Thermochemical investigation of RE2O3-SeO2 systems. III. Yttrium selenium oxides in the pseudo-binary system

Article abstract of DOI:10.1023/A:1019970630727

Ternary pure phases in the thermodynamical equilibrium Y₂Se_xO_3+2x in the pseudo-binary Y₂O₃-SeO₂ system have been synthesized by solid state reactions and characterized by X-ray powder diffraction, IR spectroscopy and DSC measurements. A new phase Y₂Se_3.5O₁₀ was found besides the known phases Y₂Se₄O₁₁, Y₂Se₃O₉ and Y₂SeO₅. The thermal decomposition properties of the compounds have been determined by total pressure measurements and their thermodynamics data have been derived from their decomposition functions and C_p values. The phase diagram and the phase barogram have been established.

Full text of DOI:10.1023/A:1019970630727

Journal of Thermal Analysis and Calorimetry, Vol. 69 (2002) 301–316
THERMOCHEMICAL INVESTIGATION OF
RE O –SeO SYSTEMS
III. Yttrium selenium oxides in the pseudo-binary system
2
3
2
*
M. Zhang-Presse and H. Oppermann
Institut für Anorganische Chemie der Technischen Universität Dresden, Mommsenstr. 13,
D-01062 Dresden, Germany
Abstract
Ternary pure phases in the thermodynamical equilibrium Y
–SeO system have been synthesized by solid state reactions and characterized by X-ray pow-
der diffraction, IR spectroscopy and DSC measurements. A new phase Y 10 was found besides
the known phases Y Se Se and Y SeO . The thermal decomposition properties of the
compounds have been determined by total pressure measurements and their thermodynamics data
2 x
Se O3+2x in the pseudo-binary
Y
2
O
3
2
2
Se3.5O
2
4
O
11, Y
2
3
O
9
2
5
p
have been derived from their decomposition functions and C values. The phase diagram and the
phase barogram have been established.
Keywords: C
p
measurement, phase diagram, thermal decomposition, thermodynamic data, yttrium
selenium oxides
Introduction
Rare earth selenoxides have attracted special interests due to their optical, magnetic
and catalytical properties. In the earlier researches on the rare earth selenides four ter-
nary compounds Y Se O , Y Se O , Y Se O and Y SeO have been reported in the
2
4
11
2
3
9
2
2
7
2
5
binary system Y O –SeO [1–3]. Pedro et al. [1] synthesized Y Se O with solid state
reaction and obtained Y Se O and Y SeO through decomposition of the former
2
3
2
2
4
11
2
3
9
2
5
phase. The characterization of the compounds by X-ray and IR spectroscopy showed
that they belong to different structure families. RE Se O (where RE=rare earth ele-
2
4
11
ments) were found to crystallize in three structure types, in which Y Se O was clas-
sified together with other RE Se O (RE=Sm, Eu, Gd and Lu) into one group. Pedro
2
4
11
2
4
11
et al. subdivided the RE Se O phases in two types. Y Se O and its analogous
RE Se O (RE=Gd, Dy, Ho, Er, Tm, Yb and Lu) were included in one group. All
2
3
9
2
3
9
2
3
9
RE SeO compounds (RE=Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)
form only one type of structure.
2
5
*
Author for correspondence: Prof. Dr. H. Oppermann E-mail: ilona.salzmann@chemie.tu-dresden.de
1
418–2874/2002/ $ 5.00
2002 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
©
Kluwer Academic Publishers, Dordrecht

3
02
2 3 2
PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
Castro et al. [2] described the TG and DTA of the Y Se O compounds and
3+2x
2
x
found that the decomposition of Y Se O takes place in three steps. Y Se O decom-
poses into Y Se O and gaseous SeO in the temperature region 700-800K Y Se O
9
2
4
11
2
4
11
2
3
9
2
2
3
further decomposes in the region 880–1080 K to Y SeO and SeO . Y SeO decom-
poses directly to Y O and SeO starting from 1128 K.
2
5
2
2
5
2
3
2,g
The fourth compound, Y Se O , was obtained by Niinistö et al. [3] by thermal
decomposition of Y(Se O )NO .
2
2
7
2
5
3
This paper reports our research results on the Y O –SeO system from which we
confirm the existence of three known phases Y Se O , Y Se O , Y SeO and a new phase
2
3
2
2
4
11
2
3
9
2
5
Y Se O . The phase Y Se O , described by Niinistö et al. [3] was not confirmed in our
work. A phase Y Se O with 60 mol% SeO , as it exists on the line Nd O –SeO [4] and
Sm O –SeO [5] does not exist in thermodynamic equilibrium in this system.
2
3.5 10
2
2
7
2
1.5
6
2
2
3
2
2
3
2
Synthesis and characterization of the ternary compounds
Syntheses
The ternary compounds in the Y O –SeO system are synthesized by solid state reac-
tions between corresponding stoichiometric amounts of Y O (purity 99.9%,
Riedel-de Haen AG) and SeO in evacuated quartzglass tubes, as described previ-
2
3
2
2
3
2
ously [4, 5]. X-ray powder diffraction patterns and IR spectra of end-products show
that all four phases Y Se O , Y Se O , Y Se O and Y SeO are pure. The optimal
reaction temperature for preparation of Y Se O is 300–350°C, and for Y Se O is
2
4
11
2
3.5 10
2
3
9
2
5
2
4
11
2
3.5 10
4
thesized above 450°C. The phase pure Y Se O can not be obtained until 700, while
00°C. Traces of Y Se O , detectable by X-ray, are found in Y Se O product syn-
2 3 9 2 3.5 10
2
3
9
pure Y SeO is obtained above 800°C. The SeO -rich products show white colour,
while the SeO -poorer phases are rose.
2
5
2
2
Characterizations by X-ray diffraction
All synthesized phases and products are characterized by X-ray diffraction, shown in
Figs 1 and 2, using a Siemens diffractometer D5000 with CuK radiation in 2q range
a
from 10 to 70°. From these figures we can detect the existence of four thermodynami-
cally stable phases in the binary Y O –SeO , system: Y Se O , Y Se O , Y Se O
and Y SeO , containing 80, 77.8, 75 and 50 mol% SeO , respectively.
2
3
2
2
4
11
2
3.5 10
2
3
9
2
5
2
The phase Y Se O is characterized by a group of intensive lines in the 2q range
between 24 and 34° with the strongest reflection at 28.5°. Y Se O , is characterized
2
4
11
2
3.5 10
by two strong lines at 13.5 and 14.7°, which clearly differentiate it from other phases.
The diffraction pattern of Y Se O has only two strong main reflections at 25 and
2
3
9
2
pattern of Dy SeO , which was given by Pedro et al. [1], the two stronger reflections
5.5°, it is similar to that of Dy Se O , shown by Pedro et al. [1]. Comparing with the
2 3 9
2
5
at 28 and 28.3° are also characteristic of Y SeO besides the strongest main reflection
at 29°. Therefore the statement of Pedro et al. [1], that all RE SeO must have the
same structure, cannot be confirmed.
2
5
2
5
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
303
Fig. 1 Powder diffraction patterns of yttrium selenium oxides and of the initial com-
pounds Y and SeO
2
O
3
2
Fig. 2 Powder diffraction patterns of tempered composition of 66.7 and 50–60 mol%
SeO in correlation with Y SeO and Y Se
2
2
5
2
3 9
O
J. Therm. Anal. Cal., 69, 2002

3
04
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
Figure 2 depicts the X-ray patterns of samples with the composition between
0–60 mol% SeO and of 66.7 mol% SeO (‘Y Se O ’) in correlation to that of the 50
and 75 mol% SeO phases. The figure shows that all samples are mixtures of the 50
5
2
2
2
2
7
2
and 75 mol% SeO phases. So it follows, that neither a 60 nor a 66.7 mol% SeO2
phase exist in this system. The phase Y Se O , described by Niinistö et al. [3] may not
2
2
2
7
be a thermodynamically stable phase. These conclusions are also confirmed by the
total pressure measurements.
If Y Se O samples, annealed in small ampoules at different temperatures, are
2
4
11
quickly quenched, it follows from the X-ray patterns (Fig. 3) that more reflections of
Y Se O will be shown in the patterns with increasing temperatures. The sample,
quenched at 700 K, mainly contains Y Se O . So it is obvious from this that there is
2
3.5 10
2
3.5 11
a typical peritectic of Y Se O near 700 K, which is in agreement with DTA investi-
gations.
2
4
11
Fig. 3 Powder diffraction patterns of Y
ent temperatures, 1d in correlation with Y
2 4
Se O11 after tempering and quenching at differ-
2
Se3.5O
10
Y Se O samples are also annealed at different temperatures. The X-ray pat-
terns are demonstrated in Fig. 4, showing the peritectic decomposition of Y Se O
2
3.5 10
2
3.5 10
near 775 K. The pattern near 13.5° (2q) at 500°C allows the assumption of a a–b tran-
sition of the Y Se O phase, which is also found in DTA.
2
3.5 10
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
305
Fig. 4 Powder diffraction patterns of Y
ferent temperatures, 1d in correlation with Y
2
Se3.5
O
10 after tempering and quenching at dif-
Se
2
3 9
O
Characterization by IR spectroscopy
IR spectroscopy was used as a second method for characterization of the Y Se O
phases. The samples were measured in KBr-pressed pellets on a Spekord M80.
2
x
3+2x
The IR spectra of synthesized compounds are depicted in Fig. 5. The phases
shown in their spectra are distinguishable one from each other. The region between
–
00–900 cm show the most characteristic Se–O bond vibrations, as is observed in a
1
3
large number of selenites and selenites in the reported literature [6].
Total pressure measurements
The decomposition behaviour of the yttrium selenium oxides was investigated by to-
tal pressure measurements on pure phase powder products in glass- or quartzglass-
membrane manometers, as otherwise described [5–10].
For the determination of the decomposition pressure at various temperatures we
measured the pressure step by step in intervals of 25 or 50 degree. The achievement
of the equilibrium at one temperature is very slow. Therefore, we evaluated only the
series by increasing the temperature. The total pressure functions of the phases ob-
tained in coexistence with SeO -pure neighbour phases are shown in Fig. 6. Every
function was obtained from more experiment cycles by increasing temperature. All
2
phases decompose into the next SeO -poorer coexistent solid phase and gaseous
SeO . Since the partial or sublimation pressure of the solid phases are negligible, the
total pressure was equal to the SeO -decomposition pressure.
2
2
2
J. Therm. Anal. Cal., 69, 2002

3
06
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
2 3 2
Fig. 5 IR spectra of the yttrium selenium oxides and the initial compounds Y O and SeO
2 2 4 2 2 3 9 2 5
Fig. 6 Total pressure functions of SeO , Y Se O11, Y Se3.5O10, Y Se O and Y SeO
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
307
Y Se O
11
2
4
The decomposition pressure of Y Se O is 0.133 kPa at 275°C (548 K), and it
reaches 101.3 kPa at temperature near 375°C (648 K). The decomposition normalised
to 1 mol gaseous SeO is:
2
4
11
2
2
Y Se O =2Y Se O10,s+SeO2,g
2 4 11,s 2 3.5
The decomposition pressure of Y Se O at the peritectic temperature near
25°C (798K) is not measurable with our method.
2
4
11
4
Y Se O
2
3.5 10
The measured decomposition temperature of Y Se O with 0.133 kPa pressure is
2
3.5 10
327°C (600 K) and the pressure arrives 101.3 kPa near 425°C (698 K). The transition
temperature of this phase lies outside the measurable p–T region. The decomposition
can be described by:
2
Y Se O =2Y Se O +SeO
2 3.5 10,s 2 3 9,s 2,g
Y Se O
9
2
3
This phase decomposes above 600°C (873 K) and the total pressure reaches
01.3 kPa near 825°C (1100 K). Therefore, the phase transformation at 827°C
1
2
Fig. 7 Total pressure measurements in- and outside and saturation of the SeO -rich phase
J. Therm. Anal. Cal., 69, 2002

3
08
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
(1100 K) and peritectic reaction at 1125°C (1398 K) can not be observed. The decom-
position reaction can be formulated as follows:
0
.5Y Se O =0.5Y SeO +SeO
2 3 9,s 2 5,s 2,g
Y SeO5
2
This Y O -richest phase shows a measurable decomposition pressure at a temperature
higher than 700°C (973 K). The decomposition function lies parallel and close to that
of the Y Se O phase. The decomposition can be described by:
2
3
2
3
9
Y Se O =Y O +SeO
2,g
2
3
5,s
2
3,s
If we measure the total pressure of composition from a mixture of Y Se O and
SeO out of the saturation, we obtain the function, as it is demonstrated in Fig. 7
2
4
11
2
(
tion-function, arrives at a divariant region and the goes along the Y Se O -decompo-
marked with ¨). The pressure increases first step by step along the SeO -satura-
2
2
4
11
sition line, arrives again at a divariant region and increases along the decomposition
line of Y Se O . So we may conclude that there are no more phases in equilibrium
besides SeO , Y Se O and Y Se O in the region from 77.8 mol% SeO to the bi-
2
3.5 10
2
2
4
11
2
3.5 10
2
nary phase SeO . A composition between 50 to 75 mol% SeO (see ▼ curve in Fig. 7)
2
2
shows the increasing of the pressure first along the decomposition function of
Y Se O and after a divariant period along the function of Y SeO . So we obtain from
the total pressure measurements the same information as we obtained by X-ray and
2
3
9
2
5
IR spectroscopy. Between the 50 and 75 mol% SeO phase there exist no more
phases, neither a 60 mol% SeO phase, obtained in the Nd O –SeO and Sm O –SeO
2
2
2
2
3
2
2
3
systems [4, 5], nor a 66.7 mol% SeO phase, reported by Niinistö et al. [3].
2
p
Fig. 8 C functions of yttrium selen oxides
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
309
J. Therm. Anal. Cal., 69, 2002

3
10
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
Heat capacities of Y Se O
3+2x
2
x
The heat capacities of Y SeO and Y Se O were determined by DSC measurements
with a DSC 404 (Netzsch) from 323 to 773 K. Since the another two phase Y Se O
11
2
5
2
3
9
2
4
and Y Se O decompose above 400 and 500 K [4], we used a DSC 111 (Setaram) in
lower temperature region from 150 to 473 K. The curves, which are presented in
Fig. 8, are obtained from average values of several single determinations. The coeffi-
2
3.5 10
–
cients of the fit-function C =a+bT+cT +dT are listed in Table 1. In Table 2 the ex-
2
3
p
perimental values are compared with the data, obtained by the Neumann–Kopp rule.
The agreement between the experimental and estimated values are good at lower tem-
peratures, while the differences increase with increasing temperature.
2
=a+bT+cT +dT )
3
Table 1 Coefficient of the C
a/
p
functions of Y
2
SeO3+2x (C
p
3
b×10 /
–5
c×10 /
6
d×10 /
T region/
K
Error/
Phase
–1 –1
–1 –2
K
–1 –3
–1 –4
K
–1 –1
K
J mol
K
J mol
J mol
K
J mol
J mol
Y
Y
Y
Y
2
2
2
2
SeO
Se
5
140.51
150.81
–12.25
–83.60
–36.44
323–773
323–673
153–473
153–373
±2.5
3
O
9
308.60
238.69
193.59
37.20
368.84
638.70
–64.48
–30.12
–18.24
±4.2
±6.3
±6.3
Se3.5
O
10
–167.20
–376.20
4 11
Se O
Evaluation of the total pressure measurements
The enthalpy of formation and standard entropy of phases Y SeO , Y Se O and
Y Se O were obtained by evaluation of the total pressure measurements with the de-
2
5
2
3
9
2
4
11
termined C functions via the second law.
The obtained functions for the decomposition reactions of the phases (Fig. 6)
correspond to the decomposition pressure pSeO2 in the form:
p
B
lg p(atm)= A+
T
and give through the combination with the Gibbs–Helmholtz equation and
0
0
Van’t-Hoff equation the decomposition enthalpy DH and entropy DS at the mid-
dle temperature T:
ZT
ZT
0
Z,T
0
Z,T
DS
DH
lg p=
-
4
.575 4.575T
From this we can derive the enthalpy of formation and the standard entropy.
Y SeO5
2
The evaluated pressure function for the decomposition:
Y SeO =Y O +SeO
2,g
2
5,s
2
3,s
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
311
is given by
lgp(atm)=(8.67±0.22) – (10850±250)/T
from which it follows that
0
Z,1200
–1
DH
=207.5±4.2 kJ mol
–1
=166.1±4.2 J mol K
0
Z,1200
–1
DS
0
So we obtain the enthalpy H (Y SeO , 1200) and entropy S (Y SeO , 1200) via
0
2
5,s
2
5,s
the Hess law:
0
H (Y SeO , 1200)=H (Y O , 1200)+H (SeO , 1200)–DH
0
0
0
Z,1200
2
5,s
2
3,s
2,g
0
0
0
0
S (Y SeO , 1200)=S (Y O , 1200)+S (SeO , 1200)–DS
Z,1200
.
2
5,s
2
3,s
2,g
and with the data in Table 3 to:
0
H (Y SeO , 1200)= –2065.2±12.5 kJ mol
–1
2
5,s
0 –1
S (Y SeO , 1200)=433.0±12.5 J mol K
–1
2
5,s
The values at T are converted to 298 K with the determined value C
p
–
1
Y SeO ,700)=202.7±2.5 J mol K and Kirchoff’s law:
–1
(
2 5,s
0
–1
DH (Y
SeO5,l, 298)= –2248.1±16.7 kJ mol
–1
5,l
B
2
0
S (Y SeO , 298)=150.2±18.4 J mol K
–1
2
Y Se O
9
2
3
The p–T function for the decomposition
.5Y Se O =0.5Y SeO +SeO
2,g
0
2
3
9,s
2
5,s
can be given in the temperature region from 873 to 1098 K with
lgp(atm)=(9.81±0.30)–(10750±300)/T
The enthalpy and entropy of decomposition at middle temperature 1000 K are
the following:
0
Z,1200
–1
DH
=205.4±6.3 kJ mol
–1
=187.9±6.3 J mol K
0
Z,1200
–1
DS
The enthalpy and entropy are then obtained via the Hess law at 1000 K:
0
H (Y Se O , 1000)=H (Y SeO , 1000)+2H (SeO , 1000)–2DH
0
0
0
Z,1000
2
3
9,s
2
5,s
2,g
0
S (Y Se O , 1000)=S (Y SeO , 1000)+2S (SeO , 1000)–2DS
0
0
0
Z,1000
2
3
9,s
2
5,s
2,g
With the derivated data of Y SeO at 1000 K
2
5
J. Therm. Anal. Cal., 69, 2002

3
12
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
0
–1
–1
H (Y SeO , 1000)=–2105.8±20.9 kJ mol
2
5,s
0 –1
S (Y SeO , 1000)=395.8±23.0 J mol K
2
5,s
to
0
H (Y Se O , 1000)=–2663.9±41.8 kJ mol
–1
2
3
9,l
0 –1
S (Y Se O , 1000)=666.9±43.9 J mol K
–1
2
3
9,l
These data converted with the determined data C (Y Se O ,600)=326.4±
p
2
3
9,l
–
1
–1
.18 J mol K to the standard data at 298 K:
4
0
–1
DH (Y
Se
O
9,s, 298)= –2892.8±50.2 kJ mol
–1
B
2
3
0 –1
S (Y Se O , 298)=271.5±58.5 J mol K
2
3
9,s
Y Se O
2
3.5 10
The enthalpy of formation and standard entropy of Y Se O are obtained via the
third law. The pressure functions show (Fig. 6) an overlapping for Y Se O and
Y Se O near 400°C (673 K). This would lead to an instability of the Y Se O phase
2
3.5 10
2
3
9
2
3.5 10
2
3
9
at 400°C, which we cannot detect from DSC measurement. So we assume, that the
adjustment of the equilibrium is too slow at low temperature and the obtained slopes
of functions (Fig. 6) are too steep.
From the decomposition pressure pSeO2 = –2.026 kPa for the reaction:
2
Y Se O =2Y Se O +SeO
2 3.5 10,s 2 3 9,s 2,g
at 600 K (Fig. 6) we can obtain the free reaction enthalpy
0
Z,650
–1
DG
=20.7±4.2 kJ mol
The standard entropy for Y Se O follows via the solid state reaction
2
3.5 10
Y O +3.5SeO = Y Se O
3.5 10,s
2
3,s
2,s
2
with the tabulated data (Tab. 5) as
0 –1
S (Y Se O , 298)=332.6±8.4 J mol K
–1
2
3.5 10,s
–
with the determined value C (Y Se O , 450)=356.4±41.8 J mol K we obtain the
1
–1
p
2
3.5 10
entropy at 650 K via Kirchhoff’s law:
0 –1
S ( Y Se O , 650)= –610.9±12.5 J mol K
–1
2
3.5 10,l
–
1
–1
By using the calculated value S°(Y Se O , 650)=526.3±43.9 J mol K and the
2
3
9,l
data in Table 3 the decomposition entropy can be obtained according to the following
equation
0
Z,650
0
0
0
DS
=2S (Y Se O9,l, 650)+S (SeO2,g, 650)–2S (Y Se3.5O )
2 3 2 10,l
as:
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
313
0
Z,650
–1
=131.3±108.7 J mol K
–1
DS
The Gibbs–Helmholtz equation leads to
0
Z,650
–1
DH
=106.2±117.0 kJ mol
0
So the enthalpy H (Y Se O , 650) can be followed by using the Hess law via
2
3.5 10,l
0
H (Y Se O , 650) =H (Y Se O , 650) + 0.5H (SeO , 650) – 0.5DH
0
0
0
Z,650
2
3.5 10,l
2
3
9,l
2,g
to
0
H (Y Se O10,l, 650)= –2877.3±56.4 kJ mol
–1
2
3.5
At last we obtain, with the quoted C , the standard enthalpy:
p
0
–1
DH (Y
Se3.5O10,l, 298)= –3002.0±58.5 kJ mol
B
2
*
Table 3 Thermodynamic data for SeO
2
and Y
2
O
3
[11]
Compound
SeO2,g
SeO2,s
2 3,s
Y O
0
H1
200
–1
kJ mol
–62.3±4.2
–73.6±4.2
–89.5±4.2
–94.9±4.2
–1793.4±4.2
–1812.1±4.2
–1856.8±4.2
–1868.9±4.2
0
H1
000
0
H7
00
0
H6
00
0
DH2
98
–126.7±4.6 [12]
–224.5±0.6 [14]
–1903.2±4.2 [13]
0
–1 –1
K
S1
J mol
333.6±4.2
323.1±4.2
303.9±4.2
295.9±4.2
264.6±0.4 [12]
265.0±4.2
241.6±4.2
197.3±4.2
178.5±4.2
99.1±0.3 [12]
200
0
S1
000
0
S7
0
S6
0
S2
00
00
98
66.6±0.4 [12]
*
Converted to T analoque Barin [11] on the basis of the data tabulated by Gluschko [12, 13] for 298 K.
–1
–
We assume error ±4.2 kJ mol respective ±4.2 J mol
1
–1
K
Y Se O
11
2
4
We obtain the data of Y Se O by the second law in using the above obtained data of
Y Se O . The thermal decomposition
2
4
11
2
4
11
2
Y Se O =2Y Se O10,s+SeO2,g
2 4 11,s 2 3.5
is described by the total pressure function
lgp(atm) = (14.36±0.60) – (9200±400)/T
0
Z,600
–1
=176.0±8.4 kJ mol and DS
0
Z,600
–1
=274.9±12.5 J mol K
–1
We obtain with DH
0
H (Y Se O , 600)=H (Y Se O , 600)+0.5H (SeO , 600)–0.5DH
0
0
0
Z,600
and
2
4
11,s
2
3.5 10,s
2,g
J. Therm. Anal. Cal., 69, 2002

3
14
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
0
S (Y Se O , 600)=S (Y Se O , 600)+0.5S (SeO , 600)–0.5DS
0
0
0
Z,600
2
4
11,s
2
3.5 10,s
2,g
0
with the tabulated data (Table 3) and with DH (Y Se O , 298)= –2958.1±
2
3.5 10,s
–
1
0
.9 kJ mol (determined by solution calorimetry, [15]) and H (Y Se O ,600)=
7
–
2
3.5 10,s
–1 –1
2853.5±10.0 J mol as well as S (Y Se O ,600)= 538.9±70.2 J mol K will be
–
1
0
2
3.5 10,s
the enthalpy of formation and entropy of Y Se O at 600°C derived:
2
4
11
0
H (Y Se O11,s, 600)= –2990.3±66.9 kJ mol
–1
2
4
0
–1
S (Y Se O , 600)=549.8±78.6 J mol K
–1
2
4
11,s
Via the Kirchoff’s law and with the determined C (Y Se O ,400)=
p
2
4
11,l
–
1
–1
77.8±6.3 J mol K at 298 K the enthalpy of formation
3
0
–1
DH (Y
Se
O
11,s, 298)= –3104.5±69.0 kJ mol
B
2
4
and standard entropy
0 –1
S (Y Se O , 298)=285.3±83.6 J mol K
–1
2
4
11,s
are obtained.
DTA and phase diagram of the system Y O –SeO
2
2
3
Different thermoanalytical investigations are used besides the total pressure measure-
ments for the construction of the phase diagram of the Y O –SeO system. A
2
3
2
2 4 2
Fig. 9 DSC curves of compositions from Y Se O11 and SeO
J. Therm. Anal. Cal., 69, 2002

ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y
2
O
3
–SeO
2
315
2 3 2
Fig. 10 Phase diagram of the pseudo-binary Y O –SeO system
Labsys TM DSC 12 equipment was used for the measurements. The ternary phases,
compositions of the ternary phases of Y O and SeO are filled in DSC quartzglass
2
3
2
–
ampoules, evacuated and sealed. The samples are heated with a rate of 10 K min in
1
reference to Al O . The ‘onset-temperatures’ of effects for the corresponding compo-
sitions are given in Fig. 10, from which we may obtain the following conclusions:
2
3
1
. There exist four thermodynamically stable phases: Y Se O , Y Se O ,
Y Se O and Y SeO in the pseudo-binary Y O –SeO system. There are no thermal
2
4
11
2
3.5 10
2
3
9
2
5
2
3
2
signals between 0–50 mol% SeO2.
. Y Se O melts peritectically at 425±5°C (698°K) and results to solid
Y Se O and SeO -rich melt. This finding is in good agreement with X-ray and IR
2
2
4
11
2
3.5 10
2
spectroscopy results.
. Investigations of compositions from Y Se O to SeO between 82.5 and
3
2
4
11
2
9
intensity increases with increasing SeO concentration. The other signal is at
1 mol% SeO show two endothermic signals. One signal is at 325±5°C (598 K), its
2
2
4
tions between 92.5 and 97.5 mol% SeO show also two endothermic effects, for
25±5°C (698 K), its intensity decreases with growing SeO concentration. Composi-
2
2
9
2.5 mol% SeO at 320°C (593 K) and 347°C (620 K), for 95 mol% SeO at 329°C
2 2
(
(
602 K) and 357°C (630 K) and for 97.5 mol% SeO at 325°C (598 K) and 369°C
642 K). With increasing of the SeO concentration the intensity of the first effect de-
2
2
creases and the second increases (Fig. 9). These lead to an obvious eutectic reaction
between Y Se O and SeO at 325°C (598 K) with eutectic point (E ) at 91.5±5 mol%
SeO2.
2
4
11
2
1
4. Y Se O melts peritectically at 525±5°C (798 K) (P ). This value is also in
2 3.5 10 2
good agreement with the results, obtained by annealing. The phase shows an endo-
thermic transition (U ), with a small effect at 485±5°C (758 K).
5. Y Se O melts peritectically at 1125±5°C (1398 K) (P ) and shows a kind of
transition (U ) at 1100°C (1373 K).
1
2
3
9
3
2
J. Therm. Anal. Cal., 69, 2002

3
16
2 3 2
ZHANG-PRESSE, OPPERMANN: PSEUDO-BINARY SYSTEM Y O –SeO
6. The SeO -poorest phase Y SeO in the pseudo-binary Y O –SeO system is
2 2 5 2 3 2
thermodynamically stable over the whole temperature region up to 1200°C (1473 K).
7. The melting temperature of SeO (S ) is determined as 391±2°C (664 K) in
agreement with earlier results [10].
2
1
*
* *
We thank Ms. J. Krug, H. Kempe, H. Dallmann and J. Schubert for cooperation and the Deutsche
Forschungsgemeinschaft (DFG) Bonn for financial support.
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1
2
3
4
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1
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