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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
X-RAY
CHARACTERIZATION
OF COMPOUNDS IN THE SrO-PbO
SYSTEM
W. Wong-Ng, J. P. Cline, L. P. Cook and W. Greenwood
Ceramics Division
National Institute of Standardsand Technology
Gaithersburg, MD 20899
ABSTRACT
Conventional and high-temperature x-ray diffraction (HTXRD) were used as the principal
techniquesfor the investigation of the crystal chemistry and phase equilibria of the compounds
in the SrO-PbO, system. Two binary oxides were confirmed in this system: SrPbO,, and Sr,PbO,,
The SrPbO, phase melts around 865 “C and was found to undergo phase tranformations from
orthorhomic to tetragonal to cubic structure at 425 - 450 “C and 815 “C respectively. A
discontinuity in the changeof lattice parametersat 815 “C indicates a first order phasetransition
from tetragonal to cubic. No phase transformation was observed for the Sr,PbO, phase, which
melts at around 1075 “C. An x-ray diffraction pattern of the cubic SrPbO, phasemeasuredat 880
“C is also reported.
INTRODUCTION
Phase equilibrium diagrams provide essential “road maps” for processing of high-temperature
ceramic materials. A compilation of the phaseequilibrium diagrams of the multi-component Bi-PbSr-Ca-Cu-0 (BSCCO) system and its quaternary, ternary and binary subsystemsreveals an
absenceof data for the binary PbO,-SrO system [ 1,2]. Since an extensive database is useful for
the superconductorcommunity, we have undertaken the study of the crystal chemistry and phase
equilibria of this system.
In 1970, Keester and White [3] reported the presenceof two compoundsin the SrO-PbO, system,
namely, SrPbO,, and Sr,PbO,. SrPbO, has a perovskite structure type, and like many other
perovskites, undergoesphasetransformation at high temperatures.At = 450 “C, a transformation
from orthorhombic to tetragonal was reported, and another one from tetragonal to cubic was
predicted around 700 “C [3]. However, no further experiments were reported at temperatures
higher than 600 ‘C, due to possible atmospheric interaction.
The Sr,PbO, phaseis important in the processingof high T, superconductorphasesin the BSCCO
system [4]. For example, during the processing of the 110 K 2223 [(Bi,Pb):Sr:Ca:Cu],
(Sr,Ca),PbO, was often found as an impurity. For this phase, a complete solid solution range can
be preparedbetween Ca and Sr end members [5,6]. Furthermore, the formation of a liquid phase
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Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
Copyright(C)JCPDS-International Centre for Diffraction Data 2000, Advances in X-ray Analysis, Vol.42
assoicatedwith the presence of (Sr,Ca),PbO, was found to promote the formation of the 2223
phase [7].
The main goal of this study was to investigate phase transformations of SrPbO, and SrzPbO,,
using conventional powder x-ray diffraction, high-temperature x-ray diffraction (HTXRD) and
differential thermal analysis (DTA) techniques. In order to avoid material deterioration due to
atmospheric reaction, the DTA and HTXRD characterizations were studied under purified dry
air. The secondgoal was to investigate the melting temperatures of SrPbO,, and Sr,PbO, using
DTA. Since reference x-ray powder diffraction patterns are important for phase identification,
and the pattern of the cubic SrPbO, phaseis currently absentin the ICDD Powder Diffraction File
(ICDD-PDF [8]), a third goal was to report an x-ray pattern of this phase taken at hightemperature.
EXPERIMENTAL
METHODS
Conventional solid statemethods were employed to prepare SrPbO, and Sr,PbO,, Stoichiometric
amounts of PbO and SrCO, were used as the starting material. Sampleswere weighed, mixed,
pressedand heat-treatedat 750 “C and 800 “C with intermediate grindings and pressings. For
the analysis of the phase content of the samples prepared, an automated conventional x-ray
diffractometer which was equipped with a theta-compensation slit and CuKa radiation was
employed. Data processing was achievedby using the Siemens’ software suite.
High-temperature x-ray diffraction studies were conducted using a Siemens D5000 powder
diffractometer with a 0-8 geometry. With this geometry, the samplecan be maintained stationary
in a horizontal fashion. A Btihler HDK2.3 furnace utilizing a platinum heating strip was employed
for the high-temperatureexperiments. A PID controller, using a type-S thermocouple was welded
to the underside of the heating strip, was used to regulate the temperature. The sample was
mounted in the form of a dispersion, using methanol as the liquid, onto the Pt strip (- 2-4 mg/cm2)
and allowed to dry. The diffractometer is further equipped with variable divergence slits and an
MBraun scaningposition sensitive detector (PSD). An Ni filter was mounted on the PSD window
to filter the copper Ka radiation used for the experiments. X-ray data were collected from 15 O
to 95 “20 at an interval of 0.02 “20. A scantime of 20 min was used for each diffraction pattern
to achieveadequatesignal-to-noiseratio. During theseexperiments, in order to minimize chemical
reaction, purified air was dried with CaSO,, and any residual CO, was removed with Ascerite.
Gasflow was maintained at =:100 cm3/min near ambient pressure throughout the measurements.
Simultaneous DTA/TGA experiments were performed using a Mettler TA-1 thermoanalyzer.
Sampleswere placed in high density MgO crucibles, and an a-alumina referencewas used. The
system was calibrated against the melting point of Au (1063 “C). The DTA/TGA system was
arranged to allow a fresh flow of purified air (with CO, and moisture removed) past the sample
during analysis. DTA and DTG event onset temperatures were determined by the usual method
which utilized the intersection of the baseline with me extension of the linear region of the rising
peak slope. Event temperatureswere estimatedto have standarduncertaintiesof less than 10 “C.
A heating rate of 4 “Urnin was used. Temperatures of melting eventshave been selectedbased
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on the first heating cycle.
Lattice parametersof the samplesat elevatedtemperatureswere obtainedusing the NBSLSQ leastsquaresrefinementsprogram [9]. The Pt diffraction peaks arising from the Pt strip were used as
referencefor calibration. The thermal expansion coefficient of Pt is taken as 9 x low6A/‘C [lo].
At 25 “C, a,,, = 3.923 1 A (PDF 4-802 ), and at an elevated temperature, the expandedlattice
parameter a’,,, is calculated as
+t) = a,,,[1+0.000009(aT)],
where AT is the difference between the temperature at which the experiment was conductedand
25 “C. At high-temperatures, theoretical Pt peaks were calculated from the derived lattice
parameters, alcRj.
RESULTS
AND DISCUSSION
SrPbO, and Sr,PbO, were confirmed to be the only binary oxides in the SrO-PbO, system. In the
following, the crystal chemistry and crystallographic of these two phaseswill be discussed.
(1) SrPbO,
DTA results indicated that two endothermic eventstook place between 600 “C and 1100 “C. The
lower temperature one took place at = 8 15 “C and the higher temperature one (960 “C)
correspondsto the melting of the sample.
The structure of SrPbO, at room temperature is orthorhombic, with spacegroup Pbnm. The cell
parameterwas reported to be a=5.8595(5) A, b=5.9568(5) A and c= 8.3253(6)w [11,12]. The
structure is consistent with other A2+B4+03orthorhombically distorted perovskites [13]. In this
structure, PbO, octahedrawere found to be tilted with respect to each other (Figure 1).
A seriesof high-temperature x-ray diffraction patterns from 100 “C to 900 “C, with 28 running
from 15’ to 95 o was obtained. These x-ray diffraction patterns confirm that two phase
transformation events took place over this temperature range. The lower temperature one was
found to be from 425 “C to 450 “C (agreed with that of =: 450 “C by Keester and White [3]),
while the higher temperature one was found to be at 800°C to 820 “C (this event was observed
by DTA to be at 8 15 “C). Figures 2a and 2b show an enlarged portion of the 20 region around
52 O. The gradual merging of peaks with increasing temperature is evident. Change in cell size
with temperature and the orthorhombic to tetragonal to cubic transformation were observed by
monitoring the orthorhombic { 132)) (024)) and (3 12) reflections of the powder pattern at an
temperature intervals of 50 “C. Between 425 “C - 450 “C, the { 132) and (024) reflections of
the orthorhombic phase merged and transformed into (211) of the tetragonal cell. The(312)
reflection remained unchanged in position but then constituted the tetragonal { 112}. This
observation
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Fig. 1. Crystal structure of orthorhombic SrPbO, [ 1l] showing the PbO, octahedratilting with
respect to each other. The outline of the unit cell is shifted by ( YZ, % , Yz).
I
52.0
52.6
53.2
53.8
54.4
3
X.0
Fig. 2a. X-ray diffraction pattern of SrPbO, in the region of 52”-55 O28 from 200 “C to 475 “C.
Phasetransformation from orthorhombic (0) to tetragonal (T) at = 425 “C - 450 “C is observed
from the merging of three peaks (132, 024 and 312) to two ((211,121) and 112).
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112
52.0
52.6
A
53.2
33
1
82O'C(C)
8OO'C0
53.8
54.4
3
$
55.0
(“1
Fig.2b. X-ray diffraction pattern of SrPbO, in the region of 52 “-55 o 28 from 600 “C to 820 ‘C.
Phasetransformation from tetragonal (T) to cubic (C) at 820 “C is evident from the merging of
two peaks ((211,121) and 112) to one (211, 121,112).
confirms those of Keester and White [3]. However the transformation from tetragonal to cubic
phaseappearsto take place at a much higher temperaturethan the estimatedtemperatureof ~700
“C. It is clear from Fig. 3 that at about 820 oC, the {211} and { 112) reflections merged into the
{ 112) peak of the cubic cell. From the combined information of x-ray diffraction (peaksmerging)
and DTA events (endothermic peaks at 820 “C and 960 “C) indicated that 815 “C is the
temperature at which this tetragonal to cubic transformation takes place.
Table 1 gives the results of least-squareslattice parameters of SrPbO, from 650 “C to 880 “C
using the diffraction peaks of 20 up to 80” 28. Lattice parameter refinementsbelow 600 “C were
not attempted since Keester and White [3] have reported theseparametersup to = 600 “C. Figure
3 shows a plot of the lattice parametersas a function of temperature. While the axis at the top of
the figure showsa trend of smooth yet small increaseas a function of temperature,the bottom two
axes illustrate dramatic merging events. The data points representedby open circles were those
reported by Keester and White [3], and the filled ones are those obtained from the present study.
The first order orthorhombic to tetragonal transition at = 450 “C was already demonstratedby
Keester and White [3]. The abrupt changesof lattice parameter at around 800 “C - 820 “C can
be further seen from Fig. 4, where the unit cell volume is plotted as a function of temperature
between 600 “C - 880 “C. The tetragonal and cubic unit cell volumes indeed fall on two separate
straight lines. The gap between the two straight lines suggeststhat the volume changesbetween
the two crystal systemsis not continuous and the transformation is implied to be also of first order.
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1
0
I
I
200
I
I
n3n;rature,.
I
600
I
I
I
800
"C
Fig. 3. Cell parametersof SrPbO, versus temperature (from room temperature to 880 “C ). The
open circles are the experimental points by Keester and White (1970) ; the filled circles are those
obtained from the present study. discernible first-prder phasetransformations at around 450 “C
and 815 “C are observed.
Temperature
(“C)
Fig. 4. Plot of cell volume of the tetragonal (T) forms to cubic (C) from = 600 “C to 880 “C. A
clear break between two straight lines is featured in this plot, implying a first order phase
transition.
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361
Table 1. Lattice parameters of SrPbO, as a function of annealing temperature. (T) is used to
representthe tetragonal structure, and (C) for the cubic structure.
Tmp(“C)
a (4
c(A)
V(A3)
600(T)
650(T)
700(T)
750(T)
800(T)
820(C)
840(C)
860(C)
880(C)
4.1925(13)
4.197(2)
4.199(4)
4.2016 (15)
4.206(4)
4.2149(4)
4.2161(5)
4.2175(3)
4.2196(3)
4.212(2)
4.212(2)
4.214(4)
4.216(4)
4.217(2)
74.04(14)
74.19(6)
74.31(12)
74.44(6)
74.60(11)
74.88(2)
74.94(3)
75.02(2)
75.100(12)
The x-ray diffraction pattern of the cubic phasetaken at 880 “C is shown in Table 2. Indexing was
achieved based on a cubic cell with Pm3m spacegroup (by analogy with SrTiO, [14]) and with
a = 4.1296(3) A. The reported intensities are of the integrated values. Figure 5 shows the
strcuture of cubic SrPbO,. Compared to Fig. 1, it is seen that at high temperature, the PbO,
octahedra are well aligned with respect to each other, which gives rise to the higher cubic
symmetry.
Table 2. Reference powder pattern of the cubic SrPbO, taken at a temperature of 880 “C.
Indexing was basedon a cubic cell of Pm3m spacegroup and with a=4.1296(3$. The reported
intensity values are integrated.
h
k
1
dobs
2eobs
2%,
Int
1
1
1
2
2
2
2
3
3
3
2
0
1
1
0
1
1
2
0
1
1
2
0
0
1
0
0
1
0
0
0
1
2
4.2206
2.9833
2.4359
2.1095
1.8868
1.7224
1.4917
1.4063
1.3342
1.2721
1.2179
21.032
29.933
36.865
42.839
48.202
53.121
62.150
66.409
70.535
74.542
78.426
21.040
29.927
36.870
42.834
48.190
53.131
62.183
66.423
70.530
74.535
78.464
21.0
100.0
4.3
31.2
16.5
45.0
13.6
6.2
12.1
3.2
2.6
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Fig. 5. Crystal structure of the cubic SrPbO, [15] showing the octahedral PbO,. Sr is surrounded
by 12 oxygens.
(2) Sr,PbO,
The DTA experiment showed that only one endothermal peak was observedat 1075 “C, which
corresponds to the melting of this compound. The Sr,PbO, phase is therefore stable up to its
melting temperature without phase transformation.
Fig. 6. Crystal structure of Sr,PbO, showing the edge-sharing PbO, octahedral environment.
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Sr,PbO, has an orthorhombic Pbam structure [ 15, 161. Figure 6 illustrates the PbO, octahedral
chains edge-sharingalong the c-axis. Figure 7 shows the Sr coordination environment. Sr has a
6+ 1 coordination which can be viewed as a monocappedtrigonal prism. The six shorter distances
form a trigonal prism, while the longer Sr-0 one represents a cap on one of the side face. In
contrastto the SrPbO, phasein which the PbO, octahedraand Sr ions can be realigned in a more
symmetrical fashion as the temperature increases, the SrO, arrangements in Sr PbO 4are still
stable at higher temperature and do not rearrange to form other configurations.
a
‘CL b
Fig. 7. Crystal structure of Sr,PbO, showing the monocappedtrigonal prism (6 + 1
coordinations) environment around the Sr ions.
SUMMARY
No phasetransformation was observed for the Sr,PbO, phase. Two phasetransformations were
confirmed for SrPbO,. The lower temperature one which took place at 425 “C - 450 “C
correspondsto the orthorhombic to tetragonal transformation. The higher temperature one (815
“C, corroborated by DTA and HTXRD) corrresponds to a first order tetragonal to cubic
transformation. The temperature regimes of the three structures can be rearranged as follows:
25 “C <T<450”C
450°C <T<820”C
820°C <T
orthorhombic
tetragonal
cubic
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1The purpose of identifying the equipment in this article is to specify the experimental procedure.
Such identification does not imply recommendation or endorsementby the National Institute of
Standardsand Technology.
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[l]. J.D. Whitler and R.S. Roth, ‘Phase Diagrams for High T, SuperconductorsI”, The American
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[3]. K.L. Keester and W. B. White, “Crystal Chemistry and Properties of Phasesin the system
SrO-PbO-0” J. solid State Chem., 2 (1970) 68-73.
[4]. W. Wong-Ng, L.P. Cook, F. Jiang, Greenwood, W., U. Balachrandran, and M. Lanagan,
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at 7.5% 02,” J. Mater. Res., 12 (1997) 2855-2865.
[5]. W. Wong-Ng, J.A. Kaduk, and W. Greenwood, “Crystal Structure and X-ray Powder
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[8]. PDF, Powder Diffraction File produced by ICDD, 12 Campus Blvd., Newtown Square, PA.
19073-3273.
[9]. D.E. Appleman and H.T. Evans, Jr. Report #PB216188, U.S. Department of Interior, National
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[13]. R.D. Shannon and P.E. Bierstedt, “Single-Crystal Growth and Electrical Properties of
BaPbO,, ” J. Amer. Cerm. Sot., 53 (1970) 635-642.
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[15]. A. Teichert, and H. Mtiller-Buschbaum. “Zur Kristallstruktur von Ca,PbO,, ” Z. Anorg.
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