Aluminum doped zirconia nanopowders: Wet-chemical synthesis and structural analysis by Rietveld refinement

Article abstract of DOI:10.1016/j.materresbull.2007.10.015

Alumina/zirconia nanopowders, with up to 20 mol% Al₂O₃, were prepared by wet-chemical synthesis technique, using controlled hydrolysis of alkoxides. The as-synthesized powders are amorphous, have very high specific surface area and the corresponding particle size smaller than 4 nm. Amorphous powders with 0, 10 and 20 mol% Al₂O₃ crystallize at 460, 692 and 749 °C, respectively, as a single-phase tetragonal zirconia, without any traces of alumina phases. Rietvled refinement of X-ray diffraction data, used for the detailed structural analysis of annealed nanopowders, showed that the high-temperature zirconia phase is stabilized due to the formation of ZrO₂/Al₂O₃ solid solutions. High solubility of alumina in the tetragonal zirconia (up to 28.6 at% Al³⁺) and stabilization of tetragonal zirconia solid solution up to high temperature (as high as 1150 °C) were also confirmed.

Full text of DOI:10.1016/j.materresbull.2007.10.015

Materials Research Bulletin 43 (2008) 2727–2735
www.elsevier.com/locate/matresbu
Aluminum doped zirconia nanopowders: Wet-chemical
synthesis and structural analysis by Rietveld refinement
a,
b
b
ˇ
*
´
´
´
Vladimir V. Srdic , Srpan Rakic , Zeljka Cvejic
a Department of Materials Engineering, Faculty of Technology, University of Novi Sad, Serbia
b Institute for Physics, Faculty of Natural Sciences, University of Novi Sad, Serbia
Received 22 May 2007; received in revised form 17 September 2007; accepted 19 October 2007
Available online 30 October 2007
Abstract
Alumina/zirconia nanopowders, with up to 20 mol% Al2O3, were prepared by wet-chemical synthesis technique, using
controlled hydrolysis of alkoxides. The as-synthesized powders are amorphous, have very high specific surface area and the
corresponding particle size smaller than 4 nm. Amorphous powders with 0, 10 and 20 mol% Al2O3 crystallize at 460, 692 and
749 8C, respectively, as a single-phase tetragonal zirconia, without any traces of alumina phases. Rietvled refinement of X-ray
diffraction data, used for the detailed structural analysis of annealed nanopowders, showed that the high-temperature zirconia phase
is stabilized due to the formation of ZrO2/Al2O3 solid solutions. High solubility of alumina in the tetragonal zirconia (up to 28.6 at%
Al3+) and stabilization of tetragonal zirconia solid solution up to high temperature (as high as 1150 8C) were also confirmed.
# 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures; A. Oxides; B. Sol–gel chemistry; D. Crystal structure
1. Introduction
Zirconia (ZrO2) is widely used in various applications (as catalysts, electrolyte for solid oxide fuel cell, oxygen
sensor, thermal barriers, biomaterials, tough ceramics, etc.) because of its good mechanical properties, high oxygen
ionic conductivity and high chemical stability. Pure zirconia exists in three different crystal structures, i.e. monoclinic,
tetragonal and cubic. At room temperature, only the monoclinic form is stable. The high-temperature modifications of
zirconia (tetragonal and cubic) can be stabilized at room temperature with different dopants [1–4]. Cations used for the
stabilization typically have a lower charge state, a larger ionic size and a higher ionicity than Zr4+ [5]. The stabilization
with trivalent dopants is achieved by their incorporation in the fluorite-type structure by substituting Zr4+ cations and
creating oxygen vacancies to maintain local charge balance [1,3]. Oxygen vacancies associated with Zr4+ ions can
provide stability for tetragonal and cubic zirconia as strong covalent nature of the Zr–O bond favors seven-fold
coordination of Zr4+. It is suggested [5] that dopant ionic size determines the preferred location of oxygen vacancies.
Thus, oversized trivalent cations (such as Y3+, Gd3+ and Yb3+) tend to form eight-fold coordination with oxygen,
leaving oxygen vacancies to the Zr4+ ions. On the other hand, undersized trivalent cations (such as Fe3+ and Cr3+) are in
six-fold coordination and thus in competition with Zr4+ ions for oxygen vacancies. These are the reasons why the
* Corresponding author. Tel.: +381 21 450 3665; fax: +381 21 450 413.
´
E-mail address: srdicvv@uns.ns.ac.yu (V.V. Srdic).
0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.10.015

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stability of the high-temperature modifications of zirconia (tetragonal and cubic) with undersized dopants is much
lower than with oversized dopants.
Al3+ belongs to the group of undersized trivalent dopants, but is apparently too small (0.52 Å) to substitute
extensively for Zr4+ (0.84 Å), resulting in rather limited equilibrium solubility [6,7]. However, recent developments of
different nano-processing techniques in which precursors are mixed on the molecular level, have enabled the
formation of different structures with many zirconium and aluminum ions connected through metal–oxygen–metal
linkages [8–15]. Crystallization in these conditions allows the synthesis of phases with various forms of metastability,
including extended mutual solubility. Thus, different alumina/zirconia solid solutions were prepared by wet-chemical
(simultaneous hydrolysis of aluminum and zirconium alkoxide [8] or chloride [9], spray pyrolysis of aqueous solutions
of zirconium acetate and aluminum nitrate [12]) as well as gas phase synthesis methods (condensation of nanoparticles
formed in vapor phase by laser-evaporation of alumina and zirconia microparticles [13] or oxidation of aluminum and
zirconium alkoxide vapors [15]). In these samples stabilization of the high-temperature zirconia phases was obtained
and structures were characterized with a single-phase solid solution having stoichiometry Zr(1ꢀx)AlxO(2ꢀx/2) [12,13].
The aim of the presented work is to investigate effects of alumina on the characteristics of zirconia nanoparticles
prepared by wet-chemical synthesis, starting from alkoxides. Results will provide further insight into nature of the
metastable solid solutions formed in ZrO2/Al2O3 system.
2. Experimental
2.1. Sample preparation
Pure ZrO2 and ZrO2/Al2O3 nanopowders were prepared by wet-chemical synthesis technique, i.e. controlled
hydrolysis of alkoxides. Zirconium alkoxide (Zr(OC3H7)4, Fluka, Switzerland) and aluminum-alkoxide (Al(OC4H9)4,
Fluka, Switzerland) were mixed in appropriate ratio under inert atmosphere, dissolved in anhydrous ethanol and
hydrolyzed with distilled water under acidic condition (pH ꢁ 2) and at room temperature (alkoxide, water and
hydrochloric acid molar ratio was 1:4:0.8). Aqueous sols were obtained by continuous evaporation (at ꢁ80 8C) and
replacing the ethanol with distilled water without changing the concentration and pH. The hydroxides were
precipitated by slowly adding the aqueous sols to a well-stirred ammonium hydroxide solution (pH > 11). The
precipitated powders were washed with distilled water until there was no indication of residual Clꢀ (qualitatively
determined by adding a few drops of the wash effluent to a AgNO3 solution), and than three times with absolute
ethanol to remove free water and replace the particle surface hydroxyl with ethoxy groups. After washing, the gel
nanopowders were filtered, dried at 120 8C for 1 day and annealed in air at different temperature up to 1150 8C for 1 h,
and finally dry milled with zirconia balls (TOSOH).
2.2. Characterization
The specific surface area of the as-synthesized and annealed alumina/zirconia nanopowders was measured by low-
temperature nitrogen adsorption according to the BET method using a Quantachrom Autosorb-3B instrument. The
particle size was calculated assuming spherically shaped particles by dBET ¼ 6=ðr˙SvÞ, where r is the density and Sv the
specific surface area of the sample.
Thermal analysis of as-synthesized nanopowders was performed with Bähr STA 503 instrument in temperature
interval 20–800 8C and heating rate of 10 8C/min in stationary atmosphere.
The X-ray diffraction measurements were performed with a Siemens D5000 instrument using Ni-filtered Cu Ka
radiation, produced at 40 kVand 30 mA. The XRD data were recorded with a collection mode of 30 s/step and a step
size of 0.058 2u over the angular range from 20 to 1108 2u. Complete analysis of the X-ray data, i.e. refinement of
structural parameters (such as atomic coordinates, occupancies and lattice parameters) and microstructural parameters
(such as particle size and lattice strain) were obtained using Rietveld’s powder structure refinement analysis with
computer program ‘‘FullProf 2003’’ [16]. The program ‘‘FullProf 2003’’ is able to refine simultaneously both
structural and microstructural parameters through a last squares minimization technique. The background was defined
by a six-parameter polynomial and refined simultaneously with the zero-point and scale. Both, the instrumental and
sample intrinsic profiles were described by a convolution of Lorentzian and Gaussian components, and the TCH-
pseudo Voigt profile function was used. Parameters characterizing the instrumental resolution function were obtained

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Table 1
Notation and characteristics of as-synthesized nanopowders
Sample notation
Composition
Surface area (m2/g)
(at% Al3+
)
(at% Zr4+
)
0-A/Z
10-A/Z
20-A/Z
0
20
35
100
80
65
Amorphous
Amorphous
Amorphous
293.7
354.0
397.9
from a LaB6 standard powder sample by the appropriate Rietveld analysis and kept constant during refinements.
Refinements were undertaken in space group P21/c for monoclinic ZrO2 with all atoms in general positions, in space
group P42/nmc for tetragonal ZrO2 with Zr and O atoms in the special positions 2(a) and 4(d), respectively, and in
space group Fm3m for cubic ZrO2 with Zr and O atoms in the special positions 4(a) and 8(c), respectively. Refinements
were made with the assumption that some of Al atoms are on Zr sites in the tetragonal or cubic zirconia lattice and the
rest form a separate alumina phase (amorphous and/or crystalline transitional h, g or u-Al2O3). Occupation numbers
were calculated according to the assumption that in both tetragonal and cubic ZrO2, Al atoms randomly occupy the Zr
sites in lattice and charge balance is achieved by formation of vacancies on the O sites. Space group Fd3m is used for
the transitional h and g-Al2O3 phases and C2/m for the u-Al2O3. Broadening of the diffraction peaks was analyzed
through the refinement of the regular TCH-pseudo Voigt function parameters and the multipolar functions.
The alumina contents in the as-synthesized nanopowders were determined by energy dispersive X-ray analysis
(EDX), using a scanning electron microscope, SEM JEOL 6460LV and appropriate standards.
3. Results
The as-synthesized nanopowders were prepared in such a way that the pyrolysed product would contain 0, 20 and
35 at% Al3+ relative to Zr4+, which are close to the values of 0, 10 and 20 mol% Al2O3 used in this paper as the nominal
compositions (Table 1).
The as-synthesized powders are amorphous, have very high specific surface area (Table 1) and the corresponding
particle size, calculated from BET data, smaller than 4 nm. The increase of the specific surface area with the alumina
content from 293.7 m2/g for the powder 0-AZ to 397.9 m2/g for the powder 20-AZ (Table 1) indicates the size-
reducing effect of alumina on zirconia particles.
Fig. 1 shows DTA curves with a sharp exothermic peak resulting from the crystallization of a zirconia phase.
Position of the peak depends on alumina content and is at 460 , 692 and 749 8C for the 0-AZ, 10-AZ and 20-AZ
samples, respectively. The selected XRD patterns, shown in Figs. 2–4, confirm this observation. The pure zirconia
powder annealed at 500 8C for 1 h consists of a mixture of the monoclinic ZrO2 and the high-temperature zirconia
phase (the tetragonal or cubic zirconia). On the other side, in the ZrO2/Al2O3 nanopowders annealed at relative high
temperatures (ꢂ1000 8C) there are no traces of the monoclinic phase (i.e. the transition t ! m ZrO2 started even after
heating at 1150 8C in the 10-AZ nanopowder). In addition, XRD patterns of the annealed ZrO2/Al2O3 nanopowders
have only reflection belonging to the zirconia phase without any traces of crystalline alumina phases. This means that
alumina addition stabilizes the high-temperature zirconia phase, what is already mentioned in literature [8,10,12–15].
An another effect of alumina addition to zirconia is reduction of particle size (Fig. 5). Thus, in the powder 0-AZ
annealed at 850 8C the average particle size is higher than 50 nm, whereas in the powder 20-AZ annealed at the same
temperature the average particle size is only about 10 nm and reaches 25 nm after heating at 1000 8C.
4. Discussion
According to the presented results it is clear that addition of alumina to zirconia stabilizes the high-temperature
zirconia phase. This could be explained by the formation of ZrO2/Al2O3 solid solutions. However, due to the
considerable line broadening and overlapping of the corresponding X-ray reflections, it was not possible to determine
whether the high-temperature phase (denote with * in Figs. 3 and 4) has the tetragonal or cubic symmetry. In addition,
there was no any information about type of ZrO2/Al2O3 solid solution and portion of Al3+ ions which could be

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Fig. 1. DTA curves for alumina/zirconia nanopowders with 0, 10 and 20 mol% Al2O3.
Fig. 2. XRD patterns of pure zirconia nanopowders, 0-AZ, annealed at 400 and 500 8C (letters t and m denote tetragonal and monoclinic ZrO2
phases, respectively).
incorporated in the zirconia lattice. Because of that the structural analysis of X-ray diffraction data was done with the
Rietveld analysis. Refinements were performed with the pure monoclinic ZrO2 phase, the tetragonal or cubic ZrO2
structure having some of Al atoms on Zr sites and the corresponding quantity of the amorphous and/or transitional
alumina phase (formed by the rest of Al3+ ions). The obtained results are listed in detail in Table 2 (for the 10-AZ
nanopowders) and Table 3 (for the 20-AZ nanopowders), and one typical refined XRD pattern with weighted
difference plot is illustrated in Fig. 6.
For the 10-AZ nanopowders annealed at 700 8C the best fit was obtained when all Al atoms are on Zr sites of the
dominant zirconia phase (Table 2). However, even with the Rietveld analysis it was difficult to distinguish whether the
high-temperature zirconia phase has the tetragonal or cubic symmetry. It is well known that tetragonal zirconia is
formed if small amount of oversized trivalent dopants is added, whereas for the stabilization of cubic phase higher
concentration is necessary (>8 mol% M2O3) [2,3]. This boarder corresponds very closely to the structure having
almost every second Zr4+ ion in seven-fold coordination. On the other side, undersized trivalent dopants take oxygen
vacancies from Zr4+ ions and provide only half as many Zr4+ ions with seven-fold coordination, for the same dopant
content [5]. Thus, it is expected that for the stabilization of cubic zirconia phase M2O3 content should be above

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2731
Fig. 3. XRD patterns of alumina/zirconia nanopowders, 10-AZ, annealed at different temperatures (* indicates high-temperature ZrO2 phases and
letter t tetragonal ZrO2 phase).
Fig. 4. XRD patterns of alumina/zirconia nanopowders, 20-AZ, annealed at different temperatures (* indicates high-temperature ZrO2 phases and
letter t tetragonal ZrO2 phase).
15 mol%. This is the reason why it is believed that the 10-AZ nanopowder annealed at 700 8C, having only 10 mol %
of Al2O3, consists of tetragonal zirconia, with very low tetragonality, c/a = 1.0108.
Rietveld analysis for the 10-AZ nanopowders annealed at 850, 950 and 1050 8C showed that the best fits were
obtained with the tetragonal solid solution and almost all Al atoms on Zr sites. Changes of lattice parameters and unit
cell volume with annealing temperatures are presented in Figs. 7 and 8, respectively. The changes are very pronounced
at low annealing temperatures, whereas at 1050 8C lattice parameters and unit cell volume reach values characteristic
for the pure zirconia. On the other side, refinement of the XRD data showed that the 10-AZ sample annealed at
1150 8C has small portion of tetragonal phase (13.1 vol%) and decreased portion of Al atoms on zirconia sites. This
clearly indicates on simultaneous t ! m ZrO2 transition and segregation of alumina phases.
Rietveld refinement for the 20-AZ nanopowder annealed at 750 8C showed that the best fit was obtained with
tetragonal solid solution and when almost all aluminum ions (27.0 at% Al3+) are incorporated in zirconia lattice.
However, the difference in the refined data obtained with cubic and tetragonal phase is relatively small. The observed
high solubility of Al3+ in tetragonal lattice was not expected, since aluminum is trivalent undersized dopant, having
much smaller size than Zr4+. When Al3+ ions substitute Zr4+ (having eight-fold coordination) in zirconia lattice, they

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2732
Fig. 5. Particle size, estimated from XRD and BET, as a function of annealing temperature of 0-AZ, 10-AZ and 20-AZ nanopowders.
introduce oxygen vacancies and in same time tend to form six-fold coordination. The six-fold coordination for
aluminum can be achieved if Al3+ ions incorporate mostly in the surface region of nanoparticles. Cations that reside on
the surface (shell of a particle) are expected to have a lower coordination than those in the bulk (core of a particle). For
small particles (less that 10 nm) the surface area becomes significantly large, thus, high quantity of Al3+ ions can be
incorporated in surface region [17]. This could be the reason for the observed high solubility of Al3+ in zirconia lattice.
However, it is surprising that even of the observed high solubility the structure of the 20-AZ powder annealed at 750 8C
is characterized with tetragonal zirconia. This can be explained as follows. The six-fold coordination for Al3+ ions can
also be achieved if Al3+ ions take oxygen vacancies from Zr4+. In this case decreasing of Al-coordination number by
taking oxygen vacancies from Zr4+ will decrease the number of Zr4+ ions with seven-fold coordinated. Thus, it seems
that in the 20-AZ nanopowder annealed at 750 8C portion of Zr4+ ions with seven-fold coordination is relatively high to
Table 2
Structural parameters of annealed alumina/zirconia nanopowders, 10-AZ, determined from XRD data by Rietveld refinement
850 8C
950 8C
1050 8C
P42/nmc
1150 8C
Space group
Composition
P42/nmc
P42/nmc
P21/c
P42/nmc
13.08%
86.92%
Lattice parameters
a (Å)
b (Å)
c (Å)
b (8)
3.5936(19)
5.1407(20)
3.5927(22)
5.1747(20)
3.5943(21)
5.1902(18)
5.1561
5.1885
5.3193
98.76
3.6019(19)
5.1929(18)
Occupation parameters
N(Zr)
N(O)
N(Al)
0.418(3)
0.838(4)
0.082(3)
0.417(3)
0.841(4)
0.083(3)
0.425(3)
0.870(4)
0.075(3)
0.426(2)
0.053(2)
Thermal coefficients
U2b [Å2)
0.007(1)
0.018(3)
0.012(2)
0.008(2)
0.004(1)
0.022(3)
0.018(3)
0.025(4)
U4d [Å2)
Agreement factors
cRp%
cRwp%
RB%
x2
8.37
10.10
1.77
1.27
8.27
8.90
1.12
1.43
7.73
8.38
1.49
1.52
9.15
10.60
1.30
1.84

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Table 3
Structural parameters of annealed alumina/zirconia nanopowders, 20-AZ, determined from XRD data by Rietveld refinement
750 8C
850 8C
1000 8C
Space group
P42/nmc
P42/nmc
P42/nmc
Lattice parameters
a (Å)
c (Å)
3.5841(21)
5.1258(18)
3.5853(20)
5.1489(19)
3.5944(19)
5.1884(18)
Occupation parameters
N(Zr)
N(O)
N(Al)
0.365(3)
0.933(5)
0.135(3)
0.357
0.852
0.143
0.393
0.800
0.107
Thermal coefficients
U2b (Å2)
0.016(5)
0.069(3)
0.012(4)
0.046(3)
0.009(3)
0.019(3)
U4d (Å2)
Agreement factors
cRp%
cRwp%
RB%
x2
7.54
8.24
1.40
1.45
10.90
12.40
2.13
7.79
8.91
1.57
1.79
1.44
Fig. 6. XRD pattern of the 20-AZ nanopowder annealed at 850 8C refined by Rietveld analysis.
considerably decrease the tetragonality (c/a = 1.0110), but is still not enough to completely stabilize cubic zirconia
phase.
Rietveld analysis for the annealed 20-AZ nanopowder showed that solubility of alumina in tetragonal zirconia is
very high (ꢁ28.6 at% Al3+ at 850 8C) and decreases with temperature to 21.4 at% Al3+ at 1000 8C. The observed
decrease in solubility is accompanied with continuous increase of tetragonality and unit cell volume (Figs. 7 and 8),
but not with the appearance of transitional alumina phases. Thus, no any traces of alumina phases were observed even
in the 20-AZ powder annealed at 1000 8C. Segregation starts, most probably, in the surface region of small particles,
but the formed structures are too small to be detected by XRD.
The fraction of Al on Zr sites in the tetragonal zirconia lattice as a function of temperature and alumina content is
presented in Fig. 9. The fraction is equal to the doping level for the 10-AZ nanopowder annealed at temperature up to
950 8C. In the powder with higher alumina content only a fraction of Al3+ ions is remaining on the Zr sites, and the
fraction continuously decreases with temperature. These observations are in agreement with data already published
before [15] for CVS nanopowders. However, the main difference is in the solubility maximum, which is much higher
in the case of WCS nanopowders.

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2734
Fig. 7. Lattice parameters (cell constant a—solid lines; cell constant c—dash lines) as a function of annealing temperature for different alumina/
zirconia nanopowders.
Fig. 8. Unit cell volume as a function of annealing temperature for different alumina/zirconia nanopowders.
Fig. 9. Fraction of Al3+ ions on Zr sites in the tetragonal zirconia lattice, obtained by Rietveld refinement. Corresponding curve obtained for CVS
nanopowder [15] is presented with dash line.

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5. Conclusions
Alumina/zirconia nanopowders were prepared by controlled hydrolysis of alkoxides. The as-synthesized powders
are amorphous, have very high specific surface area (with corresponding particle size smaller than 4 nm) and
crystallize in tetragonal alumina/zirconia solid solution after heating at 700–750 8C. Rietvled refinement of X-ray
diffraction data enabled the detailed structural analysis of annealed nanopowders. In the nanopowders with 10 mol%
Al2O3, annealed at lower temperatures, the tetragonal solid solutions with almost all Al atoms on Zr sites were formed,
whereas at 1150 8C simultaneous t ! m ZrO2 transition and segregation of alumina phases were observed. The
nanopowder with 20 mol% Al2O3, annealed at 750 8C characterizes also the tetragonal solid solution, but relatively
high solubility of alumina in zirconia. However, the observed high solubility (28.6 at% Al3+) is still not enough to
completely stabilize cubic zirconia phase. Solubility of alumina in tetragonal zirconia decreases with temperature and
is accompanied with continuous increase of tetragonality and unit cell volume.
Acknowledgement
The research was supported by the Serbian Ministry of Science, Project ‘‘Synthesis of nanopowders and processing
of ceramics and nanocomposites for application in novel technologies’’, No. 142059.
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