Thermodynamic properties of Pb2PtO4 and PbPt2O4 and phase equilibria in the system Pb-Pt-O

Article abstract of DOI:10.1016/j.jallcom.2009.03.051

The compounds Pb₂PtO₄ and PbPt₂O₄ were synthesized from an intimate mixture of yellow PbO and Pt metal powders by heating under pure oxygen gas at 973 K for periods up to 600 ks with intermediate grinding and re

Full text of DOI:10.1016/j.jallcom.2009.03.051

Journal of Alloys and Compounds 481 (2009) 228–232
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Thermodynamic properties of Pb PtO and PbPt O and phase equilibria in the
2
4
2
4
system Pb–Pt–O
K.T. Jacob , G. Rajitha , G.M. Kaleb
a,∗
a
a
Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India
Institute for Materials Research, University of Leeds, Leeds LS2 9JT, UK
b
a r t i c l e i n f o
a b s t r a c t
Article history:
2 4 2 4
The compounds Pb PtO and PbPt O were synthesized from an intimate mixture of yellow PbO and
Received 17 February 2009
Accepted 11 March 2009
Available online 24 March 2009
Pt metal powders by heating under pure oxygen gas at 973 K for periods up to 600 ks with intermediate
grinding and recompacting. Both compounds were found to decompose on heating in pure oxygen to PbO
and Pt, apparently in conflict with the requirements for equilibrium phase relations in the ternary system
Pb–Pt–O. The oxygen chemical potential corresponding to the three-phase mixtures, Pb2PtO4 + PbO + Pt
and PbPt2O4 + PbO + Pt were measured as a function of temperature using solid-state electrochemical cells
incorporating yttria-stabilized zirconia as the solid electrolyte and pure oxygen gas at 0.1 MPa pressure
as the reference electrode. The standard Gibbs free energies of formation of the ternary oxides were
derived from the measurements. Analysis of the results indicated that the equilibrium involving three
condensed phases Pb2PtO4 + PbO + Pt is metastable. Under equilibrium conditions, Pb2PtO4 should have
decomposed to a mixture of PbO and PbPt2O4. Measurement of the oxygen potential corresponding to
this equilibrium decomposition as a function of temperature indicated that decomposition temperature
in pure oxygen is 1014(± 2) K. This was further confirmed by direct determination of phase relations in
the ternary Pb–Pt–O by equilibrating several compositions at 1023 K for periods up to 850 ks and phase
identification of quenched samples using X-ray diffraction (XRD), scanning electron microscopy (SEM) and
energy dispersive X-ray spectroscopy (EDS). Only one ternary oxide PbPt2O4 was stable at 1023 K under
equilibrium conditions. Alloys and intermetallic compounds along the Pb–Pt binary were in equilibrium
with PbO.
Keywords:
Thermodynamic properties
Electromotive force (emf)
Thermal analysis
Thermodynamic modeling
Oxide materials
©
2009 Elsevier B.V. All rights reserved.
c = 0.6306 nm [3]. The compound is an insulator with Pt4+ ions in
octahedral environment. PtO6 octahedra are edge shared along the
c-axis direction to form rutile-type chains. The Pb2+ ions are stacked
1
. Introduction
As part of studies on the thermodynamics of interaction between
lead containing ferroelectric oxides and Pt electrodes, phase rela-
tions in the system Pb–Pt–O were explored and thermodynamic
properties of the two ternary oxides present at ambient pres-
in rows in the channels between the chains. The compound PbPt O4
2
is a metallic conductor and has triclinic crystal structure (space
group P 1¯ , Z = 2) with lattice parameters a = 0.61173, b = 0.66489 and
◦
◦
◦
sure were determined. Anderson et al. [1] have reported PbPt O4
c = 0.55523 nm, ˛ = 97.195 , ˇ = 108.827 , ꢀ = 115.213 [4]. Pt is in two
2
oxidation states; Pt2+ is in square planar and Pt in octahedral envi-
4+
formation at the interface between lead–zirconate–titanate (PZT)
solid solution and Pt during PZT deposition using high-throughput
modified molecular-beam epitaxy (MBE) technique. There are
three known ternary oxides in the Pb–Pt–O system; Pb Pt O6.5,
Pb PtO4 and PbPt O . The cubic pyrochlore Pb Pt O with lat-
tice parameter a = 1.0291 nm has been synthesized at 973 K and
pressure P/P = 3000, where P represents standard pressure [2].
The other two compounds are stable at ambient pressure. The
compound Pb PtO4 has orthorhombic crystal structure (space
group Pbam, Z = 2) with lattice parameters a = 0.9115, b = 0.7941 and
ronment. PtO groups are columnar stacked along the c-axis. These
4
stacks are held by other planar PtO4 groups to constitute Pt O8
3
2
2
sheets. The sheets are linked together by PtO6 octahedra to form
a three-dimensional network. The Pb atoms are surrounded by six
oxygen forming distorted octahedra. The metallic conductivity is
consistent with the short Pt–Pt bonds in the columnar stacks of PtO4
groups along the c-direction [5]. In the literature, there is neither
thermodynamic data for the two ternary oxides nor information of
phase relations in the ternary system Pb–Pt–O.
2
2
4
2
2
6.5
◦
◦
2
Earlier studies on Pb-containing systems of interest in the
processing of piezoelectric ceramics were concerned with ther-
modynamic properties of lead zirconate [6], lead titanate [7],
lead aluminates [8], lead ruthinate [9] and Pb(Zr,Ti)O3 (PZT)
solid solution [10]. Phase equilibria in the systems Pb–Ru–O [9],
∗ Corresponding author.
E-mail addresses: katob@materials.iisc.ernet.in, ktjacob@hotmail.com
K.T. Jacob).
(
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.03.051

K.T. Jacob et al. / Journal of Alloys and Compounds 481 (2009) 228–232
229
PbO–RuO –TiO [11] and RuO –TiO2 in air [12] have been delin-
2
2
2
eated. Also available is a thermodynamic analysis of the interaction
between Pb(Zr,Ti)O3 solid solutions and RuO2 electrodes as a func-
tion of PZT composition, temperature and oxygen partial pressure
[
13].
2. Experimental aspects
2.1. Materials
The compounds Pb2PtO4 and PbPt2O4 were synthesized from yellow lead
monoxide (PbO), platinum metal and pure oxygen gas at high temperature. PbO
and Pt powders, each of purity greater than 99.9%, were thoroughly mixed in the
appropriate stoichiometric ratios in an agate mortar, mixture compacted at 100 MPa
pressure in a steel die, and the pellet heated under flowing oxygen gas at 973 K
until completion of reaction. At intervals of ∼90 ks the pellets were quenched in
liquid nitrogen, reground and recompacted for further heat treatment. After each
quench, powders were examined at room temperature by X-ray diffraction (XRD)
to assess the progress of reaction. The formation of dark brown Pb2PtO4 was com-
plete in ∼300 ks, whereas ∼540 ks was required for dark grey PbPt2O4. The Pb and
Pt content of the compounds were determined by chemical analysis and the oxygen
content by mass loss measurement during hydrogen reduction at 823 K. The com-
pounds were found to be essentially stoichiometric. The XRD patterns of the two
compounds recorded in this study were almost identical to those reported in the lit-
erature [3–5]. The lattice parameters for PbPt2O4 obtained in this study (a = 0.61168,
◦
◦
◦
b = 0.66495 and c = 0.55510 nm, ˛ = 97.196 , ˇ = 108.819 , ꢀ = 115.225 ) showed very
minor difference from the values given by Obbade et al. [4].
The intermetallic compounds PbPt3 and PbPt, required for phase equilibrium
studies on the system Pb–Pt–O, were prepared by melting a mixture of metals in the
required ratios at 1673 and 1273 K, respectively. The compound PbPt3 was annealed
at 1073 K and PbPt at 1023 K for ∼20 ks. XRD confirmed formation of the compounds.
2.2. Decomposition studies
Thermal stability of Pb2PtO4 and PbPt2O4 was investigated using differential
thermal analysis (DTA) and thermo-gravimetric analysis (TGA) in pure oxygen gas
both on heating and cooling. Examined by XRD and EDS were the residue obtained
after DTA/TGA experiments. In a few experiments, after recording the decomposition
at high temperature, the samples were rapidly cooled under flowing argon gas and
the residue examined by XRD and EDS.
Fig. 1. Schematic diagram of the cell assembly.
2
.3. Electrochemical measurements
is a stream of oxygen gas. The finely dispersed Pt catalyzes the conversion of oxy-
gen molecules in the gas to oxygen ions in the solid electrolyte at the triple phase
boundary. Such electrodes work well above 873 K. Better catalysts are required for
lower temperatures. Periaswami et al. [15] have shown that RuO2 dispersed on the
YSZ surface can lower the response temperature to 773 K. In this study, an attempt
was made to enhance the temperature range of measurement by using spheroidized
RuO2 dispersion. A few drops of 10% aqueous solution of RuCl3 was introduced over
the inner flat surface of the YSZ tube. The tube was heated to 1073 K for 300 s under
flowing air. Obtained by this treatment was a highly adherent dispersed RuO2 black
deposit. Pressed against this surface by an alumina tube was a Pt mesh with an
attached Pt lead.
The three-phase mixtures were compacted in YSZ crucibles to form the work-
ing electrodes of the three cells. The components of the mixture were taken in the
molar ratio 1:1:1.5, with an excess of the phase that decomposed at high temperature
to establish the oxygen pressure over the electrode. The solid electrolyte tube was
pressed against the mixture, with a Pt mesh sandwiched in between. The assembly
was enclosed in an outer quartz tube. At the cold end, the gap between the neck of
the quartz tube and the YSZ tube was sealed with De Khotinsky cement as shown
in the diagram. After assembling the cell the quartz tube was evacuated through a
side arm to a pressure of 0.1 Pa and then flame-sealed. The entire assembly shown in
Fig. 1 was placed inside a vertical resistance furnace, with the electrodes located in
the even-temperature zone (± 1 K). The cement seal located at the top of the assem-
bly was maintained at room temperature throughout the measurements. A Faraday
cage made of stainless steel foil was placed between the furnace tube and the cell
assembly. The foil was earthed to minimize induced emf on cell leads. The temper-
ature of the cell was measured with a Pt/Pt–13%Rh thermocouple, checked against
the melting point of gold. A high-impedance digital voltmeter with a sensitivity of
The emf of the following cells were measured as a function of temperature from
75 K at intervals of 25 K until the emf became marginally negative, which indicated
8
that the oxygen pressure over the working electrode was above atmospheric:
Pt, PbPt2O4 + PbO + Pt/(Y2O3)ZrO2/RuO2, O2(0.1 MPa), Pt
(1)
(2)
(3)
Pt, Pb2PtO4 + PbO + Pt/(Y2O3)ZrO2/RuO2, O2(0.1 MPa), Pt
Au, Pb2PtO4 + PbPt2O4 + PbO/(Y2O3)ZrO2/RuO2, O2(0.1 MPa), Au
The cells are written such that the right-hand side electrodes are generally pos-
itive. In each cell the oxygen pressure at the reference electrode on the right-hand
side was higher than that over the working electrode consisting of three condensed
phases, except at the highest temperature when the dissociation pressure marginally
exceeded that of the reference gas. The upper temperature limit was signalled by
marginally negative value of the emf.
Initially measurements were made on cells (1) and (2). As discussed later, an
analysis of the electrochemical measurements indicated that the working electrode
of cell (2) was in metastable equilibrium. Hence, cell (3) was designed based on
stable equilibrium phase mixture at the working electrode. Gold was used as an
inert electrical leads, since use of Pt could change phase relations at least at the
points of contact between the electrical lead and the measuring electrode.
The apparatus used for electrochemical measurements was similar to that
described earlier [14] with minor variations. Shown in Fig. 1 is a schematic dia-
gram of the cell assembly. Since the oxygen partial pressures over the electrodes
of the cell were expected to be high, a closed system was used for measurements.
The three-phase electrode was placed in a stabilized zirconia crucible, which was
sealed in an evacuated quartz tube. The equilibrium oxygen partial pressure was
established by the decomposition of the ternary oxide present at the electrodes at
high temperature. Used as the reference electrode was pure oxygen gas at standard
pressure. Yttria-stabilized zirconia (YSZ) tube, containing 12 mol% Y2O3, closed at
one end, was used as the solid electrolyte. YSZ is an oxygen ion conductor with ionic
transport number greater than 0.999 at the temperatures and oxygen partial pres-
sures encountered at the electrodes of the cells used in this study. The tube was
vacuum tested for leaks and found to be impervious.
0.01 mV measured the cell potentials.
2.4. Isothermal section at 1023 K of Pb–Pt–O phase diagram
The isothermal section was explored by equilibrating compacted mixtures of
metals/intermetallics and oxides at 1023 K for prolonged periods up to 850 ks and
phase identification after quenching in liquid nitrogen or chilled mercury. The
phase composition of the samples was unaltered by further heating. During the
total equilibration period, the samples were quenched twice, ground to −325 mesh,
and recompacted using a steel die for further heat treatment. Chosen for study
were 11 samples representing nine compositions inside the ternary triangle. In
The oxygen reference electrode is generally fabricated by platinizing the sur-
face to the YSZ tube, spheroidizing the thin deposit by heat treatment, and pressing
a Pt gauze with an attached Pt lead against it. Passed over the platinized surface

2
30
K.T. Jacob et al. / Journal of Alloys and Compounds 481 (2009) 228–232
two cases, samples of the same average composition were prepared using differ-
ent starting materials. The samples containing metallic phases were contained in
YSZ crucibles and sealed in evacuated quartz tubes for equilibration. Oxide mix-
tures were equilibrated under pure oxygen gas. Optical microscopy (OM), scanning
electron microscopy (SEM), energy dispersive spectroscopy (EDS) and powder X-ray
diffraction (XRD) at room temperature identified the equilibrium phases present in
the quenched samples.
3. Results and discussion
3
.1. Decomposition of Pb PtO4 and PbPt O4
2
2
DTA of Pb PtO in pure oxygen gas showed an endothermic peak
2
4
corresponding to decomposition, starting at 1086 K and ending at
131 K, with maximum height at 1112 K. The decomposition was not
1
reversible since no peak was observed during cooling. The exami-
nation of the residue by XRD after cooling in oxygen indicated the
presence of yellow PbO with orthorhombic structure and Pt metal.
The same phases were identified when the decomposition products
were cooled under argon gas. TGA in oxygen confirmed the onset of
decomposition at 1085 K; the total mass loss during decomposition
corresponded to the loss of one molecule of diatomic oxygen gas
from Pb PtO .
2
4
DTA of PbPt O in pure oxygen gas exhibited an endothermic
2
4
decomposition peak, starting at 1111 K and ending at 1160 K, with
peak maximum at 1141 K. No peak was observed during cooling. The
examination of the residue after cooling in oxygen and argon indi-
cated the presence of yellow PbO and Pt. TGA in oxygen confirmed
the onset of decomposition at 1110 K; the mass loss during decom-
position corresponded to the escape of one and a half molecules of
oxygen gas from the compound.
Fig. 2. Variation of the emf of cells (1)–(3) with temperature.
The oxygen potential (ꢂꢃO2 = RT ln PO ) at the working elec-
2
trode of cell (1) defined by the reaction,
PbO(ortho) + 2Pt + 1.5O → PbPt O
(7)
2
2
4
3.2. Electrochemical measurements
can be computed from the emf since there is negligible mutual
solubility between the solid phases. The standard Gibbs energy of
The emf of cell (1) was measured in the temperature range from
75 to 1125 K, cell (2) from 875 to 1100 K, and cell (3) from 875
formation of PbPt O4 from yellow PbO with orthorhombic struc-
2
8
ture, Pt metal and oxygen gas according to reaction (7) is related to
the oxygen potential:
to 1025 K. After a change in temperature, the emf of cells (1) and
2) became steady in 3–10 ks after the attainment of thermal equi-
(
ꢂG (± 385)/J mol = 1.5ꢂꢃꢀ = −6FE1 = −276,951 + 249.94T
◦
−1
librium; longer periods were required at the lower temperatures.
The response of cell (3) was very sluggish, requiring up to 80 ks to
register steady values. Constancy of emf was checked over 120 ks.
The reversibility of the cells was checked by microcoulometric
titration in both directions. The emfs were found to gradually return
to the same value after essentially infinitesimal displacement of the
equilibrium oxygen potential of the working electrode to higher
and lower values for cells (1) and (2). The response of cell (3) after
titration was too sluggish for the method to be effectively used,
except at the higher temperatures. The reversibility of the cells was
also tested by temperature cycling. The same emf was registered
when approached from lower and higher temperatures.
7
O
2
(8)
The “second-law” enthalpy of formation of PbPt O4 from PbO,
2
Pt and O2 according to reaction (7) at a mean temperature
of 1000 K is −276.95 (± 1.45) kJ/mol. The corresponding entropy
change is −249.94 (± 1.42) J/mol K. The decomposition tempera-
ture of PbPt O computed using Eq. (8) in pure oxygen at standard
2
4
pressure is 1108(± 2) K, and in air 1028(± 2) K. The decomposition
in pure oxygen derived from emf measurements is in good agree-
ment with the values of 1111 and 1110 K derived from DTA and TGA
results obtained during heating. Dynamic techniques, where tem-
perature is continuously varied, tend to register decomposition at
temperatures slightly higher than that obtained from a series of
essentially equilibrium measurements at constant temperatures. It
is to be noted that the onset temperature, rather that peak max-
imum temperature, for decomposition in DTA and DTGA studies
correlates better with thermodynamic measurements. The decom-
The reversible emf of the three cells is plotted versus temper-
ature in Fig. 2. The emfs of all the cells decrease linearly with
increasing temperature. The least-squares regression analysis gives
the following expressions for the temperature-dependence of emf:
E1(± 0.66)/mV = 478.40–0.43175T
E2(± 0.54)/mV = 477.26–0.4408T
E3(± 0.64)/mV = 461.72–0.4549T
(4)
(5)
(6)
position temperature of PbPt O4 in air (1028 K) obtained from
2
electrochemical measurements compares with values of 1057 K [4]
and 1023 K [5] reported in the literature using DTA.
The oxygen potential at the working electrode of cell (2) and the
standard Gibbs energy change for the reaction,
The quoted uncertainty limits correspond to twice the standard
error estimate (2ꢁ). Since the ionic transport number of YSZ solid
electrolyte is greater than 0.999 under the experimental conditions
used in this study, the emfs are related to the difference in oxygen
chemical potentials at the two electrodes by the Nernst equation.
Further, as pure oxygen at 0.1 MPa pressure was used as the ref-
erence electrode, the emf directly gives the chemical potential of
oxygen at the working electrode.
2
PbO(ortho) + Pt + O → Pb PtO
(9)
2
2
4
computed from the emf are given by,
ꢂG (± 210)/J mol = ꢂꢃꢀꢀ = −4FE2 = −184,194 + 170.12T (10)
◦
−1
9
O
2

K.T. Jacob et al. / Journal of Alloys and Compounds 481 (2009) 228–232
231
the equilibrium between three condensed phases represented by
reaction (9) is metastable. The formation of PbPt O4 from Pb PtO4
2
2
appears to very sluggish and does not occur in the time frame of
DTA/TGA measurements in oxygen. Heating Pb PtO4 beyond its
2
equilibrium decomposition temperature results in its decomposi-
tion to PbO and Pt. Electrochemical proof for this hypothesis comes
from measurements on cell (3). The oxygen potential at the working
electrode of this cell defined by the reaction,
3
PbO(ortho) + PbPt O + 0.5O → 2Pb PtO
(13)
2
4
2
2
4
ꢂG (± 125)/J mol−1 = 0.5ꢂꢃOꢀꢀꢀ = −2FE3 = −89,098 + 87.78T
◦
1
3
2
(14)
is almost identical to that calculated from the results from cells (1)
◦
−1
and (2) [ꢂG13/J mol = −91, 437 + 90.3T], although the separate
enthalpy and entropy terms show some difference. The decom-
position of Pb PtO4 under true equilibrium conditions defined by
2
Fig. 3. Overlapping three-phase fields generated by the decomposition of Pb2PtO4
and PbPt2O4 in pure oxygen.
reaction (13) occurs at 1014(± 2) K in pure oxygen and at 945(± 2) K
in air. Since the emf of cell (3) was measured over a smaller range
of temperature, greater reliance is placed on measurements on
cell (2) for deriving thermodynamic properties of Pb2PtO4. The
results obtained in this study clearly demonstrate that metastable
equilibria are as valuable as stable equilibria in determining ther-
modynamic properties of complex oxides. Direct phase equilibrium
studies provide further evidence of correct phase relations is the
system Pb–Pt–O.
The first (temperature-independent) term on the right-hand
side of Eq. (10) gives the enthalpy of formation of Pb PtO4 from
2
PbO, Pt and O2 according to reaction (9) [−184.19(± 0.93) kJ/mol] at
a mean temperature of 988 K. The second (temperature-dependent)
term with a change in sign gives the corresponding entropy of
formation [−170.12 (± 0.93) J/mol K]. The decomposition temper-
ature of Pb PtO corresponding to reaction (9) in pure oxygen at
Thermodynamic properties of PbPt2O4 and Pb2PtO4can be eval-
uated at 298.15 K from the results of this study, using average values
2
4
0
.1 MPa pressure is 1083(± 2) K and in air 1006(± 2) K. This result
◦
from thermodynamic measurements is in good accord with direct
decomposition studies using DTA/TGA, which indicate the onset of
decomposition at 1086 K in pure oxygen. Bettahar et al. [3] reported
a value of 1008 K in air using DTA.
of 18.84, 12.56 and 6.28 J/mol K for change in heat capacity (ꢂC ) for
P
reactions (7), (9) and (13), respectively [16]. The standard enthalpy
and entropy of formation of PbPt2O4 according to reaction (7) at
298.15 K are −290.2(± 3.3) kJ/mol and −272.74 (± 3) J/mol K, respec-
tively. For Pb2PtO4 the standard enthalpy and entropy of formation
according to reaction (9) at 298.15 K are −193.0(± 2.7) kJ/mol and
−185.17(± 2.5) J/mol K. The standard enthalpies of formation from
Three-phase equilibria defining the oxygen potentials at the
working electrodes of cells (1) and (2) are displayed on a Gibbs tri-
angle in Fig. 3. It is seen that the two three-phase regions overlap in
violation of the topographical rules of construction of ternary phase
diagrams. This suggests that one of the three-phase fields may
correspond to metastable equilibrium. To identify the metastable
equilibrium, the standard Gibbs energy change for the reaction,
◦
−1
elements at 298.15 K are ꢂH
(± 3.6)/kJ mol = −508.2 for
2
98.15
◦
−1
PbPt O , and ꢂH 98.15(± 3)/kJ mol = −829.1 for Pb PtO . The
2
4
2
4
2
◦
−1 −1
corresponding standard entropies are S2
(± 3.4)/J mol
K
=
98.15
◦
−1 −1
1
86.93 for PbPt O4 and
S
(± 2.8)/J mol
K
= 199.1 for
2
2
98.15
3
Pb PtO + Pt → 4PbO + 2PbPt O
(11)
Pb PtO . The auxiliary thermodynamic data used in the calculation
2
4
2
4
2
4
◦
−1
o
are ꢂH
(± 0.63)/kJ mol = −218.06 and
S
(± 0.21)/J ·
was computed from the results obtained in this study (Eqs. (8) and
298.15
298.15
mol · K−1 = 68.7 for PbO [17], S
−
1
◦
(± 0.4)/J mol
K = 205.15 for O2 gas [17].
−1 −1
K
= 41.63
(
10)):
298.15
◦
−1 −1
ꢂG (± 740)/J mol−1 = −1320 − 10.48T
◦
for Pt [18], and S
(± 0.04)/J mol
298.15
(12)
1
1
It would be useful to confirm the derived values for enthalpy of
formation and entropy of the two ternary oxides by more direct
calorimetric techniques.
The result indicates that under equilibrium conditions a mixture
of Pb PtO + Pt must react to give a mixture of PbO + PbPt O . Thus,
2
4
2
4
Table 1
Experimental details of phase equilibrium studies at 1023 K.
Expt. no.
Materials used for sample
preparation
Average composition of sample
Duration of
equilibration (ks)
Phases identified after
equilibration
Techniques used
XPt
XPb
XO
1
2
3
4
5
6
7
8
9
Pb2PtO4
Pt + Pb2PtO4
Pb + PbPt2O4
Pt + Pb2PtO4
Pt + PbPt + PbPt2O4
Pt + PbPt2O4
Pb + PbPt3 + PbO
Pb + PbPt3 + PbO
PbPt + Pb2PtO4
PbPt + Pt + PbPt2O4
Pt + Pb2PtO4
0.1447
0.184
0.2
0.4
0.4
0.2842
0.272
0.4
0.2
0.2
0.571
0.544
0.4
0.4
0.4
0.4
0.2
0.2
0.2
850(in O2)
824
768
674
739
785
797
821
835
PbO + PbPt2O4
PbO + PbPt2O4
PbO + Pt
XRD, EDS
XRD
XRD, EDS
XRD
XRD
XRD
XRD, EDS
XRD, EDS
XRD
XRD, EDS
XRD
PbO + PbPt2O4 + Pt
PbO + PbPt2O4 + Pt
PbPt2O4 + Pt
PbO + PbPt + Pb0.69Pt0.31(l)
PbO + PbPt + PbPt3
PbO + PbPt + PbPt3
PbO + PbPt3 + Pt
PbO + PbPt2O4 + Pt
0.5
0.1
0.25
0.374
0.374
0.5
0.55
0.426
0.426
0.3
1
0
0.2
0.2
835
772
11
0.7
0.1

2
32
K.T. Jacob et al. / Journal of Alloys and Compounds 481 (2009) 228–232
Based on the thermodynamic data obtained in this study, phase
relations at 973 K were computed by invoking the Gibbs energy
minimization principle. The result is shown in Fig. 5. At this temper-
ature, two ternary oxides are present and an additional three-phase
field involving Pb PtO , PbPt O and PbO appears on the diagram.
2
4
2
4
4. Summary and conclusions
Thermodynamic properties of two lead platinum oxides were
determined using solid-state cells at high temperatures. The for-
mation reactions and the corresponding Gibbs free energies are:
PbO(ortho) + 2Pt + 1.5O → PbPt O
2
2
4
◦
−1
ꢂG (± 385)/J mol = −276, 951 + 249.94T
2
PbO(ortho) + Pt + O → Pb PtO
2
2
4
◦
−1
ꢂG (± 210)/J mol = −184, 194 + 170.12T
Fig. 4. Experimentally determined isothermal section of the phase diagram for the
system Pb–Pt–O. The symbol × identifies the average composition of equilibrated
samples.
Products formed by the decomposition of PbPt O are equi-
2
4
librium phases, whereas Pb PtO4 yields metastable products.
2
Equilibrium phase relations in the system Pb–Pt–O were deter-
mined by isothermal equilibration at 1023 K for extended periods.
Using estimated heat capacities, the standard enthalpies of
formation from elements and standard entropies of the two
3.3. Phase diagram for the system Pb–Pt–O at 1023 K
The average composition of the samples equilibrated at 1023 K,
◦
ternary oxides at 298.15 K are derived. For PbPt O , ꢂH (298.15) =
2
4
the elements and compounds used in preparing the samples, dura-
tion of equilibration, phases identified in quenched samples at room
temperature and the techniques used for identification are pre-
sented in Table 1. The isothermal section of the phase diagram
composed from the results obtained in this study is shown in Fig. 4.
f
◦
−
508.2(± 3.6) kJ/mol and
S
= 186.75(± 3.4) J/mol K, and
298.15
◦
◦
for Pb PtO ꢂH (298.15) = −829.1(± 3) kJ/mol and
S
=
2
4
f
298.15
198.7(± 2.8) J/mol K.
Only one ternary compound PbPt O4 was identified at 1023 K. The
Acknowledgements
2
compound Pb PtO , which was a constituent of some of the sam-
2
4
ples before equilibration, appears to have disappeared during the
long equilibration. A three-phase field involving the solid phases
PbPt O , PbO and Pt was identified.
K.T. Jacob wishes to thank the Indian National Academy of Engi-
neering, New Delhi, for support as INAE Distinguished Professor.
G. Rajitha wishes to thank the University Grants Commission of
India for the award of Dr. D.S. Kothari Postdoctoral Fellowship which
facilitated this research.
2
4
Along the binary Pb–Pt, two intermetallic compounds PbPt and
3
PbPt and a liquid phase (0 ≥ XPt ≥ 0.39) were identified, in reason-
able agreement with the binary phase diagram for the system [19].
All the alloy and intermetallic phases were found to coexist with
PbO.
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Fig. 5. Isothermal section of the phase diagram for the system Pb–Pt–O at 973 K
calculated from thermodynamic data obtained in this study.