Thermodynamic investigations in the Bi-O system

Article abstract of DOI:10.1023/A:1010110002235

With the use of various calculation methods, unknown thermodynamic properties (TP) of Bi₂O₅ and BiO₂, as well as the temperature dependencies of reduced Gibbs energy (TDRGE) were determined. With the help of thermodynamic

Full text of DOI:10.1023/A:1010110002235

Journal of Thermal Analysis and Calorimetry, Vol. 61 (2000) 289–303
THERMODYNAMIC INVESTIGATIONS IN THE Bi–O
SYSTEM
*
1
G. Moiseev , N. Vatolin and N. Belousova
1
2
1
Institute of Metallurgy, Ural Division of the Russian Academy of Sciences, 101, Amundsen Str.
6
20016, Ekaterinburg
Krasnoyarsk State Academy of Nonferrous Metals and Gold, 95, Krasnoyarsk Workers Str.
2
6
60025, Krasnoyarsk, Russia
Abstract
2 5
With the use of various calculation methods, unknown thermodynamic properties (TP) of Bi O and
BiO
With the help of thermodynamic simulation (TS) methods at 300–1500 K, common P=10 Pa the
thermal decompositions of condensed Bi , BiO , Bi and BiO have been investigated in initial
atmosphere of O and Ar. Every condensed substance was presented as the individual phase.
It was found that Bi oxides have temperature stability fields and also districts of possible
mixture formation. During equilibrium heating of Bi oxides the various types of phase transfor-
2
, as well as the temperature dependencies of reduced Gibbs energy (TDRGE) were determined.
5
2
O
5
2
2 3
O
2
x
O
y
x
O
y
mations were observed. The characteristics of some transformations were estimated.
Keywords: decomposition, mixture, oxide, phase transformation, standard enthalpy of formation,
thermodynamic simulation
Introduction
According to [1–4] in the Bi–O system condensed oxides BiO, Bi O , BiO and Bi O
can exist. The thermodynamic properties (TP) and temperature dependencies of the
reduced Gibbs energy (TDRGE) for BiO and Bi O are known, and introduced into
2
3
2
2
5
2
3
the databases IVTANTERMO and ASTRA.BAS [5]. For others oxides the known TP
are presented in Table 1. It is clear, that those TP are not enough for calculation of the
TDRGE of Bi O and BiO .
2
5
2
The lack of and TDRGE for the above mentioned oxides does not permit reliable
thermodynamic investigations into the systems containing bismuth and oxygen (high
temperature superconductors, phases with ‘fast ionic transport’ [6, 7] and so on). As a
rule, in published works (for example [6–10]) only the properties of condensed Bi O
2
3
are taken into account. According to above mentioned references, this oxide is the
most stable in the Bi–O system. The data about temperature dependencies of partial
pressures of gaseous components over Bi O are known and summarized in [11]. But
2
3
*
Dedicated to Prof. Šesták in recognition of his outstanding activity in thermal analysis
1
418–2874/2000/ $ 5.00
2000 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht
©

Table 1 Thermochemical properties and thermodynamic functions (TDRGE) for Bi
2
O
3
and BiO
2
o
0
o
o
∆
H2 ,/
S
,/
H
−H /
T
decomposition
/
*
*
*
p
C at T>Ttr/
–1 –1
98
298
298
0
Oxide
Bi
–1
–1
–1
–1
a
b
c
kJ mol
J K mol
J mol
K
J K mol
–
185[13]
2
O
5
139.3±10.8
23230
423[3]
161.16
69.07
15.978
13.432
45.979
142.22
–
728.1
5
73[3]
538[2]
78[1]
BiO
2
–335.2
74.3±7.7
11784
3.700
75.60
5
*
–2
–1
2
3
Coefficients q
i
in equation Φ (T)=q
1
+q
2
3 4 5 6 7
lgx+q x +q x +q x+q x +q x
T /K
q
1
q
2
q
3
q
4
q
5
q
6
q
7
2
98–423
23–6000
98–573
73–6000
121.548
116.468
57.7669
62.6359
38.5001
33.986
–0.00549424
0
0.977894
0.493134
0.254093
0.275511
19.085
0
0
2 5
Bi O
4
2
5
0
16.00
0
0
0
0
0
0
0
16.5001
18.0601
–0.0004427
BiO
2
0
*
–3
5
–2
–1
–1
p
a, b, c are coefficients of equation: C =a+b⋅10 T–c⋅10 T /J K mol
–4
*
–1
–1
2 5 2
Φ (T) in cal K mol for Bi O and BiO (x=10 ⋅T/K)

MOISEEV et al.: Bi–O SYSTEM
291
we did not find in the available literature the full data about the thermal and thermo-
dynamic stability of condensed Bi O . For others oxides those characteristics evi-
2
3
dently are unknown too.
Therefore the goals of this work were:
•
the determination of the unknown TP and TDRGE for condensed Bi O and
BiO ; introduction of those data into the database ASTRA.OWN [12] for ap-
2
5
2
plication in computer modeling;
a study of temperature and composition dependencies of condensed BiO,
Bi O , BiO and Bi O thermal decomposition.
•
2
3
2
2
5
Methods and procedures
Different calculation methods and procedures (for example [12 ]) were used for de-
termination of the unknown TP of Bi O and BiO . The essence of the main methods
are given below, together with the calculation results.
2
5
2
Thermal decomposition of condensed Bi O was studied with the use of thermo-
x
y
dynamic simulation (TS) methodology, which was described in detail in [5, 14, 15].
In the system ‘Bi O +O (Ar)’ we take into account the thermodynamic functions of
condensed Bi, BiO, Bi O , gaseous Bi, Bi , Bi , Bi , BiO, Bi O , O , O and Ar [5],
x
y
2
2
3
2
3
4
4
6
2
condensed Bi O , and BiO (this work). Every condensed substance was present as an
individual phase. The study was carried out from 300 to 1500 K with the steps of 10
2
5
2
5
to 100 K in initial O or Ar atmosphere, common P=10 Pa with the application of
program ASTRA.4 [5].
2
Determination of the TP and TDRGE for Bi O and BiO
2
2
5
Bi O
2
5
0
The S2 values were calculated:
–
98
from the empirical relation [16]:
0
98
logS2 ≈0.875+0.725logn
(1)
(2)
0
–1
–1
where S2 is in [cal mol K ]; n is the number of atoms in the molecule;
98
–
from the equation [17]
0
98
S2 ≈AlogM +B
where M is molecular mass of compound, A and B are empirical constants in the row
0
of similar compounds (values of S for As O and Sb O were taken from [18]).
2
98
2
5
2
–1
5
0
–1
The average value of S2 equal to 139.4±10.8 J mol K we have taken as reliable.
98
0
0
The value of increment H2 −H was calculated with the use of equation [19]
98
0
0
298
S
0
98
0
0
298
–1
H2 −H ≈204S exp−
, cal g-atom
(3)
0
2
3.5
J. Therm. Anal. Cal., 61, 2000

2
92
MOISEEV et al.: Bi–O SYSTEM
0
0
where, S2 =S /n and n is the number of atoms in the molecule.
For determination of the dependence C (T), first we calculated the C
98
298
0
value.
p
p 298
For it we used:
the equation [12]:
–
0
Cp ≈AlogM +B
(4)
2
98
0
where M, A and B are the same as in Eq. (2). (Values of C
were taken from [18, 20]);
for As O and Sb O
2 5 2
p 298
5
–
calculation procedures based on the equations:
0
0
0
C
C
(Bi O )≈C
(Bi O )+2C
(O)
p 298
2
5
p 298
2
3
p 298
(
5)
0
0
0
(Bi O )≈2C
(BiO)+3C
(O)
p 298
p 298
2
5
p 298
0
0
where C
and
(Bi O ) and C
(BiO) were taken from the database ASTRA.BAS [5]
p 298
2
3
p 298
–
3
5
–2
C (T )(O)≈20.04+4.07 ⋅1 0 T −9.42 ⋅1 0 T
(6)
p
–
1
–1
in J mol K were taken according to Dr. H. Yokokawa (private communication);
–
with the use of the empirical equation [16]
–
3
–1
3
–2
C (T )≈(7n+2)−(1 .1 6n−1) ⋅1 0 T +(1.333n−66) ⋅1 0 T ,
(7)
p
–
1
–1
in cal mol K , where n is the number of atoms in the molecule.
0
–1
–1
The average value Cp (Bi O )=114.2±5.6 J mol K was taken as reliable and
used for calculation of C (T) using the method, described in [21]. In result we found:
2
5
2
98
p
–
3
5
–2
–1
–1
C (Bi O )≈161 .1 61+15.978 ⋅1 0 T −45.979 ⋅1 0 T , J mol K
(8)
p
2
3
According to [3], the Bi O decomposition finished at 423 K; the change of
enthalpy at this temperature was taken as equal to 0.
2
5
BiO2
0
For the estimation of S values we used: – the Latimere method [12]; – Eq. (1);
2
98
0
–
Periodic System (a variant of the Birkengaime method [12]); – the additive method:
comparison of S2 (MO ) (M = Hf, Sb, Pb, W) on row, period and diagonal of the
98
2
0
98
0
298
0
298
S2 (BiO )≈S (Bi O )−S (BiO),
2
2
3
(
9)
0
98
0
298
0
298
S2 (BiO )≈S (Bi O )+S (Bi O ))/4
2
2
5
2
3
0
The values of S for Bi O and BiO were taken from the database ASTRA.BAS
0
2
98
2
3
–1
–1
[
taken as reliable.
The value of H2 −H was calculated with the use of Eq. (3).
The values of C
of procedures described above for Bi O , and are equal to:
5], S2 (Bi O ) was taken from this work, and the average=74.3±7.7 J mol K was
98
2
5
0
0
98
0
0
and the function C (T) have been determined with the help
p 298
p
2
5
J. Therm. Anal. Cal., 61, 2000

MOISEEV et al.: Bi–O SYSTEM
293
0
C
(BiO )≈68.9±4 .1 and
p298
2
(
10)
–
3
5
–2
−1 −1
C (BiO )≈69.07+13.432 ⋅1 0 ⋅T −3.7 ⋅1 0 ⋅T , Jmol K
p
2
The decomposition temperature of BiO is 573 K according to [3]; the change of
enthalpy at this temperature was taken as equal to 0.
2
0
For the determination of standard enthalpies of formation (∆H2 , SEF) of Bi O
and BiO , various methods and estimations have been used.
98
2
5
2
First, we used the approximate ratios
Tph.tr (Sb O )
Tph.tr (SbO2 )
2
5
SEF(Sb O )
SEF(SbO2 )
2
5
≈
,
≈
,
(11)
Tph.tr (Bi O ) SEF(Bi O ) T (BiO ) SEF(BiO )
2
5
2
5
ph.tr
2
2
using known data for Sb O and SbO taken from [4, 18, 20], and the estimated SEF
values of Bi O and BiO were equal to –566.4 and –13.4 kJ mol , respectively.
Then we used the universal linear approximation rules (ULAR) [22] (Fig. 1),
2
5
2
–
1
2
5
2
–
1
and determined SEF values of Bi O and BiO equal to –615.3 and –307.2 kJ mol ,
2
5
2
respectively. These values were calculated from linear regression equations, con-
structed with the use of known SEF for Bi O and BiO [5]:
2
3
left side:
0
–1
H ( j)≈−307.5X (Bi), kJ g-atom
(12)
at
right side:
0
–1
H ( j)≈205(X (Bi)−1), kJ g-atom
(13)
at
In the last stage, we used the agreement procedure (AP) [23] on the supposition
that only the SEF values for BiO and Bi O are correct [5].
The SEF values for Bi O and BiO oxides were determined to be equal –728.1
2
3
2
5
2
–
1
and –335.2 kJ mol , respectively.
The agreement procedure [23] allows correction of known, and determination of
unknown, SEF values of related inorganic substances by use of the following func-
tion:
0
H ( j)
at
–1
–1
Ψ( j)=
, kJ g-atom sort
(14)
ϕ j
where we named Ψ(j) as the ‘sort standard enthalpy of formation’ (SSEF), for the j-th
relative compound (in our case Bi O );
x
y
0
∆H
( j)
0
298
–1
H ( j)=
, kJ g-atom
(15)
at
nj
where n is the number of atoms in a molecule of the compounds;
j
J. Therm. Anal. Cal., 61, 2000

2
94
MOISEEV et al.: Bi–O SYSTEM

x 
ϕ =


(16)
j

y


j
where we named ϕ as the ‘sort’ of the j-th related compound (in our case for Bi O
j
2
5
ϕ=2/5=0.4; for BiO ϕ=1/2=0.5, and so on).
It was found that, for related compounds in definite systems, if the values of SEF
2
were determined correctly, then in the coordinates ‘lgΨ – ϕ ’ we must observe linear
dependencies
j
j
lgΨ ≈A+bϕ
(17)
j
j
In our case
–
1
–1
lgΨ =2.6784−0.6584ϕ , kJ g-atom sort
(18)
j
j
and calculations of SEF for Bi O and BiO were made with the use of Eq. (18). Note,
2
5
2
that application of the ULAR to analysis of reference data gave an average diver-
gence between calculated and reference data of not more than ±7.5% [22]; whereas
for application of the AP, divergence was not more than ±2.5% [23].
The results of the various calculations and estimations of SEF values for Bi O
x
y
oxides are presented in Table 2. It is seen from Table 2 and Fig. 1, that if we take the
SEF for Bi O and BiO [5] as reliable, then the value of the SEF for Bi O estimated in
this work [13] is not correct. We thus take as reliable the SEF for Bi O and BiO cal-
2
3
2
5
2
5
2
culated with the use of the AP.
Fig. 1 The results of the use of universal linear approximation rules – ULAR [22]
0
(
1
curve 1) and AP [23] (curve 2) for the determination of H for Bi
2
O
5
and BiO
2
;
at
0
0
–4 – H for BiO, Bi
2
O
3
, Bi
2
O
5
and BiO
2
; • – 1 and 2 – H for BiO and Bi
2 3
O
at
at
0
according to [5]; • – 3 – for Bi
to the ULAR; o – H for Bi
2
O
5
according to [13]; o – H for Bi O according
x y
according to the AP
at
0
x
O
y
at
J. Therm. Anal. Cal., 61, 2000

Table 2 Known data and results of the estimation and calculation of SEF for bismuth oxides
0
at
–1
∆
H /kJ g-atom
0
–1
0
298
–1
∆
H298/kJ mol
∆H /kJ mol
Oxide
BiO
estimation
on Eq. (11)
on ULAR,
Eqs (12), (13)
on AP
Eq. (18)
known data
taken as reliable
known data
*
*
–209.4
–104.7
–
–102.5
–123.0
–87.9
–104.7
–115.7
–104.7
–111.7
–209.4
–578.3
–728.1
–335.2
*
*
Bi
Bi
2
O
O
3
5
–578.3
–115.7
–
**
**
2
–185
–
–26.43
–80.9
–71.1
BiO
2
–
–102.4
*
*–
according to [5];
according to [13]
*
–
2 3
Table 3 Mole fraction of Bi O
A – in mixtures of BiO
2
+Bi
2
O
3
(initial system BiO
2
+Ar)
T/K
400
500
600
700
800
820
840
860
880
1.0
–
12
15
–8
–5
–2
–2
–1
–1
–1
X(Bi
2
O
3
)
3
)
3
)
1.284⋅10
6.536⋅10
8.143⋅10
1.247⋅10
5.806⋅10
1.196⋅10
2.60⋅10
6.74⋅10
B – in mixtures of Bi+Bi
2 3
O (initial system BiO+Ar, 300 –1220 K)
T/K
600
700
800
900
1000
1100
1200
1220
–
–12
–10
–8
–6
–5
–3
–3
X(Bi
2
O
1.962⋅10
2.815⋅10
7.58⋅10
6.198⋅10
2.256⋅10
7.64⋅10
1.995⋅10
3.35⋅10
C – in mixtures of Bi+Bi
2 3
O (initial system BiO+Ar, 1240–1500 K)
T/K
1240
1260
1280
1300
0.5080
1320
1340
1400
1500
X(Bi
2
O
0.5035
0.5047
0.5060
0.5105
0.5140
0.5287
0.5970

2
96
MOISEEV et al.: Bi–O SYSTEM
All the properties for Bi O and BiO that are taken as reliable are presented in
2
5
2
Table 1. They were used for the determination of the TDRGE values for those oxides
with the help of the subprogram TERMOS [5]. The numerical coefficients of the ap-
proximating polynomials of the TDRGE are given in the second part of Table 1.
The thermal decompositions of BiO, Bi O , BiO and Bi O
5
2
3
2
2
Some results of investigations are presented in Figs 2 to 5 and in Tables 3 to 5.
Fig. 2 The systems Bi
in the interval 450–1500 K; 1–3 – X(Bi
of systems enthalpies (I=f(T)); T and T
the transformations in Eqs (19) and (20), and the changes of system enthalpies
near T and T , respectively
x
O
y
+O
2
; Changes of compositions and energetic characteristics
), X(BiO ) and X(Bi ); 4 – changes
, ∆I and ∆I are the temperatures of
2
O
5
2
2 3
O
A
B
A
B
A
B
Fig. 3 The system Bi
2
O
5
+Ar; Changes of compositions and energetic characteristics in
), X(BiO ) and X(Bi ); 4 – I=f(T);
≅ 460 K is the temperature at the end of phase BiO formation;
the interval 400–1500 K; 1–3 – X(Bi
2
O
5
2
2 3
O
T
T
A
2
B
≅ 880 K – the same for the Bi
2
O
3
phase
The equilibrium heating of Bi
O
x y
in O
2
(Fig. 2, Table 4)
In surplus oxygen (the initial content of O in every system was equal to 14 mass%)
oxides BiO, Bi O and BiO are transformed into Bi O by 300 K. This phase existed
2
2
3
2
2
5
J. Therm. Anal. Cal., 61, 2000

MOISEEV et al.: Bi–O SYSTEM
297
Fig. 4 The system BiO +Ar; Changes of compositions and energetic characteristics in
the interval 700–1500 K; 1 and 2 – X(BiO ) and X(Bi
2
2
2 3 A
O ); 3 – I=f(T); T ≅ 880 K
is the temperature at the end of phase Bi O formation
2
3
Fig. 5 The system BiO+Ar; Changes of compositions and energetic characteristics in
the interval 300–1500 K; 1–3 – X(BiO), X(Bi ) and X(Bi); 4–I =f(T); T ≅ 1230
K and ∆I are the temperature of phase transformation and the change of system
enthalpy nearly T
2
O
3
A
A
A
up to 480 K. Between 480 and 930 K the phase BiO and at T>930 K the phase Bi O
3
2
2
were observed.
The sequence of phase composition changes of the oxides can be connected with
the transformations:
Bi
2
O
5
→2BiO
2
(cd)+0.5O
2
, 480 K
(19)
(20)
2
BiO →Bi O (cd)+0.5O , 930 K
2
2
3
2
The equilibrium heating of Bi O in Ar (Fig. 3 to 5, Tables 3 and 4)
x
y
Every bismuth oxide under these conditions (initial content of Ar was equal to 14
mass%) has its own type of decomposition.
J. Therm. Anal. Cal., 61, 2000

2
98
MOISEEV et al.: Bi–O SYSTEM
Bi O +Ar (Fig. 3, Table 4)
2
5
In the interval 300 to 460 K the condensed phases Bi O and BiO can coexist. The mole
2
5
2
fraction of BiO in the mixture, – X(BiO ), – with rise of temperature increase to:
2
2
T/K
300
400
420
440
460
1.0
–
7
–2
–2
–1
X(BiO
2
)
9.984⋅10
1.392⋅10
7.757⋅10
3.427⋅10
Between 460 and 880 K the lone condensed phase in system is BiO , and at
T>880 K it is Bi O . Consequently, in contrast with the Bi O +O system, an initial phase
2
2
3
2
5
2
of Bi O in Ar can exist only with a mixture of phase BiO ; the temperature range of exis-
tence for the individual phase BiO moves to the low temperature side. These differences
are natural results of the atmosphere change and are easily explained.
2
5
2
2
BiO + Ar (Fig. 4, Tables 3A and 4)
2
The phases BiO and Bi O can coexist at 300–880 K, and a rise of temperature is ac-
companied by an increasing X(BiO ) in a mixture of phases (Table 3A). At T>880 K
sesquioxide of bismuth is the lone condensed phase in the system.
2
2
3
2
BiO+Ar (Fig. 5, Tables 3B,C and 4)
Between 300 and 1230 K, a rise of temperature in a mixture of BiO + Bi O resulted
2
3
–
18
–3
in an increase of X(BiO ) from ∼ 1⋅10 (300 K) to ∼ 3.35⋅10 (1230 K) (Table 3B). A
possible reason for it is disproportion of part of the ‘mother’ phase
2
3
BiO(cd)→Bi O (cd)+Bi(g)
(21)
2
3
Between 1240 and 500 K, the coexisting phases Bi O and Bi were observed as a re-
sult of the possible transformation:
BiO(cd)→Bi O (cd)+Bi(cd)
2
3
3
(22)
2
3
at about 1230 K (Table 3C). A decrease of the metallic bismuth content in the mixture
Bi O +Bi with rise of temperature can be explained by bismuth evaporation.
2
3
Bi O +Ar
2 3
Over all temperature intervals this oxide is practically the lone condensed phase.
Only between 300 and 500 K we observed the existence of BiO , (X(BiO ) in the mix-
2
2
–
6
ture Bi O +BiO being equal to ∼ 3.3⋅10 ).
2
3
2
In Table 4, the composition and temperature intervals of existence of the phases
are summarized, and the T and ∆H values of some of the phase transformations are
given. The last mentioned characteristics were determined according to the methods
described in [14, 15]. Let us remember that in the temperature range of a phase trans-
formation, the full enthalpy of system (∆I) changes by a leap (∆I, Fig. 2 to 5). Be-
cause we can calculate numerically by computer experiments not only the function
I=f(T), but also the molar content of every condensed phase (and gaseous component
J. Therm. Anal. Cal., 61, 2000

Table 4 Temperature ranges of existence of oxides and characteristics of some phase transformations during equilibrium heating of bismuth
oxides
Bi
x
O
y
+ O
2
3
00–480 K
Bi
480 K
→2BiO
∆H≈66.5 kJ mol Bi O5
480–930 K
930 K
→Bi
∆H≈45.9 kJ mol BiO2
930–1500 K
Bi
Bi
2
O
5
2
+0.5O
2
2BiO
2
2
O
3
+0.5O
2
–
1
–1
–1
2
O
5
2
BiO
2
2 3
O
–1
∆
H≈32.9 kJ mol BiO
2
∆H≈91.8 kJ mol Bi
2
O
3
Bi
2
O
5
+ Ar
880 K
→Bi
∆H≈48 kJ mol– BiO2
3
60–460 K
+BiO
460–880 K
880–1500 K
2
BiO
2
2 3 2
O +0.5O
–1
Mixture Bi
2
O
5
2
2
2
BiO
2
Bi
2
O
3
–
1
∆
2 3
H≈96 kJ mol Bi O
Bi
2
O
3
+ Ar
3
00–500 K
Mixture Bi +BiO
X(BiO )≈6⋅10
500–1500 K
Bi
2
O
3
2
O
3
–
–
–
–
–
–
–
7
2
BiO
2
+ Ar
3
00–880 K
880–1500 K
Bi
Mixture Bi +BiO
2
O
3
2 3
O
BiO + Ar
3
00–1230 K
1230 K
BiO→Bi +0.5Bi
∆H≈25.7 kJ mol BiO
3
2 3
O
–1
Mixture BiO+Bi
2
O
3
–
–
–
–1
∆
2 3
H≈38.4 kJ mol mixture Bi+Bi O

3
00
MOISEEV et al.: Bi–O SYSTEM
also) at every temperature, we can determine ∆H for the phase transformation from
the equation
∆
I
–1
∆
H≈ , kJ mol
(23)
mT
where m is the number of moles of the j-th substance below the phase transformation
temperature, or the number of moles of phase transformation products at the tempera-
ture of its completion.
T
Discussion
Before interpreting these results, we must note a few important initial conditions and
assumptions that we used.
(
i). We take as reliable the TP and TDRGE for condensed BiO and Bi O from
the data base ASTRA.BAS [5]. With the application of the SEF of BiO and Bi O ,
2
3
2
3
and assuming that in the Bi–O system can exist the thermodynamically stable phases
Bi O and BiO , we calculated, with the use of AP [23], the most important properties,
2
5
2
–
rect, we have used only agreed values of the SEF for Bi O oxides. An independent
SEF, – for those oxides. Consequently, if the SEF for BiO and Bi O [5] are not cor-
2 3
x
y
analysis with the use of a ‘rough’ approximation (Eq. (11)) and the ULAR [22]
Fig. 1) gives us the definite assurance, that the SEF values of Bi O oxides are deter-
(
x
y
mined more-or-less correctly. But, for ‘full’ assurance, it is necessary to measure the
SEF for Bi O oxides in precise experiments, as well as the other thermochemical
x
y
properties.
ii). For the study of the Bi O thermal decomposition, we used the assumption
(
x
y
that every condensed compound can be present as an individual phase. This assump-
tion was made for the determination of the principal behaviours of the bismuth oxide
and, of course, is only a preliminary and ‘ideal’ point of view on the objects of this in-
vestigation.
The following speculations must be reasonable if we take into account the above
mentioned notes.
Under the chosen conditions of computer experiments we can conclude, that, in
the Bi–O (Ar) systems between 300 and 1500 K, the individual oxides Bi O , BiO ,
Bi O , BiO as well as the mixtures Bi O +BiO , Bi O +BiO , BiO+ Bi O and Bi O +
2
5
2
2
3
2
5
2
2
3
2
2
3
2
3
Bi, can exist.
In atmospheres with surplus O , all initial oxides would be oxidized at 300 K up
to the ‘highest’ oxide Bi O , and with increasing temperature, the degree of bismuth
2
2
5
oxidation changes from +5→+4→+3.
In an inert gaseous atmosphere, there is also a common tendency of lowering of
the average degree of the bismuth oxidation with increasing temperature, but it is ac-
companied by formation of oxide and oxide–bismuth mixtures over definite tempera-
ture intervals.
J. Therm. Anal. Cal., 61, 2000

MOISEEV et al.: Bi–O SYSTEM
301
Under the chosen conditions, the oxide Bi O is most stable, but not the oxide
2
3
BiO, as would be expected if the results of thermal decomposition investigations of
many oxides of less-common metals [4, 5, 11, 14, 15] are taken into account. But at
the same time, the results of our study agree with well known experimental facts (see
for examples [1–4, 9–11], that the oxide Bi O is the most stable oxide in the Bi–O
2
3
system.
The values of the partial pressures of O , Bi and Bi O calculated during the TS
of thermal decomposition of Bi O (cd) in inert atmosphere agree satisfactorily with
2
4
6
2
3
experimental data [11] (Table 5).
2 3
Table 5 Comparison of calculated partial components pressures over Bi O with experimental
data [11] (in MPa)
P(O
2
)
P(Bi)
4
P(Bi O)
T/K
calc.
4.27⋅10
8.71⋅10
[11]
calc.
[11]
calc.
[11]
–
–
9
–10
–11
–10
–11
–11
9
00
4.59⋅10
1.47⋅10
6.10⋅10
8.10⋅10
8.97⋅10
9
–8
–9
–8
–9
–9
1
000
1.55⋅10
5.71⋅10
2.04⋅10
5.20⋅10
5.26⋅10
This work is probably the first investigation of the thermal stability of bismuth
oxides over a wide temperature interval and under different gas atmospheres. There-
fore we cannot compare the results with known data. According to the review in [11]
only the partial pressures of some components over Bi O (cd) were measured at about
2
3
9
00 to 1200 K. However, the results of our study are only a first numerical estimation,
as according to [11] in the gas phase over condensed Bi O together with Bi÷Bi , BiO
and Bi O (their thermodynamic functions taken into account at the TS) also volatile
2
3
4
4
6
(
BiO) , where n=2÷4, Bi O , Bi O , Bi O and Bi O can exist. For these substances
the thermodynamic functions have not yet been determined.
n
2
3
4
5
3
2
3
The majority of these data are new. In our opinion, the most interesting of them
is the possibility of formation and coexistence of bismuth oxides with different de-
grees of oxidation at the same temperature. This observation we interpreted as the
mixtures of phases, but we can also interpret it as the formation and existence of
phases of changing composition, i.e. solid (or liquid) solutions in which molten and
crystalline oxide phases can coexist. But these problems can be solved in special in-
vestigations, where we must find (or determine) all temperature boundaries ‘soli-
dus–liquidus’ and use definite solution models.
Conclusions
With the help of various calculation methods, the unknown TP and TDRGE of Bi O5
and BiO have been determined. The thermal decompositions of BiO, Bi O , Bi O
2
2
2
3
2
5
and BiO have been studied from 300 to 1500 K in initial O and Ar atmospheres. The
compositions of the condensed phases and the consequences and characteristics of
some phase transformations were determined.
2
2
J. Therm. Anal. Cal., 61, 2000

3
02
MOISEEV et al.: Bi–O SYSTEM
The results of this investigation agree with known information, about Bi O behav-
iour at equilibrium heating. It was found that in the Bi–O(Ar) systems can coexist not
only individual oxides, but also bismuth oxides, with different degrees of oxidation.
2
3
*
* *
This work was carried out with the financial support of the Russian Fund of Fundamental Investiga-
tions (project 99-03-32707).
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