the samples were
retrieved and characterized by XRD. The details of these equilibrations are given in Table 3.
3
.3 Additional phase equilibrations
To determine the phase fields in the ternary Bi-Fe-O system, equilibrations were
carried out by taking appropriate mixtures of bismuth and iron metal powders and their
oxides. After thorough mixing, the starting materials were made into pellets. The pellets
were placed in zirconia crucibles which were hermetically sealed inside copper tubes. The
sealing was carried out by pulsed tungsten inert gas welding inside an argon atmosphere
glove box containing < 5 ppm of oxygen and moisture. The samples in copper tubes were
then equilibrated by placing them in a stainless steel vessel under flowing argon gas. The
temperature and total duration of equilibrations were 1023 K and 480 h, respectively. All the
equilibrations were carried out with one intermediate grinding at the end of 50% of the total
duration of heating, pelletization and hermetic sealing in copper tubes. Equilibrations were
also carried out at 773 K with a total equilibration time of 960 h. Initially, these
equilibrations were carried out in the vacuum sealed quartz tubes with samples contained in
alumina crucibles. However, in most of the cases, the quartz tubes were severely attacked.
Analysis of the deposits on the quartz tubes showed significant amounts of bismuth
indicating the attack was due to Bi O vapor. Since Cu O is thermodynamically less stable
2
3
2
°
than Bi O [∆G for the reaction: 6 Cu (s) + Bi O (s)
→
2 Bi (l) + 3 Cu O (s) is 24.6 kJ
2
2
3
2
3
-1
-1
mol at 773 K and 3.3 kJ mol at 1023 K.], copper tubes were used to contain the samples
taken in zirconia crucibles. The products obtained after equilibrations were analyzed by
XRD. The compositions of the samples taken for equilibrations are shown in Fig. 2. The
details of these experiments are given in Tables 4 and 5.
9

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To confirm the phases that co-exist with liquid bismuth, samples were equilibrated
with an excess of liquid bismuth also. Porous pellets of samples containing Bi metal powder
as a constituent were prepared and were kept immersed in an excess of molten bismuth
metal taken in alumina crucibles as described in section 3.2.3. Bismuth metal was taken as a
constituent of the sample pellet to ensure good contact between the solid phases and the
excess liquid metal in the crucible during equilibrations. These samples were prepared inside
the argon atmosphere glove box. The samples were then taken outside the glove box,
vacuum sealed in quartz ampoules and equilibrated. Since the pellets were immersed in
excess liquid Bi, vaporization of Bi O was minimum, and therefore they could be sealed in
2
3
quartz ampoules. The initial overall compositions of the pellets are shown in Fig. 2. The
composition and the initial phases present in the samples taken for equilibration at 1023 K
are given in Table 4. The corresponding data on the samples equilibrated at 773 K are given
in Table 5.
3
.4. Oxygen potential measurements by emf method
With a view to study the temperature range of stability of BiFeO and measure the
3
Gibbs energy of formation of BiFeO and Bi FeO , four emf cells using yttria stabilized
3
25
39
zirconia solid electrolyte were constructed, and oxygen potentials were measured.
-)W, Sample-1 |YSZ| (O , 1atm), Pt (+)
(
(I)
2
(
-)W, Sample-2 |YSZ| (O , 1atm), Pt (+)
(II)
2
(
-)W, Sample-3 |YSZ| (O , 1atm), Pt (+)
(III)
(IV)
2
(-)W, Sample-4 |YSZ| (O , 1atm), Pt (+)
2
For cell I, Bi, Bi Fe O and BiFeO (containing a small amount of Bi FeO ) mixture
2
4
9
3
25
39
was used as the sample electrode (sample 1), and for cell II, a mixture of Bi, Bi Fe O and
2
4
9
Bi FeO was used as the sample electrode (sample 2). Two sample electrodes with
2
5
39
1
0

ACCEPTED MANUSCRIPT
different compositions but falling within the section bound by Bi, BiFeO and Bi FeO
39
3
25
were used for measurements with cell III and IV (samples 3 and 4). Emf values were
measured in the temperature range of 773 to 1023 K. The details of the emf measurements
are given in Table 6.
The details of the experimental assembly of the galvanic cell are discussed in Ref.
[
9]. One end closed yttria stabilized zirconia (YSZ) solid electrolyte tube having a flat
bottom (13 mm OD, 9 mm ID and 300 mm long) supplied by M/s Nikkato Corporation,
Japan was used. The reference electrode of the cell was prepared by applying platinum paste
(M/s Eltecks Corporation, India) over the inner bottom surface of the electrolyte tube and
heating it at 1373 K for 2 h in the air which resulted in a uniform and porous platinum film
over the electrolyte surface. Pure oxygen gas was used as the reference. For sample
electrodes, tungsten wire was used as an electrical lead. A sample electrode was prepared by
mixing the starting materials in approximately equal volume ratios and by pelletizing it. The
entire cell assembly was taken inside an argon atmosphere glove box. An appropriate
quantity of Bi metal was melted in an alumina crucible, and the sample pellet was then
forcibly dipped in liquid bismuth by placing the solid electrolyte tube over it in such a way
that the lower end of the tungsten lead was dipped in liquid Bi. After slow cooling, the entire
cell assembly was taken outside the glove box. During emf measurements, high purity argon
and oxygen gases were passed through the sample and reference compartments, respectively.
The cell assembly was placed in the constant temperature zone of a furnace. A 50 mm long,
hollow cylindrical stainless steel block was also placed in the constant temperature zone of
the furnace to further enhance the uniformity of the temperature in the zone. Using this
arrangement the cell temperature could be controlled within ± 0.2 K using a PID temperature
controller. The stainless steel block was grounded to avoid any ac pickup in the emf signal.
1
4
The cell emf was measured using a high impedance electrometer (input impedance >10 Ω,
1
1

ACCEPTED MANUSCRIPT
M/s Keithley, U.S.A, Model-6514) and the temperature was measured using a multimeter
(M/s Agilent Technologies, Malaysia, Model- 34970A Data acquisition/switch unit). Data
were recorded using a computer interface. Attainment of equilibrium was tested by shorting
the two electrical leads and testing for the restoration of the pre-test emf. The readings were
recorded when the cell emf was stable within ± 0.05 mV for at least 6 h. The thermo emf due
to dissimilar electrical leads viz., platinum and tungsten were measured separately by
forming a junction between the two electrical leads followed by the measurement of the
thermo emf between them as a function of temperature. Using this data, the measured cell
EMF was corrected to derive the actual EMF.
Before and after completion of the emf measurements (which involved multiple
heating and cooling cycles), the sample electrodes were analyzed by XRD.
4
4
. Results and discussion
.1. Characterization of the ternary compounds
The XRD patterns of Bi Fe O prepared by solid-state reaction method and co-
2
4
9
precipitation route and Bi FeO prepared by solid-state reaction method matched exactly
2
5
39
with the patterns reported in JCPDS files 74-1098 and 46-0416, respectively [33]. Phase
pure sample of BiFeO could not be prepared by both the methods. XRD pattern of BiFeO3
3
sample showed the presence of a small amount of Bi FeO also. Except for the peaks of
2
5
39
Bi FeO , all other peaks in the XRD pattern obtained for the BiFeO sample matched with
2
5
39
3
the pattern, 77-4078 reported in JCPDS file. The XRD patterns of the compounds prepared
by the solid-state reaction of oxides are compared with their PCPDF patterns in Fig. 3.
SEM/EDS analysis of Bi FeO and Bi Fe O showed uniform phase distribution with no
25
39
2
4
9
impurity phases, and the composition was found to be homogeneous with 5% uncertainty.
1
2

ACCEPTED MANUSCRIPT
The analysis results of ICP-AES showed that the total metallic impurities are less than 2
ppm and Bi to Fe ratios in Bi FeO and Bi Fe O were 25.0 ± 1.4 and 0.50 ± 0.03.
2
5
39
2
4
9
4
.2. Experiments to determine the temperature range of stability of BiFeO3
.2.1. TG/DTA and DSC experiments with BiFeO3
4
The thermogram obtained by TG/DTA of BiFeO from 298 to 973 K did not show
3
any weight change as well as any thermal event both on heating and cooling. XRD pattern of
BiFeO after the TG/DTA experiment also did not show any change from its XRD pattern
3
taken prior to the experiment. The thermogram from DSC experiment in the temperature
range of 298 to 823 K also gave the same results. This could mean either BiFeO is
3
thermodynamically stable in the temperature range of study or the kinetics of its
decomposition is too slow that it did not decompose within the short dwell time of the
sample during TG/DTA and DSC experiments at high temperatures.
4
.2.2. Equilibration of BiFeO in the vacuum and air at different temperatures
3
As described in section 3.2.2, nine samples of BiFeO were simultaneously
3
equilibrated for prolonged periods in a temperature gradient furnace in the vacuum to
determine its temperature range of stability. Similar equilibrations were carried out in the air
also. The results obtained are given in Tables 1 and 2. As mentioned in section 4.1, phase
pure BiFeO could not be prepared and it always contained a small amount of Bi FeO .
3
25
39
XRD patterns obtained after equilibrating the samples in the vacuum and air are shown in
Figs. 4 and 5, respectively. The pattern of the samples equilibrated at temperatures below
7
63 K in the vacuum (samples 1 to 3 in Table 1) was identical to that of the starting material,
i.e., a mixture of BiFeO (major phase) and Bi FeO (minor phase). A similar feature is
3
25
39
observed for samples equilibrated in the air at 757 K and below (samples 10 to 13 in Table
). In the case of the sample which was equilibrated in the vacuum at 763 K, the intensity of
2
1
3

ACCEPTED MANUSCRIPT
XRD lines of Bi FeO increased along with those of Bi Fe O . Simultaneously, a reduction
2
5
39
2
4
9
in the intensity of the lines corresponding to BiFeO was also observed. This shows the
3
decomposition of BiFeO to the neighboring compounds, namely Bi FeO and Bi FeO .
3
25
39
2
9
But this decomposition was not complete even after 480 h. A similar feature is observed in
the sample equilibrated in the air at 807 K. In the case of the samples equilibrated at 805 and
8
71 K in the vacuum, and 853 K in the air, BiFeO had decomposed completely to
3
Bi FeO and Bi FeO . In the case of sample equilibrated at 943 K in the vacuum and at
2
5
39
2
9
9
41 K in the air, the presence of a small amount of BiFeO could be identified although
3
major phases are Bi FeO and Bi Fe O . In the case of samples equilibrated at higher
2
5
39
2
4
9
temperatures, BiFeO was the major phase although small quantities of the other two phases
3
could be identified. These results indicate that the kinetics of decomposition of BiFeO is
3
very sluggish and it appears as a metastable compound at low temperatures. It appears as a
thermodynamically stable phase only at temperatures around 940 K. These results are in
agreement with those reported by Maurya et al. [30] and Egorysheva et al. [31]. In their
work, the authors observed the formation of BiFeO at temperatures between 873 and 973 K.
3
Although BiFeO could be prepared as a metastable phase by heating the hydroxides at 773
3
K for shorter periods (section 3.1), it is found to decompose, although not to completion, on
prolonged heating at 763 K (section 3.2.2) [Table 2 shows reactions between Bi O and
2
3
Fe O only]. This could presumably because of the small Gibbs energy change for the
2
3
decomposition reaction given below and its slow kinetics resulting in metastability of
BiFeO at low temperatures:
3
4
9 BiFeO3 12 Bi Fe O + Bi FeO
→
2 4 9 25 39
The sluggish kinetics of decomposition of BiFeO as well as its formation from the
3
decomposition products have been exemplified by the results of the emf measurements also
1
4

ACCEPTED MANUSCRIPT
(discussed in section 4.4). Further experiments are required to determine the exact
temperature above which BiFeO becomes a thermodynamically stable phase.
3
4
.2.3. Equilibration of samples containing BiFeO with liquid Bi
3
XRD patterns obtained after equilibrating mixtures of Bi, Bi Fe O and BiFeO in
2
4
9
3
liquid Bi at 873 and 1023 K for 480 h are shown in Fig. 6, and the results are summarized in
Table 3. At 873 K, the intensity of the lines corresponding to BiFeO had drastically
3
reduced, and the intensities of the lines of Bi Fe O and Bi FeO had increased. At 1023 K,
2
4
9
25
39
the intensity of the lines of Bi FeO had decreased, and those of BiFeO had increased.
2
5
39
3
This is in agreement with the observations made during equilibrations of BiFeO in the air
3
and vacuum described in section 4.2.2.
XRD patterns obtained after equilibrating mixtures of Bi, BiFeO and Bi FeO in
3
25
39
liquid Bi at 873 and 1023 K are shown in Fig. 7 and the results are given in Table 3. At 1023
K, BiFeO had not decomposed, but at 873 K, it decomposed almost completely and the
3
other two phases, i.e., Bi Fe O and Bi FeO had formed. This is again in agreement with
2
4
9
25
39
the results of long term equilibration of BiFeO in the air and vacuum described in the
3
earlier section. These results also confirm that BiFeO is metastable at low temperatures and
3
would become stable only at high temperatures.
4
.3 Additional phase equilibration studies
Results of long term equilibrations of samples with compositions falling in the
section bound by Bi, Bi Fe O and Bi O at 1023 K are given in Table 4. XRD analysis of
2
4
9
2
3
samples 1 and 2 after equilibration showed the presence of a major amount of Bi, Bi Fe O
9
2
4
and BiFeO and a small amount of Bi FeO . It is to be pointed out that the experiments to
3
25
39
prepare BiFeO (described in section 4.1), always yielded a small amount of Bi FeO also.
3
25
39
1
5

ACCEPTED MANUSCRIPT
XRD analysis of samples 3 and 4, after equilibration showed the presence of Bi, BiFeO and
3
Bi FeO , indicating the coexistence of these phases at 1023 K. Phases present in samples 5
2
5
39
to 7, after their equilibration, confirmed the presence of Bi-Bi FeO -Bi O phase field at
2
5
39
2
3
1
023 K. From these results, the phase fields identified in the region bound by Bi, Bi Fe O
2 4 9
and Bi O at 1023 K are: 1) Bi-BiFeO -Bi FeO and 2) Bi-Bi FeO -Bi O . According to
2
3
3
25
39
25
39
2
3
the long term equilibrations of BiFeO discussed above, BiFeO is stable at 1023 K. Present
3
3
experiments have also established the thermodynamic stability of BiFeO at this
3
temperature. However, the existence of Bi-Bi Fe O -BiFeO phase field was not evident in
2
4
9
3
any of the equilibration studies at 1023 K, but its existence can be deduced since (i) the
existence of Bi-Bi Fe O -Fe O and Bi-BiFeO -Bi FeO phase fields have been confirmed
2
4
9
2
3
3
25
39
by the long term equilibrations and (ii) the coexistence of Bi Fe O and BiFeO , and BiFeO
3
2
4
9
3
and Bi FeO has been established by long term equilibration of different molar ratios of
2
5
39
Bi O and Fe O in the air in the authors’ previous work (discussed in section 4.2 of Ref.
2
3
2
3
[
9]).
Results of equilibrations at 773 K are summarized in Table 5, and they need to be
considered in light of long term equilibrations of BiFeO discussed above. Equilibrations at
3
7
73 K also showed the same results as in the case of equilibrations at 1023 K. Samples, 8
and 9 (in Table 5) whose compositions were in the region bound by Bi, Bi Fe O and
2
4
9
BiFeO , gave a mixture of Bi, Bi Fe O , BiFeO and a small amount of Bi FeO . Sample
3
2
4
9
3
25
39
1
0, whose composition was in the region bound by Bi, BiFeO and Bi FeO , after
3 25 39
equilibration, gave a mixture of Bi, BiFeO and Bi FeO , indicating the coexistence of
3
25
39
these phases at 773 K. Samples 11 and 12, confirmed the presence of the phase field: Bi-
Bi FeO -Bi O at 773 K. From these results, the presence of the following phase fields was
2
5
39
2
3
identified at 773 K: (a) Bi-BiFeO -Bi FeO and (b) Bi-Bi FeO -Bi O . Since BiFeO is
3
25
39
25
39
2
3
3
metastable at 773 K, the phase which will be coexisting with Bi and Bi Fe O will be
2
4
9
1
6

ACCEPTED MANUSCRIPT
Bi FeO , but none of the equilibrations proved the existence of Bi-Bi Fe O -Bi FeO
39
2
5
39
2
4
9
25
phase field at 773 K. However, the existence of Bi-Bi Fe O -Fe O and Bi-Bi FeO -Bi O
3
2
4
9
2
3
25
39
2
phase fields had been established by the long term equilibration studies. Further, the
equilibration of different molar ratios of Bi O and Fe O in the air (discussed in section 4.2
2
3
2
3
of ref. 9) indicated that no other ternary compounds with their composition falling in
between Bi Fe O and Bi FeO are present at 773 K. All these observations lead to the
2
4
9
25
39
confirmation of the existence of Bi-Bi Fe O -Bi FeO phase field at 773 K.
2
4
9
25
39
Based on all the above results, the ternary phase diagram of Bi-Fe-O system at 773
and 1023 K have been constructed and are shown in Figs. 8 and 9. The results of the
experiments showed that on increasing the oxygen content of bismuth metal in equilibrium
with iron metal, iron oxides would first appear as the coexisting phases. With further
increase in oxygen content in bismuth, Bi Fe O would appear as the coexisting phase.
2
4
9
4
.4. Emf measurements
To determine the temperature above which BiFeO appear as a thermodynamically
3
stable phase in liquid Bi and also to determine the Gibbs energy of formation of BiFeO and
3
Bi FeO , oxygen potentials in liquid Bi in equilibrium with different phases were
2
5
39
measured. The details of the emf measurements are given in Table 6 and the output of the
emf cells, namely I, II, III, and IV in the chronological order of measurements along with the
dwell time at each temperature are given in Tables 7 to 10. The variation of emf output of
these cells with temperature is shown in Fig. 10. As seen in Tables 7 to 10, all the cells were
first operated at temperatures above 1000 K where BiFeO would be thermodynamically
3
stable. The values of emf and its temperature dependence in all the cells were nearly the
same, as seen from Fig. 10. As seen from Table 6, overall compositions of the samples for
1
7

ACCEPTED MANUSCRIPT
the cells were chosen such that in cell - I and II, they would lie in the region bound by Bi,
Bi Fe O and BiFeO . In cells III and IV, the overall compositions would lie in the region
2
4
9
3
bound by Bi, BiFeO and Bi FeO .
3
25
39
From the results of the experiments discussed in the earlier section, it is known that
BiFeO would appear as the stable phase at a temperature around 940 K. Hence, when
3
equilibrium is established at temperatures below about 940 K, samples in all the four cells
are expected to belong to Bi-Bi Fe O -Bi FeO phase field. At temperatures above about
2
4
9
25
39
9
40 K, the cell I and II are expected to belong to Bi-Bi Fe O -BiFeO phase field, while the
2 4 9 3
samples of cells III and IV are expected to belong to the Bi-BiFeO -Bi FeO phase field.
3
25
39
Since the temperature dependence of oxygen potentials in all the three phase fields are
expected to be different, a slope change at the temperature of appearance of BiFeO as the
3
stable phase is expected. However, as seen from Fig. 10, no significant change in slope has
been observed.
It is seen from Table 7 and 8, the final temperatures of operation of the cell I and II
were 985.5 and 1024.6 K, respectively. At these temperatures, BiFeO is expected to be
3
present as a stable phase. When the cells III and IV were cooled to room temperature from
8
01.4 and 859.6 K, respectively, BiFeO is not expected to be present. However, XRD
3
patterns of the samples retrieved from all the four cells showed the presence of Bi, BiFeO3
and Bi FeO in them. Cells I and II contained additionally Bi Fe O as seen from Table 6.
2
5
39
2
4
9
It is seen that metastable phases were present in the samples. This could be due to
insufficient dwell time at low operating temperatures of the cell for the metastable phases to
decompose completely, although emf values were stable due to the ‘equilibrium’ established
1
8

ACCEPTED MANUSCRIPT
with these phases. Due to the identical ‘equilibrium’ conditions in all the four cells, emf
values and their temperature dependence were the same. Obviously, these data could not be
used to deduce the Gibbs energy of formation of BiFeO and Bi FeO .
3
25
39
5
. Conclusions
The thermal stability of BiFeO has been examined by different experimental
3
techniques and found that it is metastable at low temperatures and become stable only
around 940 K. The isothermal sections of the ternary phase diagram of Bi-Fe-O system at
7
73 and 1023 K have been constructed based on long term equilibration studies.
Acknowledgments
The authors are thankful to Mr. Khaja Mohideen, Mr. N. Eswaran, and Mr. K.
Haridas, for their help in constructing the emf cell. The authors sincerely acknowledge Mr.
Henson Raj for his help in helium leak testing of the emf cell assembly. One of the authors
(AVM) acknowledges the Department of Atomic Energy, India, for providing financial
support.
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2
3
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0. E.M. Levin, R.S. Roth, Polymorphism of bismuth sesquioxide. II. Effect of Oxide
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3. T.M. Bruton, J.C. Brice, O.F. Hill, P.A.C. Whiffin, Flux growth of some γ-Bi O crystals
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by the top seeded technique, J. Cryst. Growth 23 (1974) 21-24.
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Chem. 15 (1975) 1-8.
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5. A. Ramanan, J. Gopalakrishnan, Low-temperature preparation of sillenite phases in Bi-M-
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6. S.F. Radaev, L.A. Muradyan, V.I. Simonov, Atomic structure and crystal chemistry of
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0.50
+
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0.50
3+
2+
0.33
sillenites: Bi12
(
Bi Fe
)
O19.50 and Bi12
(
Bi0.67Zn
)
O19.33 , Acta Cryst. B47 (1991) 1-6.
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1
7. A. Maitre, M. Francois, J.C. Gachon, Experimental study of the Bi O -Fe O pseudo-
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9. J. Lu, L.J. Qiao, P.Z. Fu, Y.C. Wu, Phase equilibrium of Bi O –Fe O pseudo-binary
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3
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0. S. Phapale, R. Mishra, D. Das, Standard enthalpy of formation and heat capacity of
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41.
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1. Q. Zhang, D. Sando, V. Nagarajan, Chemical route derived bismuth ferrite thin films and
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3. A. Molak, D. K. Mahato, A.Z. Szeremeta, Synthesis and characterization of electrical
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4. G.D. Achenbach, W.J. James, R. Gerson, Preparation of single-phase polycrystalline
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5. J. De Sitter, C. Dauwe, E. De Grave, A. Govaert, G. Robbrecht, On the magnetic
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3
2
3
9
19-920.
2
6. M.I. Morozov, N.A. Lomanova, V.V. Gusarov, Specific features of BiFeO formation in
3
a mixture of bismuth (III) and iron (III) oxides, Russ. J. Gen. Chem. 73 (2003) 1676-
1
680.
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2
2
3
7. M. Valant, A.K. Axelsson, N. Alford, Peculiarities of a solid state synthesis of
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3
8. T.T. Carvalho, P.B. Tavares, Synthesis and thermodynamic stability of multiferroic
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3
9. S.M. Selbach, M. Einarsrud, T. Grande, On the thermodynamic stability of BiFeO3,
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0. D. Maurya, H. Thota, K.S. Nalwa, A. Garg, BiFeO ceramics synthesized by mechanical
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activation assisted versus conventional solid state reaction process: A comparative study,
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3
1. A.V. Egorysheva, V.D. Volodin, O.G. Ellert, N.N. Efimov, V.M. Skorikov, A.E.
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3
2. N.A. Lemonova, V.V. Gusarov, Influence of synthesis temperature on BiFeO3
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3
2
3

ACCEPTED MANUSCRIPT
Table 1
Phases formed after equilibration of BiFeO (containing small amount of
3
Bi FeO ) in vacuum for 480 h
25
39
Sample
no.
Temperature (K) Phases identified after equilibration
1
716
722
726
763
805
871
BiFeO (major phase), Bi FeO (minor phase)
3
3
3
25
39
2
3
4
5
6
BiFeO
BiFeO
(major phase), Bi25FeO39 (minor phase)
(major phase), Bi25FeO39 (minor phase)
BiFeO , Bi FeO , Bi Fe O
9
3
25
39
2
4
Bi FeO , Bi Fe O
2
5
39
2
4
9
Bi FeO , Bi Fe O
2
5
39
2
4
9
BiFeO (minor phase), Bi FeO (major phase),
3
25
39
7
8
9
943
1034
1089
Bi Fe O (major phase)
2
4
9
BiFeO
Bi Fe
BiFeO
Bi Fe
3
(major phase), Bi25FeO39 (minor phase),
(minor phase)
(major phase), Bi25FeO39 (minor phase),
(minor phase)
2
4 9
O
3
2
4
O
9

ACCEPTED MANUSCRIPT
Table 2
Phases formed after equilibration of BiFeO (containing small amount of
3
Bi FeO ) in air for 480 h
25
39
Sample
no.
Temperature
(K)
Phases identified after equilibration
10
11
12
13
14
15
16
716
722
725
757
807
853
941
BiFeO (major phase), Bi FeO (minor phase)
3
3
25
39
BiFeO
(major phase), Bi25FeO39 (minor phase)
BiFeO (major phase), Bi FeO (minor phase)
3
25
39
BiFeO (major phase), Bi FeO (minor phase)
3
25
39
BiFeO
3
2 4 9
, Bi25FeO39, Bi Fe O
Bi FeO , Bi Fe O
9
25
39
2
4
BiFeO (minor phase), Bi FeO (major phase),
3
25
39
Bi Fe O (major phase)
2
4
9
1
7
8
1028
1085
BiFeO (major phase), Bi FeO (minor phase),
3
25
39
Bi Fe O (minor phase)
2
4
9
1
BiFeO (major phase), Bi FeO (minor phase),
3
25
39
Bi Fe O (minor phase)
2
4
9
Table 3
Results of equilibration of Bi, Bi
Bi FeO mixtures in liquid Bi
2
Fe
4
O
9
3 3
, BiFeO and Bi, BiFeO ,
25
39
Starting materials and
their mole ratio
Temperature Phases identified after
K)
(
equilibrating in liq. Bi for 480 h
Bi, Bi Fe O , BiFeO
873
Bi, Bi Fe O , BiFeO (minor phase),
2
4
9
3
2
4
9
3
(
1 : 0.6 : 2)
Overall composition
Bi FeO
2
5
39
(
1023
Bi, Bi
2
Fe
4
O
9
, BiFeO
3
, Bi25FeO39
(minor phase)
within the area bound by
Bi, Bi Fe O and BiFeO )
2
4
9
3
Bi, BiFeO3, Bi25FeO39
873
3 ,
Bi, BiFeO (minor phase) Bi25FeO39,
(1 : 1.56 : 0.1)
Bi Fe O (minor phase)
2 4 9
(Overall composition
1023
Bi, BiFeO Bi FeO
3, 25 39
within the area bound by
Bi, BiFeO and Bi FeO )
3
25
39

ACCEPTED MANUSCRIPT
Table 4 Details of equilibration of samples with compositions in the section bound
by Bi, Bi Fe O and Bi O at 1023 K
2
4
9
2
3
Phases taken
before
equilibration
Duration of
equilibration
(h)
Sample
no.
Phases identified
after equilibration
Composition
Bi, Bi Fe O , BiFeO ,
2
4
9
3
1
Bi0.280Fe0.200
O
0.520
Bi
2
O
3
, Fe, Fe
2
O
3
480
Bi FeO (minor)
25
39
Bi, Fe O , Bi O
2
3
2
3
Bi, Bi Fe O , BiFeO ,
2
4
9
3
2
3
4
Bi0.213Fe0.222
O
0.565
*
480
480
480
Bi FeO (minor)
in liq. Bi
Bi O , Fe, Fe O
25
39
Bi0.335Fe0.095O0.570
Bi,BiFeO , Bi FeO
2
3
2
3
3
25
39
Bi, Fe, Bi
in liq. Bi
2 3
O
Bi0.470Fe0.090
O
0.440
*
Bi,BiFeO
3
, Bi25FeO39
Bi, Bi Fe O ,
2
4
9
5
6
7
Bi0.550Fe0.005
O
0.445
480
480
480
Bi, Bi25FeO39, Bi
2
O
3
3
3
Bi O
2
3
2 3 3
Bi, Fe O , BiFeO
in liq. Bi
Bi0.550Fe0.005O0.445
*
*
Bi, Bi FeO , Bi O
25 39 2
Bi, Fe, Bi
liq. Bi
2 3
O in
Bi0.550Fe0.005O0.445
Bi, Bi FeO , Bi O
25 39 2
*
Overall composition of the phase mixture taken in the form of pellet which was equilibrated
with excess of liquid Bi. Samples sealed in quartz ampoules.

ACCEPTED MANUSCRIPT
Table 5 Details of equilibration of samples with compositions in the section bound
by Bi, Bi Fe O and Bi O at 773 K
2
4
9
2
3
Phases taken
before
equilibration
Duration of
equilibration
(h)
Sample
no.
Phases identified
after equilibration
Composition
Bi
2
O
3
, Fe, Fe
2
O
3
Bi,Bi
2 4 9 3
Fe O , BiFeO ,
8
9
Bi0.280Fe0.200
O
0.520
*
*
*
480
480
in liq. Bi
Bi FeO (minor)
25
39
Bi, Fe, Bi O3
Bi,Bi Fe O , BiFeO ,
2 4 9 3
2
Bi0.394Fe0.174O0.432
in liq. Bi
Bi FeO (minor)
25 39
Bi, Fe, Bi
in liq. Bi
2 3
O
Bi, BiFeO3,
Bi25FeO39
1
1
1
0
1
2
Bi0.470Fe0.090O0.440
Bi0.550Fe0.005O0.445
Bi0.550Fe0.005O0.445
480
960
480
Bi, Bi Fe O ,
2
4
9
Bi, Bi FeO , Bi O
3
25
39
2
Bi
2 3
O
Bi, Fe
2 3 3
O , BiFeO
*
Bi, Bi FeO , Bi O
25 39 2 3
in liq. Bi
*
Overall composition of the phase mixture taken in the form of pellet which was equilibrated
with excess of liquid Bi. Samples sealed in quartz ampoules.

ACCEPTED MANUSCRIPT
Table 6 Details of emf measurements
Region in which
Total
duration of
composition would
measurement
lie
Phases identified
after emf
measurement
Starting
materials
overall
No
Cell I
Bi, Bi
BiFeO3
containing
2
Fe
4
O
9
,
Bi-
22 days
2 4 9 3
Bi, Bi Fe O , BiFeO ,
Bi Fe O -BiFeO
(17 data
points)
Bi FeO (minor
2
4
9
3
25
39
(
phase)
small amount
of Bi FeO )
2
5
39
Cell II
Bi,
Bi
Bi-
3 months
Bi, Bi
Fe O (major
2 4 9
phase),
2
Fe
4
O
9
,
Bi
2
Fe
4
O
9
-BiFeO
3
(33 data
points)
BiFeO (minor phase)
3
Bi25FeO39
Bi FeO (minor
2
5
39
phase)
Cell III
Cell IV
Bi, BiFeO
3
Bi-
BiFeO –Bi FeO
39
2 months
Bi, BiFeO3, Bi25FeO39
Bi, BiFeO Bi FeO
39
(
containing
(20 data
points)
3
25
small amount
of Bi25FeO39
)
,
Bi FeO
2
5
39
Bi, BiFeO
containing
Bi-
17 days
3
3,
25
(
BiFeO
3
–Bi25FeO39
(12 data
points)
small amount
of Bi FeO )
39 ,
2
5
Bi25FeO39

ACCEPTED MANUSCRIPT
Table 7 Output of cell I as a function of temperature
No.
1
Temperature (K)
1013.3
930.0
Dwell time (h)
Emf (mV)
504.5
541.2
594.4
615.0
572.0
522.0
510.8
529.3
557.4
579.9
603.4
615.5
586.7
564.8
549.6
535.0
515.2
37
23
40
25
24
18
46
25
63
27
27
21
22
39
22
12
14
2
3
822.3
4
780.0
5
868.0
6
969.5
7
998.8
8
955.7
9
897.8
10
11
12
13
14
15
16
17
852.4
803.5
777.9
837.4
882.3
913.6
943.4
985.5