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Phase Relations in the System Ce,O,-AI,O,
in Inert and
Reducing Atmospheres
A. Cuneyt Tas*
Department of Metallurgical Engineering, Middle East Technical University, Ankara 0653 1 , Turkey
Mufit Akinc*
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Department of Materials Science and Engineering, Iowa State University, Ames, Iowa
The 1:l compound, CeAlO,, in the system Ce,O,-AI,O, has
been synthesized from the oxides and shown to have a
perovskite-like tetragonal unitacell with the lattice pararneters a = 3.763 and c = 3.792 A. A new XRD pattern is suggested for CeAIO,. This compound is shown to be stable up
to 1950°C. The 1:11 compound, CeAI,,O,,, has also been
synthesized and shown to possess a magnetoplumbite-like
hexagonal unit cellDwiththe lattice parameters a = 5.558
and c = 22.012 A. An XRD pattern is suggested for
CeAl,,O,, for the first time. The evolution of eutectic-like
microstructures was observed and reported in the Ce,O,rich side of this binary system.
Nd (1747°C) aluminates. Scott' predicted, by Rainan spectroscopy, the transition temperatures as 1047" and 1367°C for Pr
and Nd aluminates, respectively. Mizuno et al.' reported the
cerium monoaluminate to be cubic at T 2 150°C in inert atmogphere (JCPDS PDF 28-0260) with a lattice constant of 3.767 A.
They also claimed that the rhombohedral polymorph could be
synthesized at temperatures less than 100°C. More recently,
Kaufherr et a/.'"prepared CeAIO, in a flowing hydrogen atmosphere at 1000°C. The structure of the slowly cooled product was
found to be pseudo cukic, tetragonal with the lattic? parameters
a = 3.760 k 0.004 A and c = 3.787 k 0.004 A. The XRD
pattein of the tetragonal structure is not in the JCPDS files and
to our knowledge has never been published.
The second compound, cerium hexaaluminate (Ce,O,.
1lA120,), has been included in the first phase diagram for this
system suggested by Leonov et ~ l .and
, ~it has been reported to
melt incongruently in a reducing atmosphere at about 1950°C
and to be stable down to room temperature upon cooling. In the
more recent phase diagram suggested by Mizuno et ul.,' the
hexaaluminate field was indicated by dashed lines, due to the
difficulty in preparing it as a single phase; it was shown to melt
incongruently at 1890°C in a reducing atmosphere. It was
reported to bave a hexagonal st;ucture with the lattice constants
a = 5.543 A and c = 21.979 A and that the sample contained
CeAlO, and a-A1,0, as minor phases. This compound has been
labeled as Ce-P-A1,O3 in this phase diagram due mainly to the
compositional similarity with Na-p'-Al,O, (Na,O. 1 1A120,).
The synthesis and characterization of cerium hexaaluminate, in
its pure form, have been attempted, to our knowledge, only in
the above two studies in literature. In both of these, this conipound was assumed to have the p-alumina structure. A similar
type of compound (LaAl,,O,,) was reported"." to be present in
the La@-Al,O, system. It has been realized"-" that the addition of one divalent cation, such as Mg?', Mn2+,Co2+,or Ni' ' ,
to the above formula unit would produce a series of new compounds, LaMAIllOlY,
with structures quite similar to that of the
magnetoplumbite (PbFe,,O,,) phase shown to be present in
PbO-Fe,O, system. It so appeared that the divalent M ions
selected to be used as dopants should be chosen from among the
M oxides that were able to form spinel-type (MAl,O,) phases
with oc-alumina.L8According to this scheme,Is Al" substitutes
for Fe", La3'- (or Cei+) substitutes for Pb", and the divalent
cation replaces one Fe" to conserve the electrical neutrality.
The magnetoplumbite and Na-P'-alumina structures are both
composed of spinel blocks formed by the close packing of Al
and 0 atoms. The only significant difference between the two
structures is in the two mirror planes (5 = 0.25 and 0.75). Each
mirror plane in the p-alumina structure contains one large cation (Na') and one oxygen.".*" Figure 1 displays half of the unit
cells of the two structures drawn according to the atomic coordinates reported by Gasperin eta/." for LaMnAl, ,O,, and those
reported by Felschezo for NaAI, ,O,, . The non-close-packed
mirror planes join the stable spinel blocks. Moreover, the sites
I. Introduction
0
5001 1
of the two previously reported compounds of the
Ce,O,-AI,O, system, cerium monoaluminate (CeAIO,),
was first synthesized from the oxides by Zachariasen' at 1600°C
in a helium atmosphere. The crystal structure of the compound
was reported to be percvskite-like, tetragonal pseuGocubic with
a = 3.760 k 0.004 A and c = 3.787 t 0.004 A. Keith and
Roy' later confirmed the presence of this compound at 1:l
molar ratio in this system. They used Ce2(C,0,),.9H,0 and
AI,O, as the starting materials and observed the splitting in the
(1 1 1) reflection that would indicate primitive hexagonal symmetry, contrary to Zachariasen.' Roth3 observed the formation
of CeAIO, in the perovskite structure at 1600°C in abou! 1 h in
helium but with rhombohedral symmetry (a = 3.766 A, OL =
90.2"). Leo90v4 reported the synthesis of bright green, cubic
(u = 3.77 A) CeAIO, from 99.85% CeO, and hydrated aluminum oxide in inert or slightly reducing atmospheres at elevated
temperatures. It was also noted that CeAIO, would completely
decompose to CeO, and A1,0, in air at 800°C at 1 h. The binary
phase diagram given by Leonov et ~ 1for. the
~ Ce,O,-A1,0, system showed two polymorphic transformations. The first occurs
at 90°C (of unidentified nature) and the second was reported to
be a rhombic-to-cubic transition at 980" t 20°C. Kim6indexed
this compound, synthesized from Ce(C,O,),-xH,O and alumina
under vacuum at 1600"C, on the basis of a hexagonal cell of
rhombohedral symmetry LJCPDS PDF 21-0175) with the lattice
constants of CI = 5.35 A and c = 13.02 A. He reported the
X-ray density as 6.62 g/cm'. Geller and Raccah' estimated the
rhombohedral-to-cubic transition for CeAIO,, if there is any, to
occur at about 960°C by extrapolating the data they obtained for
the transition temperatures of the La (522"C), Pr (1370°C), and
NE
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R. S . Roth4ontrihuting editor
Manuscript No. 194x10. Received March 4, 1993; approved December 26, 1993.
Supported in part by the McKniglit Foundation. St. Paul, MN, Iowa State University, Ames, IA: and Middle East Technical University, Ankara, Turkey.
'Member, American Ceramic Society.
296 1
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Journal of the American Ceramic Society-Tas
3+,
Vol. 77, No. 11
and Akinc
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Mirror plane
4
b
( z = 0.75 )
02-
Spinel block
4
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Mirror plane
4
b
( z = 0.25 )
Na-beta-alumina (see Ref. 20)
Magnetoplumbite (see Ref. 17)
Fig. 1. Half unit cells of magnetoplumbiteand sodium p-alumina structures.
of the large cation and one oxygen in these planes are not fully
occupied, which leads to a disorder in the mirror planes caused
by a possible indeterminacy in the position of the large ion. The
mirror planes in the magnetoplumbite structure contain one
large cation (La3+or Ce'+) and three oxygens lying within the
plane and four aluminums at the intersection of the c-cell edge
with the mirror ulanes." The mirror planes in the magnetoplumbite-like structure were found," for LaMAl, ,O,-type
compounds, to be disordered; the single, fully occupied, ideal
site (2/3, 1/3, 1/4) for the large cation is split into two partially
occupied, close sites in the plane. This gave the large cation of
such structures a rather large volume in which it could move in
a diffuse manner. The packings of the spinel blocks are virtually
the same in both structures. The added divalent ions (M"), for
LaMAl, ,O,,-type compounds, were commonly found to localize themselves among the tetrahedrally coordinated A1 sites
within the spinel blocks17 of the structure rather than achieving
a statistical distribution over all the available A1 sites. Hence,
prior to this study, the previous literature has shown that the
lanthanide-hexaaluminates, excluding that of cerium, exhibited
structures similar to that of magnetoplumbite rather than that of
Na-P'-alumina. Moreover, there have been no attempts to suggest an XRD pattern for the pure compound Ce,O,. 11A1,0,,
although the JCPDS files did contain an entry for the divalent
cation-doped version, CeMgAl,,O,,, of the magnetoplumbite
type (JCPDS PDF 26-0872)."
The present study focuses on the structural characterization
of CeAIO, and CeAI,,O,,, using powder X-ray diffraction, and
the high-temperature phase equilibria in Ce,O,-AI,O, system in
reducing (Ar
10% H,), inert (Ar), and vacuum atmospheres.
+
11. Experimental Procedure
Compositions in the Ce,O,-AI,O, system were prepared by
mixing and stirring the appropriate amounts of the starting
oxides, CeO, (99.87%, 0.7 pm, Cerac, Milwaukee, WI) and
Al,O, (99.96%, 0.5 pm, Reynolds, Inc.) in ethanol in glass jars
for about an hour followed by 15 min of ultrasonification. The
contents of the jars were dried overnight at 70°C in air. The
recovered cakes were then calcined in air at 950°C for 12 h to
remove any remaining carbonaceous residues. The calcined
mixtures were then lightly ground in an agate mortar for about
30 min. The details of the green pellet preparation, equilibration
heatings (in reducing, inert and vacuum atmospheres), quench
practices, and DTA analyses employed were reported
elsewhere.22
After equilibration and quenching, each sample was ground
to a mean particle size of 7 km. Phase analysis was made by
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Table I. Results of Equilibration Runs in the System Ce,O,-AI,O,
Composition
CeA10,
CeAIO,
CeAIO,
CeAIO,
CeA10,
CeA10,
CeAIO,
CeAIO,
CeA10,
CeAIO,
CeAIO,
Ce,O,. 1 I A120,
Ce20,.l 1A1,0,
Ce,O,. 1 lAl,O,
Ce,O,. I 1Al,O,
Ce,O,. 1 1AI,O,
Ce,O,. 1 1A1,0,
Ce,O,.l 1A120,
12 moi% Ce,O,-88
12 mol% Ce,0,-88
74 mol% Ce,O,-26
79 mol% Ce2O,-2l
83 mol% Ce,03-17
Temp ("C)
1450
1550
1450
1550
1650
1750
1850
1950
1550
I550
1650
mol% A120,
mol% AI,O,
mol% AI,O,
mol% A1,0,
mol% A1,0,
1550
1550
1775
1880
1880
191s
1915
1700
1820
I875
1940
1940
Time (h)
50
40
50
40
20
10
3
1
50
50
25
30
30
12
2
2
0.1
0.1
12
8
4
3
3
Atmosphere
Cooling
Argon
Argon
Argon + 10%H,
Argon 1 O%H,
Vacuum
Vacuum
Vacuum
Vacuum
Argon + 10%H2
Argon
Vacuum
Argon
Argon + 1O%H,
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Vacuum
Quench
Quench
Quench
Quench
Furnace quench
Furnace quench
Furnace quench
Furnace quench
15"Clh to RT
15"C/h to RT
15"CIh to RT
Quench
Quench
Furnace quench
Furnace quench
1O"C/m to RT
Furnace quench
1OWm to RT
Furnace quench
Furnace quench
Furnace quench
Furnace quench
Furnace quench
+
Phases observed
Tetr. CeAIO,
Tetr. CeA10,
Tetr. CeAIO,
Tetr. CeAIO,
Tetr. CeAIO,
Tetr. CeAIO,
Tetr. CeAlO,
Tetr. CeAiO,
Tetr. CeAIO,
Tetr. CeAIO,
Tetr. CeAIO,
Ce,O,.l I AI,O, + A&O, + CeA10,
Ce,O,~llAI,O, + AI,O, CeAIO,
Ce,O,.llAI,O, + A1,0, + CeAIO,
Ce,O,.l 1AI20, +Al,O,
Ce,O,~llAI,O, + AI,O,
Liquid A1,0,
Ce,0,~11Al,03+ trace AI,O,
Ce,O,.l IAI,O, + CeAlO,
Liquid Ce,0,~11A1,03
CeA10, + Ce20, CeO,-,
CeAIO, Ce,O, CeO,_,
CeAIO, Ce,O, CeO,-,
+
+
+
+
+
+
+
+
November 1994
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P h a x Relations in the System Ce@-AI,O, in Inert and Reducing Atmospheres
XRD in the 5"-160" 2 range m k g a Cu-tube powder diffractometer (XDS 2000, Scintag, Santa Clara, CA) operated at 45 kV
and 30 mA with a scanning rate range of 0.125%.6" 2 /%n.
Least-squares cell refinement, indexing, and lattice constant
determinations were performed using the Appleman and
Evans" and T R E O R * ~routines. Elemental silicon (NBS 640a)
was used as an external standard.
Microstructural analyses were carried out by scanning electron microscopy (SEM, JSM-6100, JEOL, Peabody, MA) on
polished or as-is surfaces (in order not to damage the samples)
of the fired pellets. The chemical compositions of the microstructural features observed in SEM images of the polished
samples were examined with energy-dispersive X-ray spectroscopy (EDXS, Model Delta-5, Kevex, Foster City, CA) using
single-phase CeAIO, as a standard, and the information thus
obtained was believed to be accurate to within t3 at.%.
of samples of this stoichiometry consistently produced a singlephase (without any free Ce,O, and ct-Al,O, remaining) substance with a typical XRD pattern given in Table 11.' The pattern shown in this table was obtained from a sample heated at
1550°C for 50 h followed by slow cooling (15"C/h) to room
temperature (21"-23"C) in the furnace under a flow of argon +
10% H, gas. The structure has be$n determined to be tetragonal,
with lattice constants a = 3.763 A and c = 3.792 A. This phase
therefore is not the one covered by either of the previously published JCPDS files (21-0175, hexagonal and 23-0260, cubic) for
CeA10,. However, our lattice constants agree well with those
published by Zachariasen' and by Kaufherr rt al." The splitting
of the peaks and the proper observation of those were thought to
be the most important factors in the correct resolution of such a
structure. Figure 2 shows a portion of the peak analysis performed for the data of the above XRD pattern. For this structure
to be regarded as cubic, the splitting of the peaks displayed in
Figs. 2(A), (B), (D-F) should certainly be absent. Similarly, the
peak splitting observed in Figs. 2(A) and (D) and the 004:400
splitting shown in the pattern given in Table I1 should be absent
to index this structure on the basis of a hexagonal cell with
rhombohedral symmetry. The rhombohedral symmetry also
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111. Results and Discussion
Compositions of Ce,O,.AI,O, stoichiometry were heated
either in flowing purified argon (0 H,O 5 50 ppm), argon
10% H2, or under vacuum (Po>5 6 X
atm) at maximum
temperatures ranging from 1450" to 1950°C for periods in the
range of 1 to 50 h. The equilibration heat treatments carried out
in this study are tabulated in Table I. All the thermal treatments
+
23.2
23.4
+
23.6
2 theta
10
'For Tables I1 and 111, order ACSD-216 from Data Depository Scivicc, The American Ceramic Society, 735 Ceramic Place, Westetville, OH 4308 1.8720.
23.8
2 theto
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0
- , ' .
41 5
41.3
41.1
2 theta
41 7
9u
35 -
12.5
F
202
-
30'
B 25-
:
E 20-
15-
10-
5-
".
r
59 6
.
.
590
60
2 theto
602
604
70 1
703
70 5
2 theta
70 7
Fig. 2. Splitting of the XRD peaks in tetragonal CeAIO, (a = 3.763 8,c = 3.792 8).
70 9
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Journal of the American Ceramic Society-Tas
requires the tetragonal 111 reflection, as we observed in
Fig. 2(C), to split into 202 and 006 reflections of the hexagonal
system.
Our repeated attempts to observe the polymorphic transformation that was previously
to occur at 960" or 100°C
with differential thermal analysis have all failed. The DTA
traces (4"C/min) all registered the undisturbed background
from room temperature to 1950°C. Nevertheless, the enthalpy
of such a transformation, if there really was one, might be too
low to detect by DTA. We found that slow cooling (15"C/h) a
CeAIO, sample following the equilibration at 1550°C (Ar
10% H2) or 1650°C (vacuum) to 500°C (holding there for 24 h)
and then to room temperature followed by 1- to 2-h holding in
the furnace (21"-23°C) also did not produce any changes at all
in the suggested XRD pattem that would be indicative of a
polymorphic transformation. One question that still remains is
the possibility of the presence of an extremely sluggish transition that might occur below 100°C. On the other hand, it needs
to be noted that the purity of the starting materials might play a
much more important role, in the realm of high-temperature
phase equilibria and structural chemistry, than it was once
thought. For instance, it may well not be a coincidence for the
two different groups of researchers (Keith and Roy' and Kim')
to use the same starting material, i.e., cerous oxalate;
Ce(C,O,),~nH,O, in the preparation of CeAlO, and to produce
the rhombohedral "polymorph." Is the presence of carbon in the
structure causing this rhombohedral symmetry? It might still be
a possibility, which needs to be investigated, that the carbon
would not "bum out" until the temperature reaches 960"980°C. As another possibility, to our knowledge, LaA10, does
not have a reported "cubic" polymorph. In our laboratory we
+
and Akinc
Vol. 77, No. 11
also were not able to produce a tetragonal or cubic form of
LaA10,. Our quenched samples (froom above 155Q"C) of
LaAIO, always showed the (202:2.190 A)-(006:2.184 A) splitting that was a prerequisite for the rhombohedral symmetry to
set in and be identified. Therefore, considering the fact that the
significant presence of La,O, as an impurity in cerium oxides
was not so uncommon one or two decades ago, the rhombohedral LaAIO, which might form at very small quantities might
have acted as a seed which would then impose the CeAIO,
structure to acquire the rhombohedral symmetry of LaAlO, .
Research is presently under way in our labs to test this hypothesis by investigating the chemical synthesis conditions of
LaA10, and CeAlO, via aqueous precipitation.
Small portions of equilibrated, single-phase CeAlO, samples
were heated in Pt envelopes open on one surface in a flowing air
atmosphere at 250°C ( 5 days) and 1000°C (12 h) to test the stability of the compound. The XRD analysis showed that CeA10,
was still perfectly intact at 250°C; however, in the 1000°C sample, only traces of CeAlO, could be detected among the major
phases of CeO, and a-Al,O, . The same tetragonal XRD pattem
as that given in Table 11, observed in the sample heated in air at
250°C for 5 days, has excluded, again, for us the possibility of
identifying the previously reported,6 low-temperature, rhombohedral "polymorph."
If one considers the perovskite-like lattice of CeAIO, as
composed of A13+ions being at the octahedral interstices (cell
comers, CN = 6), 0'- ions at the edge centers, and finally the
large Ce'+ being at the body center (CN = 12). then it would be
noticed that the (1 11) planes of this structure will be the highest
atomic density regions because of the close packing of ceriums
and oxygens. The close-packed (1 1 1) faces of such a cell would
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Fig. 3. SEM micrographs of CeA10, samples: (A) 1S50°C, argon, 35 h, quenched (bar = I pm); (B) 1650"C, vacuum, 15 h, furnace-quenched
(bar = 1 pm); (C) 19SO"C, vacuum, 1 h, fumace-quenched (bar = 10 pm).
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November 1994
Phase Relations in the Svstem Ce,O,-Al,O, in Inert and Reducing Atmospheres
then be expected to display the hexagonal symmetry characterized by the presence of trigonal axes while being the most thermodynamically stable
especially during anomalous
grain growth. As was very recently discussed by DrofenikZ5for
the BaTiO, perovskite, predominant (1 11) faces displaying the
trigonal axes might be the surface habit during the grain-growth
stage of the densification process. We have observed such
microstructures in studying CeAIO,. Figure 3(A) shows the
SEM micrograph of the fracture surface of a CeA10, pellet
equilibrated in argon at 1550°C for 3.5 h, followed by water
quenching. The extensive grain growth and well-developed
(11 1 ) faces are clearly visible, especially in the upper righthand comer of this micrograph. This sample produced the typical "tetragonal" XRD pattern of Table 11. The micrograph of
Fig. 3(B) was taken from the fracture surface of a CeA10, sample equilibrated at 1650°C for 15 h under vacuum, followed by
furnace quenching (by shutting off the main power, 600"C/min
cooling from 1650" to IOOO'C, more sluggish exponential
decay-type cooling from 1000°C to room temperature) to retain
the high-temperature phases and features. A (1 11) face is visible at the center that is surrounded by microfacets and steps.
The sample still produces the characteristic tetragonal XRD
pattern indicating one more time that the real symmetry of a
crystal does not necessarily depend upon the symmetrical shape
and size of its apparent faces. Figure 3(C) shows the as-is surface of a CeA10, pellet heated at 1950°C for 1 h under vacuum
followed by furnace quenching to room temperature. In this
sample, it was almost impossible to find a region of the fracture
surface that shows the (1 11) faces. Apparently, recrystallization
2965
has occurred and the whole surface has been covered with
cuboids and tetragonal rhombs. The XRD pattern of this sample
still conforms to that given in Table 11.
We synthesized the binary compound of Ce,O,. 1 1A1,0,
from the starting oxides CeO, and A1,0, by heating the samples
either in argon, argon
10% H,, or under vacuum at temperatures above 1550°C. The resultant product was never pure; it
always contained some a-Al,O, as a minor phase whose quantity decreased significantly with increasing temperatures up to
1880°C. At lower temperatures, such as 1550"C, the samples
also contained small amounts of tetragonal CeAIO,. Cerium
hexaaluminate was found to be an incongruently melting compound as was predicted by Mizuno et al.' To determine the first
liquid formation temperature, a small piece of a preequilibrated
cerium hexaaluminate sample, with several sharp edges, was
hung by a molybdenum wire in the vacuum furnace that was
visible through a quartz viewing port. An IR pyrometer, calibrated just before this run against the melting point of PI, was
focused on the sample. The sample was heated at the rate of
3"C/min. The melting behavior of the chunk was continuously
monitored through the pyrometer, and at 1915" t 25°C the
sharp edges of the sample became rounded. The chunk, at this
point, began to flow and then fell as a drop onto a molybdenum
foil placed just below the wire. Following a 5-min hold at this
state, the sample was furnace-quenched to room temperature.
We assumed this temperature (i.e., the peritectic temperature)
as the incongruent melting temperature of this compound. This
reading agrees well with the temperature reported for the same
event by Mizuno eta1.9 The final sample of this experiment
+
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Fig. 4. SEM micrographs of samples of CeAI,,O,, composition: (A) and (B) backscattered electron images. 3"C/min to 1915"C,5 min, fumacequenched to RT. Darker trigonal a-alumina crystals surrounded by lighter, cerium-containing, once-liquid phase. (Bars = 100 km.) (C) 1550"C, 30 h,
quenched, ground, repelletized, 1S5O0C,30 h, quenched (bar = 10 Fm). (D) Same sample as in (C) (bar = 1 Fm).
2966
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Journal of the American Ceramic Society-Tas
showed, in its XRD analysis, a-AI,O, as the only crystalline
phase present. Figures 4(A) and (B) depict the surface morphology of this sample, showing darker trigonal alumina crystals
surrounded by a lighter, cerium-containing, once-liquid phase.
Repeating the same experiment, but this t.ime using a slow cooling from 1915°C to room temperature, yielded the most pure
hexaaluminate sample we obtained in this study. Traces of
ol-Al,O, still found in that sample would be indicative of
incomplete resorption.
We suggest an XRD pattern for cerium hexaaluminate,
CeAI, ,O,,, in Table IILt This pattern is obtained from a sample
first heated at 1550°C for 30 h, water-quenched, ground, repelletized, and then heated at 1550°C for another 30 h in a flowing
argon -t- 10%H, atmosphere, followed by quenching. The sample still contained some a-Al,O, that could easily be detected
from the pattern and very small amounts of CeA10, which
could be detected in the pattern only after a Rietveld analysis of
the diffraction data. The peaks of these minor phases are
excluded from the pattern given in the above table. Figures 4(C)
and (D) show the details of the fracture surfaces of this sample.
Platelike, large grains of CeAl,,O,, had always been the characteristic microstructural features of our hexaaluminate samples
in this study. The EDXS analyses carried out on the flat surfaces
of the grains of this sample (by using pure, single-phase
CeAIO, as a standard) gave us the mean weight percentages of
the three elements present as follows: Ce 19.0%, A1 40.8%,
and 0 40.2%. When compared with the theoretical percentages
(Ce 19.33%, A1 40.94%, 0 39.73%) that were calculated from
CeA1,,0,,, within the limits of error of EDXS analysis, these
may be considered as a good match.
and Akinc
Vol. 77, No. 11
The determination of the crystal structure among the two
likely candidates, magnetoplumbite and Na-p'-alumina, while
working with a powder sample rather than a single crystal of
cerium hexaaluminate, had been a difficult question to answer
in the scope of this study. The assessment of the degree of disorder and the extent of site splitting in the mirror planes of the
CeAI, ,O,, structure which might cause the possible, slight variations in the phase stoichiometry, as well as the exact determination of the structure type, need to await further powder
neutron or single-crystal X-ray diffraction experiments.
We have confirmed that the eutectic between CeAI, ,O,, and
CeAlO, occurred at 1785 k 15°C as was predicted by Mizuno
et al.' The equilibrium heat treatments for a sample of composition 12-mol% Ce20,-88-mol% A1,0, at 1700" and 1820°C produced a solid-state phase mixture of CeAI,,O,, and CeAIO,,
and a mixture of CeA1,,0,, and liquid, respectively, upon furnace quenching, as predicted by the available phase diagram.'
DTA runs for the same composition indicated a single endothermic event at 1785°C corresponding to the first liquid formation. The heating of one of these samples to 1820°C, soaking for
6 h at that temperature. and slowly cooling (5"C/min) to 1100°C
caused the posteutectic liquid matrix surrounding the elongated
CeAI,,O,, grains to crystallize. A sample micrograph of this
structure is given in Fig. 5(A). The XRD pattern of this sample
showed the presence of two crystalline phases, CeAIO, and
CeAI,,O,,.
We could not synthesize the R-compound that was reported
to form at 79 mol% Ce,O, and decompose at temperatures
below 1850°C by Mizuno et aLYOne probable reason for this
might be the inadequate quenching rates we had in the vacuum
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Fig.5. SEM micrographs of samples in the Ce,O,-Al,O, system: (A) 12 mol% Ce,O,. (Backscattered electron image) 1820"C, vacuum, 6 h,
S"C/min to 1 10O"C, furnace-quenched to RT. Eutectic matrix surrounding the elongated, dark a-alumina crystals. CeA10, is also present in the
sample. (B) 74 mol% Ce,O,. (Backscattered electron image) 1875"C, vacuum, 4 h, furnace-quenched. Dark CeAIO, grains, gray CeO, matrix.
(C) 79 mol% Ce,O,. 1940"C, vacuum, 3 h, furnace-quenched. Eutectic-like matrix of CeA10, and CeO,_, with large gray grains of CeO,-,.
(Bars = 10 Fm.)
I
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November 1994
Phuse Relation s in the System Ce,O,-A1,0, in Inert und Reducing Atmnsphcres
furnace that would not be rapid enough to prevent the decomposition of this compound. We were only able to furnace-quench
(i.e., the natural cooling of the furnace itself upon turning off
the power) our samples, although Mizuno et ~ 1reported
. ~ that
they were able to water-quench their samples from temperatures in excess of 1900°C. Heating a 74-mol% Ce,0,-26-mol%
A1,0, sample (preequilibrated at 1550°C for 24 h, followed by
water-quenching, grinding, and repelletizing) to 1875°C and
furnace-quenching produced a solid-state phase mixture of
CeO,_,] as shown in Fig. 5(B). The
CeAIO, and [Ce,O,
darker phase is CeA10, in this picture. The other two compositions we studied in the Ce,O,-rich region of this system
produced unique microstructures similar to that displayed in
Fig. 5(C). Upon heating the 79-mol% and 83-mol% Ce,O,
samples to 1940°C, followed by furnace-quenching, the liquid
was formed, and during cooling the submicrometer-spaced,
eutectic-like matrix evolved. At this point we choose to just
report the microstructures observed, even though they may not
represent the equilibrium conditions for this portion of the
binary. The lighter phase in Fig. 5(C)was the crystalline mixCe0,- ,], whereas the darker phase was CeAIO,.
ture [Ce,O,
Although our attempts to synthesize the orthrohombic
R-compound9 have failed, we still detected the traces of an
orthorhombic phase embedded in the backgrounds of the XRD
patterns of the above three samples. The extremely low intensities (1 to 3 out of 109) of that phasg, with the tentative lattice
constants a = 19.28 A, b = 12.35 A, and c = 6.37 A, did not
at this point warrant giving an accurate listing of the individual
reflections. It needs to be noted that the R-compound in the
Ce,O,-AI20, system reported by Mizyno et al.’ ha{ quite
differen! lattice constants: a = 9.538 A, b = 5.850 A, c =
15.205 A.
+
+
IV. Conclusions
The two binary compounds, CeAIO, and CeAI,,O,,, of the
system Ce,O,-Al,O, have been synthesized from the pure
oxides CeOz and A1,0,. It has been found that the conditions of
formation of these two compounds were not affected by the
atmospheres used in this study, namely, argon, argon + 10%
H,, and vacuum. CeAIO, was shown to be stable in a perovskite-lige tetragonal structure, with the lattice constants a =
3.763 A and c = 3.792 A, in the atmospheres used in this study
from room temperature to 1950°C. A new XRD pattern was
suggested for CeAIO,.
CeAI, ,O,, was synthesized from the oxides and was shown to
be an incongruently melting compound, confirming the findings of the previous researchers. It decomposed into a-Al,O,
and a liquid phase at 1915” It_ 25°C upon heating. An XRD pattern is suggested, for the first time, for the hexagonalocompound
CeA1,,Ol,, with the lattice constants a = 5.558 A and c =
22.012 A.
The eutectic reaction between CeAI, ,O,, and CeAlO, was
confirmed to occur at 1785” -C 15°C as reported by Mizuno et
a1.9 Although it was almost impossible for us to form the singlephase CeAI,,O,, in the solid state, we were able to form the
crystals of this compound, in the presence of a liquid phase, at a
composition of 12-mol% Ce,O, (without any unreacted
a-Al,O,), within a eutectic matrix of CeAl,,O,, and CeAIO,.
2967
Eutectic-like microstructures, with submicrometer-sized
lamellae, were observed, for the first time in any of the Ln,O,LnA10, (Ln: Lanthanide) systems studied, in the Ce,O,-rich
(74-83 mol%) side of the Ce,O,-AI,O, binary.
References
‘W. H. Zachariasen, “Crystal Chemical Studies of the 5f-Series of Elements.
XII. New Compounds Representing Known Structure Types,” Ai.tu Cry,~/u//o,~r.,
2,388-390 (1949).
’M. L. Keith and R. Roy, “Structural Relations among Double Oxide\ of Trivalent Elements,”Am. M i m w / . , 39, 1-23 (1954).
’R. S. Roth, “Classification of Perovskite and Other ARO,-Type Corrrpounds,” J Rrs. Nutl. Bur. Srurrd. (U.S.j,58, 75 (1957).
‘A. I. Leonov, “The Valence of Cerium in Synthetic and Natural Cerium Aluminates and Silicates, Part I , Compounds of the Perovskite Group,’’ /:I.. Akud.
NuukSSSR, Otd. Khini. Nuuk, 1, 8-13 (1963).
‘A. I. Leonov, A. V. Andreeva, V. E. Shvaiko-Shvaikovskii, and E. K. Keler,
“High-Temperature Chemistry of Cerium in the Systems Ce,O,-AI,O,, Cr20,.
Ga,O,,” liv. Akud. Nauk SSSR. Neorg. Muter., 2 , 5 17-23 (1966).
‘Y. S. Kim, “Crystallographic Study of Cerium Aluminate (CeAIO,),” Actu
Crystullogr., Sect. B , Strucr. Sci., 24, 295-96 (1968).
’S. Geller and P. M. Raccah, “Phase Transitions in Perovskitelike Compounds
of the Rare Earths,” Phys. Re>,.Bc Corzdens. Muter., 2, 1 167-72 ( I 970).
‘J. F. Scott, “Raman Study of Trigonal-Cubic Phase Transitions in Rare-Earth
Aluminates,” Ptiys. Rev., 183. 823-25 ( I 969).
’M. Mizuno, T. Yamada, and T. Noguchi, “Phase Diagram of the System
AI,O,-Ce,O, at High Temperature,” Yogyo Kyokuishi, 83,90-95 ( 1975).
“’N. Kaufherr, L. Mendelovici, and M. Steinberg, “The Preparation of Cerium(ll1) Aluminate at Lower Temperatures: IR, X-ray and Electron Spin Kwonance Study,” J . Less-Common Met., 107,281-89 (1985).
“M. Mizuno, T. Yamada, and T. Noguchi, “Phase Diagram of the System
AIzO,-La,O, at High Temperatures,” Yogp Kyokuishr, 82,630-36 ( 1974).
‘’R. C. Ropp and G. G. Libowitz, “The Nature of the Alumina-Rich Phase in
the System La,O,-AI20,,” J . Am. Cer-uni. Soc., 61,473-75 (1978).
”J. M. P. J. Verstegen, J. L. Sommerdijk, and J. G. Verriet, “Cerium and Terbium Luminescence in LaMgAl,,O,,,,”J . Lumin., 6,425-31 (1973).
“J. M. P. J. Verstegen, “A Survey of a Group of Phmphors, Based on Hexagonal Aluminate and Gallate Host Lattice\,” .I. E k r r o c hcni. SOC., 121, 1623-27
( I 974).
”A. Kahn, A. M. Lejus, M. Madsac, J. Thery, D. Vivien. and J. C. Bernier,
“Preparation, Structure, Optical, and Magnetic Propertie.. of Lanthanide Aluminate Single Crystals (LnMAI, ,O,,),” J . Appl. Phvs., 52, 686469 (198 I ).
IhF. Laville and A. M. Lejuc, “Crystal Growth and Characterisation of
LaMAl,,O,, Lanthanum Aluminates,” J . Cryst. Growth, 63. 426-28 (1981).
“M. Gasperin, M. C. Saine, A. Kahn, F. Laville, and A. M. Lejus, “Influence
of M2’ Ions Substitution on the Structure of Lanthanum Hexaaluminates with
Magnetoplumbite Structure,” J . Solid Srutr Chem., 54,6l-69 (1984).
“X. H. Wang, A. M. Lejus, D. Vivien, and R. Collongues, “Synthesis and
Characterisation of Lanthanum Aluminum Oxynitrides with Magnetoplumbitelike Structure,”Murer. Res. Bu//.,23, 4 3 4 9 (I988).
”C. A. Beevers and M. A. S. Ross, “The Crystal Structure of ‘Beta Alumina’
Na,O. I 1AI,O,,” Z. Kristuilogr., 97, 59-(~6( I 937).
’“J. Felsche, “The Alkali Problem in the Crystal Structure of Bcta Alumina,”
Z. K?-ista//ogr.,127,94100 (1968).
”Powder Diffraction File, Card No. 26-0872. International Centre for Diffraction Data, Newtowne Square. PA.
22A.C. Tas and M. Akinc. “Phase Relations in the System Ce,O
the Temperature Range 1150” to 1970°C in Reducing and Inert A
.I. Am. Ceruin. SOC.,in press.
”D. E. Appleman and H. T. Evans, “U.S. Geological Survey, Computer Coiltribution No. 20,” NTIS Report No. PB-216188. U.S. National Technical Information Service, Springfield, VA, 1973.
E. Werner, “Trial and Error Program for Indexing of llnknown Powder
Patterns, TREOR.” University of Stockholm, Stockholm, Sweden. 1984.
”M. Drofenik, “Origin of the Grain Growth Anomaly in Donor-Doped Bar0
ium Titanate,”/. Am. Cernni. Soc., 76, 123-28 (1993).
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