Self-controlled growth in highly stable α-Al2O 3 nanoparticles in mesoporous structure

Article abstract of DOI:10.1002/pssa.200306729

Highly stable α-Al₂O₃ nanoparticles are obtained in a mesoporous structure (10-25% porosity) by reconstructive decomposition of a mesoporous AlO(OH) x H₂O, x ～ 0.7, powder followed by annealing at 1475-1900 K. At x ～ 0.7, self-controlled AlO(OH) x H₂O → A1₂O₃ molecular decomposition occurs in a controlled desorption of H₂O through pores at 330-650 K. It was achieved by a novel hydrolysis method of Al metal with nascent surfaces in water at 295 K. Average α-Al₂O₃ crystallite size hardly grows to 30-50 nm (several hundred nanometers otherwise in a bulk sample) at 1475-1900 K. They are arranged through pores in a specific fashion as observed in TEM micrographs. A network structure forms of surface atoms in high-energy crystallites in a high configurational entropy (governs improved stability and superplasticity or other properties) by minimizing the Gibbs free energy. A metastable α-Al₂O₃ phase exists in processing at low temperature such as 1475 K. It has a modified X-ray diffraction or IR spectrum of equilibrium phase after annealing (reorders interstitial vacancies) at 1525 K or higher temperatures. It is proposed that a mobile Al³⁺ hole in a site neighboring an AlO_6-δ architect defect creates a center to a plane slipping or twin structure formation. Mesopores provide sites for crack initiation and also for crack arrest in confined cracks (support a high failure strain).

Full text of DOI:10.1002/pssa.200306729

phys. stat. sol. (a) 201, No. 3, 427–444 (2004) / DOI 10.1002/pssa.200306729
Self-controlled growth in highly stable a-Al O nanoparticles
2
3
in mesoporous structure
*
S. Rana and S. Ram
Materials Science Centre, Indian Institute of Technology, Kharagpur – 721 302, India
Received 25 November 2003, revised 23 October 2003, accepted 27 October 2003
Published online 22 January 2004
PACS 61.43.Gt, 61.46.+w, 82.60.Qr
Highly stable α-Al O nanoparticles are obtained in a mesoporous structure (10–25% porosity) by recon-
2
3
structive decomposition of a mesoporous AlO(OH) x H O, x ~ 0.7, powder followed by annealing at
2
1
475–1900 K. At x ~ 0.7, self-controlled AlO(OH) x H O → Al O molecular decomposition occurs in a
2 2 3
controlled desorption of H O through pores at 330–650 K. It was achieved by a novel hydrolysis method
2
of Al metal with nascent surfaces in water at 295 K. Average α-Al O crystallite size hardly grows to
2
3
30–50 nm (several hundred nanometers otherwise in a bulk sample) at 1475–1900 K. They are arranged
through pores in a specific fashion as observed in TEM micrographs. A network structure forms of sur-
face atoms in high-energy crystallites in a high configurational entropy (governs improved stability and
superplasticity or other properties) by minimizing the Gibbs free energy. A metastable α-Al O phase ex-
2
3
ists in processing at low temperature such as 1475 K. It has a modified X-ray diffraction or IR spectrum
of equilibrium phase after annealing (reorders interstitial vacancies) at 1525 K or higher temperatures. It
3+
is proposed that a mobile Al hole in a site neighboring an AlO6 – δ architect defect creates a center to a
plane slipping or twin structure formation. Mesopores provide sites for crack initiation and also for crack
arrest in confined cracks (support a high failure strain).
©
2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Mesoporous materials, which comprise nanoparticles distributed through pores of 2–50 nm diameter in a
specific structure, have been the subject of considerable interest owing to their potential applications in
catalysts, hot filters, adsorption media, phosphors, and microelectronics [1–7]. Thermal stability with a
controlled microstructure at this scale is one of the most important parameters of the material of
reproducible properties. It determines the activity and performance of the material in a specific device as
per its chemistry and other properties. A large surface area, pore structure, and surface chemistry play an
essential role especially in adsorption and catalysis.
In this series, alumina (Al O ) is a paradigm for fabricating mesoporous or classical ceramics [2–4].
2
3
Doping with a foreign metal oxide refines a stable microstructure and improves or adds other useful
3
+
3+
properties. Zhang et al. [4] stabilized a mesoporous γ-Al O with 1–5 mol% Ce or La . A composite
2
3
mesoporous structure allows Al O molecules access to large internal surfaces and cavities that enhance
2
3
their catalytic and adsorption activities. Nanocrystals of α-Al O are widely used as adsorbents, catalysts
2
3
or catalyst carriers, gas sensors, coating and soft abrasives because of fine size, high surface area, and
catalytic activity of their surfaces. These remarkable properties have a great deal of interest in under-
standing their origin and mechanisms in nanostructures [8–15].
*
Corresponding author: e-mail: sram@matsc.iitkgp.ernet.in
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

428
S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
2
2
γ-Al O has a lower surface energy, σ = 0.79 J/m , than σ = 2.04 J/m in α-Al O and becomes ener-
2
3
2
3
2
getically stable at surface area A ≥ 125 m /g, i.e. d ≤ 13 nm diameter in a spherical crystallite, and ther-
modynamically stable at even smaller A ~ 75 m /g at 800 K [11]. γ-Al O can maintain A ~ 150 m /g at
073 K while α-Al O coarsens at A ≤ 50 m /g at such temperatures. θ-Al O , supposed to have an in-
2 3 2 3
2
2
2
3
2
1
termediate σ value, appears at intermediate A in coarse to γ → α transformation [14]. Molecular dynam-
ics simulations [16], assuming preferential exposure of the surfaces with lowest energy, predicts γ-Al O
2
3
2
3+
as the energetically stable phase as A exceeds 125 m /g. Distribution of Al and vacancies in tetrahedral
and octahedral sites in γ-Al O [17] has a larger entropy S than in α-Al O which has only octahedral
2
3
2
3
sites [17, 18]. The γ → α transition has as large change in S as 5.7 J/K mol [11], demonstrating γ-Al O
to be the probable phase to exist in small particles, A ≥ 100 m /g, at room temperature.
2
3
2
In this paper, we report the synthesis of monolithic α-Al O in a self-confined 30–50 nm dimension
2
3
through pores in a mesoporous structure. Firmly stable particles form at a temperature as low as 1475 K and
do not grow much on raising the temperature as high as 1900 K. Pores support a metastable α-Al O struc-
2
3
ture. The equilibrium α-Al O structure occurs by a local atomic reordering at 1525 K or higher tempera-
2
3
ture. The results are analyzed and modeled with X-ray diffraction, microstructure, and vibrational spectrum.
A twin boundary structure has been seen earlier in high-resolution electron microscopic (HREM) images in
α-Al O [18–20]. It influences grain growth, thermal stability, and other macroscopic properties. Small
2
3
levels of porosity Φ (5–10%) offer superplasticity [10]. Uniformly distributed pores provide sites for both
crack initiation and crack arrest, leading to a large number of short cracks, and to higher failure strains.
2
Experiment details
A mesoporous α-Al O structure is derived from a mesoporous AlO(OH) x H O, x ~ 0.7, precursor with
2
3
2
Φ = 18% and d = 30 nm average crystallite size. As described earlier [15], it has been synthesized by a
novel hydrolysis method of a high-purity (99.9%) Al metal (in the shape of a thin plate) with nascent
surface in distilled water at 295 K. A self-induced exothermic hydrolysis reaction, Al + 2H O →
2
3
AlO(OH) + 2 H , occurs as soon as the Al metal is immersed in water. The resulting AlO(OH) instanta-
2
neously reacts with H O and converts to AlO(OH) x H O. This is filtered and dried to a powder at room
2
2
temperature in air. In sol-gel or other wet methods, a metastable AlO(OH) x H O precursor appears in a
2
gel with a larger x ≥ 1.0 [21, 22]. A refined mesoporous Al O powder (Φ = 45%) appears on dehydra-
2
3
tion and reconstructive thermal decomposition at 500–600 K. It transforms to a mesoporous α-Al O at
2
3
temperature as low as 1475 K (Table 1).
The phase analysis was carried out with X-ray diffraction of annealed samples at elevated tempera-
tures, using a P.W. 1710 X-ray diffractometer with filtered Co K radiation of λ = 0.17902 nm wave-
α
length. Their microstructures were studied with a transmission electron microscope (TEM), model-JEM
2000 CX, in conjunction with an energy-dispersive X-ray spectrum analyzer for in-situ elemental analy-
Table 1 Density, porosity (%), lattice volume (V), lattice surface area (A), and specific particle surface
area (S ) in AlO(OH) x H O, θ-Al O and α-Al O powders.
g
2
2
3
2
3
a
b
heat treatment
trade name
density
(
porosity
(%)
V
(nm )
A
Sg
3
3
3
2
2
g/cm )
(10 m /g)
(m /g)
As-received
Boehmite
1.85 (2.25)
2.50 (3.58)
2.98 (3.98)
3.20 (3.98)
3.25 (3.98)
3.60 (3.98)
18
30
25
20
18
10
0.429
0.186
0.770
0.767
0.755
0.763
3.70
3.38
4.84
4.82
4.77
4.80
108.1
200.0
57.5
48.1
45.0
33.3
1
1
1
1
1
275 K, 2 h
475 K, 2 h
525 K, 2 h
700 K, 2 h
900 K, 2 h
θ-Al O
2
3
α-Al O
2
3
3
3
3
α-Al O
2
α-Al O
2
α-Al O
2
a
The density calculated from V is given in parentheses.
b
The S value refers to spherical shape of particles of average size d as determined by the ∆2θ value.
g
1/2
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
429
sis. No impurity has been found in a detectable 0.1 at.% or still lower trace. Average d-value has been
calculated from the widths ∆2θ1/2 of the X-ray diffraction peaks with the Debye–Scherrer relation.
Mesoporous structure of the sample was studied by Φ value, obtained from the difference in observed
and theoretical values of its density and by the standard N gas sorption method at low temperature such
2
3
as 77 K. The density was measured, within an error of ± 0.01 g/cm , by displacement of N gas by Ar-
2
chimedes principle using a Penta Pyknometer (from Quanta Chrome, USA).
3
Results and discussion
3
.1 Molecular growth in mesoporous Al O structure
2 3
The hydrolysis of Al atoms by the reaction with H O molecules at the nascent Al plate surface (im-
2
–
mersed in water) occurs in a controlled manner by adding the reaction species of OH ions in successive
steps. H O molecules that come into contact with the Al surface instantaneously dissociate into H and
+
2
–
OH ions to conduct the hydrolysis as in Fig. 1.
2
+
+
The intermediate [AlO(OH)] or [AlO(OH) ] reaction species in step (1a) or (1b) readily proceeds to
2
the AlO(OH) molecule by neutralizing a total of zero electronic charge. It occurs by just releasing a
+
proton H with a minor redistribution of the internal energy. The energy, being a local property, readily
redistributes by dissipation of the excess heat through the surroundings.
Two primary driving forces, (i) the initial chemical potential µ between the reaction species and (ii) the
e
flow of the reaction species, drive the nascent AlO(OH) molecules to grow in a common direction perpen-
dicular to the reaction surface. A unidirectional polymeric structure results in molecules interconnecting
one another in successive steps as long as it is not disrupted by local perturbations. This is satisfied in a
controlled reaction at room temperature. A model structure of this is shown in Fig. 2a. As the temperature
rises above room temperature, the interstitial H O molecules locally desorb and the structure redistributes in
2
a modified Al O x H O structure with 2AlO(OH) → H O + Al O molecular decomposition (Fig. 2b).
2
3
2
2
2
3
From the TEM micrograph (Fig. 3a), a non-disrupted AlO(OH) x H O growth appears to last over
2
several molecules. It results in particles with D ~ 35 nm diameter. The particles distribute through pores
(
with 2 to 5 nm distance between particles) which develop when the reaction is disrupted by the local
temperature. Their elongated ellipsoidal shapes, with φ ~ 1.8 aspect ratio, are in accord with their growth
in a directional Al hydrolysis. The corresponding electron diffraction pattern (Fig. 3b) has four rings at
interplanar spacing dhkl of 0.492, 0.320, 0.237 and 0.190 nm in comparison to the 0.4850, 0.3198, 0.2358
and 0.1872 nm values in (002) , (112) , (213) and (314) reflections in X-ray diffraction (Fig. 4a) with
5
lattice parameters a = 0.866 nm, b = 0.506 nm, c = 0.983 nm and β = 94°34′ in C2h monoclinic structure
2n+
OH OH OH
1
2
2
nAl + nH O
+
nH
2
(1a)
(1b)
Al
Al
nH
Al
n
2
O
n+
OH OH OH OHOH OH
Al Al Al
1
+ 2
nH
2
n
OH O OH O OH O
Al Al Al
1
2
(
1c)
+
nH
2
n
Fig. 1 A schematic representation of surface hydrolysis of Al metal with nascent surface in
AlO(OH) x H O in water. The initial chemical potential µ lies perpendicular to the surface.
2
e
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

430
S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
Fig. 2 a) A model [AlO(OH) x H O] , n = 8, metalloceramic polymeric structure in association of mole-
2
n
cules one after another in a) directional Al hydrolysis. b) A polymeric structure retained in a controlled
Al O x H O phase transformation. It has a lot (x ~ 1.4) of H O molecules (shown by the solid circles)
2
3
2
2
with hydrogen bonding in a microporous structure.
[
23]. An average d = 30 nm, estimated from ∆2θ1/2 in the peaks, compares with the size of particles in
TEM, characterizing them to be single crystallites.
The model [AlO(OH) x H O] , n = 8, structure (Fig. 2a) has plenty of H O molecules, since x ~ 1.4.
2
n
2
5
The real value of n is still larger, of the order of 10 in D ~ 35 nm structure, as can be determined by
n = V/v, with V = π [ D] (φ) its average volume and v = 5.4 × 10 nm the AlO(OH) x H O mo-
4
3
1
2
3
−2
–2
3
2
lecular volume. A similar structure can be designed by a local molecular decomposition of it to Al O . In
2
3
3+
3+
Fig. 2b, it is shown with a change in Al coordination from three to four. The Al cations in AlO sites,
4
as in liquid Al O [17], involve an excess oxygen (ζ) in average Al O · x H O composition. At
2
3
2
3+ζ
2
n → ∞, ζ ~ 0.75 in the absence of oxygen vacancies. On heating, the H O molecules are liberated,
2
mostly over 500–600 K, in a porous structure. A shown in Fig. 4b, a high-energy amorphous state exists
before it transforms to polymorphic Al O . It is obtained at 600 K with an energized AlO(OH) x H O
2
3
2
with Φ as much as 30% by Al hydrolysis in H O vapour at ~310 K.
2
Part of the H O or oxygen in porous Al O x H O in Fig. 2b is slowly liberated at temperature suffi-
2
2
3
2
3+
2–
cient enough to reorder Al and O in Al O polymorphs. It occurs as early as at 800 K by forming fine
2
3
pores between growing structures in stable particles. Those H O confined in between particles of a few
2
Fig. 3 a) TEM micrograph and b) electron diffractogram in mesoporous AlO(OH) x H O.
2
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phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
431
5
2h
Fig. 4 X-ray diffraction peaks in mesoporous AlO(OH) x H O in C monoclinic crystal structure
2
(
curve a) and amorphous Al O x H O obtained from it at 600 K for 2 h (curve b). Curve (c) is a close-up
2 3 2
–1
of curve (b) indicating three halos at 21.0, 29.1, and 43.6 nm wavevectors.
nanometers become immobile and hinder the particles from recombining and growing any more. In this
case, they merely extend in tightly curved layers by enclosing the pores so as to minimize the total sur-
face and volume Gibbs free energy. This is consistent with TEM micrograph (Fig. 5a) in α-Al O ob-
2
3
tained by heating the precursor at 1475 K for 2 h. Thin platelet particles of 15–30 nm diameter appear
with pores in acicular shapes of 10–15 nm lengths with φ ~ 2. This is in a refined microstructure in the
precursor in Fig. 3a. The corresponding electron diffraction pattern (Fig. 5b) has four rings at 0.258,
0.225, 0.160, or 0.128 nm in (104), (006), (116), or (208) reflections.
Yang et al. [2] developed a similar large-pore (up to 14 nm) mesoporous Al O using a metalloce-
2
3
ramic gel with poly(alkylene oxide) block copolymer as a structure-directing agent in a solution with
3
+
AlCl . In AlCl –polymer solution in ethanol, Al cations hydrolyze and polymerize into a network in a
3
3
transparent gel at 313 K. The organic component burns out in a mesoporous Al O powder if heating the
2
3
Fig. 5 a) TEM micrograph and b) electron diffractogram in mesoporous α-Al O (metastable) powder after anneal-
2
3
ing at 1475 K for 2 h. Two typical diffraction spots in a twin structure are marked by the lines.
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
Fig. 6 A model mesoporous structure with hexagons of AlO structural units in Al O .
4
2
3
gel (dried) at 673 K. It consists of nanocrystallites within relatively thick amorphous walls of 3.5 nm
thickness. An altogether different structure with amorphous pore walls appears in silica and derivatives
through a surfactant templating [1, 2]. This approach applies with sporadic success to-non-silica oxides.
As compared to both of them, our method with Al metal hydrolysis seems to be rather simple, economic,
and versatile for a large-scale synthesis of mesoporous Al O and composites. It involves no other
2
3
chemical to promote the hydrolysis carried out in an aqueous solution with common metal salts.
A schematic model of the kind of structure we envisage in mesoporous Al O in this example is
2
3
shown in Fig. 6. It consists of discrete fragments rather than a single continuous layer as per the TEM
images. Townsend et al. [24] modeled a continuous layer structure in mesoporous carbon in terms of
carbon rings. Harris et al. [7] extended this model with pentagons and hexagons of carbon rings, which
more closely ascribe the observed curved surfaces in discrete fragments in mesoporous carbon deduced
by carbonization of sucrose. In our model, curved and closed structures in Al O result from molecular
2
3
association of Al O in AlO structural units. A molecular structure of interconnected network is retained
2
3
4
on the grain surface on recrystallizing it into Al O polymorphs.
2
3
3
.2 Controlled phase transformation in mesoporous Al O structure
2 3
Mesoporous α-Al O has a significant variation in X-ray diffraction as processed at different tempera-
2
3
tures in the 1475–1900 K range. Figure 7 compares diffractograms in the samples processed at (a)
475 K and (b) 1525 K for 2 h. As assigned in Table 2, diffractogram (a) involves a total of 17 peaks
peak intensity I ≥ 3 units) in addition to those in the corundum structure [25] in diffractogram (b) or
1
(
p
Table 3. They are relatively broad, according to d = 28 nm, at 0.4503, 0.3075, 0.2840, 0.2720, 0.2370,
.2310, 0.2250, 0.2165, 0.2025, 0.1961, 0.1910, 0.1735, 0.1490, 0.1455, 0.1440, 0.1370 and 0.1285 nm.
A similar series of peaks (Fig. 7c) lie in A2/M monoclinic θ-Al O structure (processed at 1275 K in 2 h)
0
2
3
of lattice parameters a = 0.5650 nm, b = 0.2900 nm, c = 1.1720 nm, and β = 104.5° against the reported
a = 0.570 nm, b = 0.290 nm, c = 1.180 nm, and β = 104.5° values [25]. It has 12 major peaks of 0.2839,
0
.2721, 0.2553, 0.2437, 0.2309, 0.2249, 0.2013, 0.1916, 0.1540, 0.1487, 01453, and 0.1388 nm in
d〉 = 12 nm. The peak at 0.1388 nm, instead of the 0.2720 nm in the above series, is the most intense
peak of the diffractogram.
〈
3
Adversely, the present peak series has a larger V = 0.190 nm lattice volume in 〈d〉 = 28 nm over
0
3
V = 0.186 nm in the θ-Al O in 〈d〉 = 12 nm, with a = 0.5720 nm, b = 0.2920 nm, c = 1.1750 nm, and
0
2
3
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
433
Fig. 7 X-ray diffraction peaks in nanoparticles in metastable α-Al O (curve a), usual α-Al O (curve b),
2
3
2
3
and θ-Al O (curve c) as annealed at 1475 K, 1525 K and 1275 K for 2 h, respectively.
2
3
β = 104.5°. V usually expands in the size effects in nanoparticles below a critical d = d . This is demon-
0
c
strated with a high-resolution X-ray diffractogram in (113) peak in the true corundum structure in Fig. 8.
The peak regularly shifts to smaller dhkl as d grows from 35 nm in (a) to 41 nm in (c). An increase in d
hkl
2
–
is noticed in d above 41 nm as in (e) in sample processed at 1900 K for 2 h. Possibly, a local O redistri-
bution occurs in regular AlO sites if not achieved before. It is known that α-Al O forms via θ-Al O as
6
2
3
2
3
3
+
a precursor state. Al in θ-Al O lies in AlO and AlO sites in a spinel structure with cation vacancies
2
3
4
6
mostly in AlO sites [13]. Part of these vacancies are retained in oxygen-deficient AlO6–δ, δ ≤ 2, sites on
6
the phase transformation in α-Al O at early temperatures.
2
3
A close-up of the X-ray diffraction reveals a trace of these peaks to be present on annealing at tem-
peratures as high as 1900 K. For example, in Fig. 9, seven additional peaks lie in modified positions and
intensities at 0.2810, 0.2293, 0.1957, 0.1919, 0.1765, 0.1335, and 0.1273 nm after 2 h of annealing at
1700 K. Redistribution of intensities in the peaks confirms reordering of atoms in the lattice. The most
intense reflection thus appears in (104) peak on annealing at 1475 K (Fig. 7a) while in (113) peak at
1
700 K (Fig. 9). They have the same intensity if annealing at 1525 K (Fig. 7b).
The obtained structure, which is stabilized with immobile pores, behaves as highly rigid and is non-
reactive to its further growth at these temperatures. The pores maintain a strict control of grain growth and
thus allow it in true nanocrystallites. Otherwise, at such high temperature, it rapidly grows to several hun-
dred nanometers [11, 12, 26]. This is not feasible with a non-porous precursor in which the molecules ad-
here to one another by strong hydrogen bonding. They do not dehydrate in mesoporous structure. A seeding
by 0.2–2 mol% additive of CuO, NiO, or MgO [10, 12] has been used to control submicrometer α-Al O
2
3
3+
3+
size. Spray pyrolysis with Al chloride or nitrate [27] and glycothermal treatment of Al hydroxide [28]
give a reasonably controlled α-Al O size. Nevertheless, it does not come down below 100 nm in any case.
2
3
Modified sol–gel and other new methods are being developed in order to control α-Al O size in small
2
3
crystallites [29–31]. Lin et al. [30] used an emulsion of Al(OH) gel in oleic acid to obtain α-Al O by
3
2
3
firing it at temperature as high as 1373 K. Carbon, which is liberated by the decomposition of oleic acid
during the thermal process, prevents the growth of agglomerates in a sequence of phase formation of the
©
2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

434
S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
Table 2 Interplanar spacing (d ) and relative intensities (I) in characteristic peaks in X-ray powder dif-
hkl
fraction in metastable phase in α-Al O nanocrystals.
2
3
dhkl (nm)
I
h
k
l
observed
calculated
a
0.4503
0.3476
0.3075
0.2840
0.2720
0.2550
0.2435
0.2370
0.2310
0.2250
0.2165
0.2085
0.2025
0.1961
0.1910
0.1785
0.1735
0.1601
0.1545
0.1511
0.1490
0.1455
0.1440
0.1405
0.1385
0.1370
0.1285
0.1240
0.1234
0.1190
0.4353
0.3488
0.3265
0.2995
0.2612
0.2560
0.2383
0.2344
0.2238
0.2177
0.2063
0.2090
0.2038
0.1967
0.1925
0.1744
0.1700
0.1607
0.1549
0.1518
0.1517
0.1497
0.1468
0.1407
0.1376
0.1339
0.1280
0.1245
0.1239
0.1191
15
69
5
45
58
100
54
43
38
25
12
90
35
4
22
9
40
65
16
9
10
17
6
43
45
43
7
0
0
0
1
0
1
1
1
1
0
2
1
2
2
1
0
1
1
2
0
1
2
1
1
0
1
2
1
1
2
0
1
0
0
0
0
1
1
1
0
0
1
0
0
0
2
0
1
1
1
2
0
2
2
3
2
0
0
1
2
3
2
4
3
5
4
0
1
2
6
0
3
1
2
6
4
7
6
1
8
2
6
3
4
0
5
8
b
a
a
c
c
a
a
b
a
a
c
b
c
b
a
a
a
b
c
b
b
b
10
6
3
10
9
0
The calculated dhkl are from the average lattice parameters a = 0.4765 nm and c = 1.3060 nm. The sam-
a
ple has been annealed at 1475 K for 2 h. The peaks that match with other phases are marked as θ-Al O ,
2
3
b
c
α-Al O , and θ-Al O /α-Al O .
2
3
2
3
2
3
emulsion → γ-Al O → δ-Al O → θ-Al O → α-Al O . Average d-value has been controlled to be
2
3
2
3
2
3
2
3
6
0 nm. No δ-Al O phase appears in processing by a mesoporous boehmite precursor in our work. It is
2 3
more effective in controlling grain growth in extremely small Al O crystallites of size under 50 nm at
2
3
temperatures as high as 1900 K.
3
.3 Critical dimension of thermodynamic stability in θ-Al O particles
2
3
The metastable θ-Al O forms and exists in self-confined dimension, d ≤ R , as per its total surface and
2
3
c
volume Gibbs free energies. To determine the critical value R of its thermodynamic stability, we studied
c
θ-Al O growth by heating at selected temperatures from 1100 to 1400 K for 2 h. As derived by ∆2θ ,
2
3
1/2
it appears at d ~ 10 nm and grows up to 20 nm. γ-Al O lies at d ≤ 10 nm. The BET measurements give
2
3
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435
Table 3 Interplanar spacing (d ) and relative intensities (I) in characteristic peaks in X-ray powder dif-
hkl
fraction in α-Al O nanocrystals.
2
3
dhkl (nm)
observed
I
h
k
l
calculated
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.3475
.2550
.2380
.2085
.1741
.1603
.1549
.1515
.1406
.1375
.1243
.1236
.1191
.1187
0.3484
0.2555
0.2381
0.2087
0.1742
0.1604
0.1548
0.1514
0.1406
0.1375
0.1242
0.1236
0.1191
0.1186
76
0
1
1
1
0
1
2
0
1
0
1
1
2
2
1
0
1
1
2
1
1
1
2
3
0
1
2
2
2
4
0
3
4
6
1
8
4
0
100
44
100
42
77
2
6
31
45
12
8
4
2
10
9
0
1
The calculated dhkl are from the average lattice parameters a = 0.4762 nm and c = 1.3020 nm. The sample
has been annealed at 1525 K for 2 h.
10–25 nm average particle surface area diameter, with a volume to surface shape factor of 6. It converts
to α-Al O by a sudden increase in d of 35 nm on raising the temperature to 1475 K. A similar d-value
2
3
3
+
has been reported by Wen et al. with AlO(OH) α H O precipitate by reaction of Al nitrate with NH OH
2
4
[
14]. A pure θ-Al O appears as early as at 1173 K, with a complete conversion to α-Al O , d = 45 to
2
3
2
3
5
5 nm, at 1453 K. No θ-Al O lies at d > 25 nm. It can be taken as the R value for all practical purposes.
2
3
c
Fig. 8 Shift in X-ray diffraction peak in (113) reflec-
tion in α-Al O nanoparticles with processing tempera-
2
3
ture at a) 1475 K, b) 1525 K, c) 1700 K, d) 1900 K and
e) 1900 K for 2 h, except in d) for 10 min.
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S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
Fig. 9 a) A close-up of X-ray diffraction
showing the additional peaks present for
metastable α-Al O3 in weak intensity
2
(marked by the asterisks) in the sample
annealed at 1700 K for 2 h. b) The peaks
in the usual α-Al O3 are indexed with
2
(
hkl).
Although the impurities, dopants, or porosity modify the grain growth and phase transformation, their
role in R has not been addressed so far.
c
Thermodynamically, formation of a solid particle in volume V enclosed in surface area A involves a
change in the total Gibbs free energy,
∆
G = Aσ – ∆G V,
(2)
v
with ∆G (>0) the change in the volume Gibbs free energy per unit volume on its formation from an
v
amorphous state [32]. Assuming a spherical shape of particle of radius r, it can be rewritten as
2
4
3
3
∆
G = 4πr σ − πr ∆G .
(3)
(4)
v
4
3
On dividing by the mass, m = πr ρ (with ρ the density), it turns into a simple working relation,
3
3
σ
k
m
k
∆
G =
+C ,
m
rρ
k
m
k
v
–1
with C = – ∆G ρ a constant for a polymorph k.
Fig. 10 Gibbs
free
energy
k
change ∆G in formation of a) γ-
m
Al O , b) θ-Al O , and c) α-Al O
2
3
2
3
2
3
nanoparticles in a spherical shape.
The figure in the inset portrays
k
their C energy level diagram at
m
zero surface area.
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
437
Equation (4) represents a monotonically decreasing function of release of the excess structural energy
k
m
k
m
∆
G with grain growth r. The ∆G vs. r curve in a polymorph k differs from the one in the thermody-
k
k
namic standard state (α-Al O ) primarily in C . Extrapolation of the curve to r → ∞ gives the value of C
2
3
m
m
k
m
3
by the ∆G bulk value. The simulations made with ρ = 3.67, 3.58, or 3.98 g/cm and σ = 0.79, 1.58, or 2.08
2
J/m in γ-, θ-, or α-Al O are portrayed in Fig. 10. An intermediate σ-value of twice that in γ-Al O is taken
2
3
2
3
k
in θ-Al O for lack of its known value. A relative C value, i.e. 178.4 J/g in γ-Al O or 17.1 J/g in θ-Al O ,
2
3
m
2
3
2
3
α
used with respect to the Cm value (assumed to be zero as usual) in bulk α-Al O reproduces the experimen-
2
3
tal R = 10 or 25 nm values at which they execute the phase transformation to θ-Al O or α-Al O . The
c
2
3
2
3
γ
1
78.4 J/g value derived for C compares with the ∆H = 132 ± 20 J/g enthalpy proposed in hypothetical
m
coarse-grained γ-Al O relative to bulk α-Al O using molecular dynamics (MD) simulations [11].
2
3
2
3
The intersections in curves (a) γ-Al O and (b) θ-Al O with curve (c) α-Al O at points A and B in
2
3
2
3
2
3
γ
θ
Fig. 10 determine the critical S = 129.1 and S = 105.9 J/g surface energies (after the corrections for
m
m
k
γ
θ
the Cm values) below which the particles exist in the involved structures. A = 163.5 and A =
7.0 m /g are obtained by their abscissas. This is in agreement with the MD simulations that γ-Al O
becomes energetically stable as A exceeds 125 m /g [11]. As shown by the energy level diagram in the
m
m
2
6
2
3
k
m
2
inset to Fig. 10, both of them are surface-stabilized high-energy polymorphs with respect to α-Al O . γ-
2
3
Al O , involving the maximum energy, nucleates and grows first in early sizes to point A. In a non-
2
3
disrupted growth, the resulting particles convert and grow in θ-Al O as an intermediate phase at moder-
2
3
k
2
ate energy between A and B. α-Al O appears after point B at A < 67 m /g (d > 25 nm). It involves γ-
2
3
m
k
m
k
m
and θ-Al O phase selection sequence as per S energy. Differences in S allow the formation and exis-
2
3
tence of a particular phase at a given temperature.
3
.4 Vacancy-stabilized α-Al O nanoparticles and twin structure
2
3
3
+
The Al vacancies (holes) and oxygen-deficient AlO6 – ε sites determine a metastable α-Al O structure.
2
3
Let us consider an ideal close packing of the atoms in an ideal α-Al O structure and then point out small
2
3
3+
2–
Fig. 11 Distribution of Al cations and holes in a) O layer and b) simple hexagonal lattice in α-Al O .
2
3
A , A , and A are the basal hexagonal cell vectors. The lattice vectors a , a , and a describe a rhombo-
1
2
3
1
2
3
hedral subcell (after Kronberg [19]).
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

438
S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
3+
Fig. 12 A local model distribution of Al cations
and holes in AlO and AhO sites in a plane (hkl) in
6
6
three feasible configurations: a) all AlO sites, b)
6
two AlO and two AhO sites, and c) three AlO
6
6
6
and one AhO sites. A modified plane (a) in d
6
hkl
spacing appears in (b) while it splits into two modi-
fied planes in a twin structure in (c).
deviations that arise in these defects. It consists of R3c hexagonal unit cell with Z = 18 Al O molecules
2
3
3
+
2–
2–
per hexagonal lattice. Distribution of Al and holes in two O layers is given in Fig. 11a. The O forms
a near-close-packed hexagonal sublattice and Al occupies two-thirds of the AlO sites. The unit cell
Fig. 11b) has four consecutive O layers in average 20 Al cations and 11 holes. A , A , and A are the
3
+
6
2
–
3+
(
1
2
3
unit lattice vectors in the basal plane.
Fig. 13 Curing
oxygen-deficient
structural units from (a) AlO6 – δ
,
δ = 2, to (b) AlO by a reaction with
6
2–
(
c) O in the mobile AhO hole in a
6
metastable α-Al O . As given in (d)
2
3
θ-Al O , an enhanced dimension
2
3
appears of AlO6 if it exists with
AlO6 – δ, δ ≤ 0, in a high-energy con-
figuration.
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
439
3
+
2–
An Al hole (Ah), a region of localized negative charge, is bounded by outer electrons in O in AhO6.
The cations and holes are arranged in a manner to give a maximum separation of like charges and a mini-
3+
2–
mum separation of unlike charges as per the Al and O bonding, so that the total charge is neutralized.
The AhO group thus has a manifested AlO dimension. As shown in Fig. 12, a substitution of AhO in part
6
6
6
of the AlO sites raises the separation between the layers from d to d ′. Two different d and d ′ sets of
6
hkl
hkl
hkl
hkl
h
h
planes may result at a small f < 0.36 fraction of holes, insufficient to fill all of their positions (f = 11/31 ≅
.36) in the four layers. In this case, at least one layer gets its modified position. A single set of d or d ′
planes exists at f = 0 or 0.36. No stable structure exists with holes at f > 0.36. An AlO defect causes
0
hkl
hkl
h
h
6–δ
3+
moderate d ′ as it has a smaller size than AlO , e.g., Al has 0.0675 nm ionic size in AlO while
hkl
6
6
0
.0530 nm in AlO [33].
4
This model structure describes the X-ray diffraction in metastable α-Al O (Table 2), with the same
2
3
R3c hexagonal structure as in bulk α-Al O . It has larger a = 0.4765 nm and c = 1.3060 nm over the
2 3
equilibrium a = 0.4742 nm and c = 1.2930 nm values after 2 h of annealing at 1700 K. A non-porous
bulk α-Al O has a = 0.4758 nm and c = 1.2991 nm [25]. The excess volume is relieved by local reorder-
2
3
ing of atoms in the equilibrium configuration by a structural relaxation [18, 34]. The peaks in metastable
α-Al O in (003), (005), (006), (110), and (112) reflections have relatively larger d by 0.005 to
2
3
hkl
0
.015 nm over the average values calculated from the average a and c values. These are the hole-sensi-
tive reflections. The (103), (004), and (206) peaks have lower dhkl values by up to 0.019 nm over the cal-
culated ones. A large atomic relaxation, as much as 0.1 nm, is shown in α-Al O boundary planes through
2
3
3+
atomistic simulation of the grain boundary structure [20]. It was realized quite early [19, 35] that the Al
displaces towards the vacant sites. The cation sheets are thus puckered or staggered rather than flat. It
distorts AlO in different Al–O bonds (Fig. 13).
6
In Fig. 14, electron diffractograms (a) and (b) in a mesoporous α-Al O of TEM (c) have a number of
2
3
twins as marked by the asterisks. They lie at off positions of the regular diffraction points in the regular
Fig. 14 Twin structures in electron diffractograms (a)
and (b) in a mesoporous powder of α-Al O (metastable)
2
3
of TEM micrograph (c). The sample has been processed
by 2 h of heating at 1475 K.
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

440
S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
arrays. The results are in accord with the basal slip and twinning observed in HREM images in α-Al O3
2
[18, 20]. Each of the two boundaries in a basal twin has a different two-dimensional structure. This arises in
“
splitting of the basal plane” in two dhkl sets in AhO and AlO size mismatch. Our model gains further
6
6
2–
support by the fact that the grain boundary or facet planes in α-Al O are found to be dense in O anions
2
3
[
36], i.e., they have significant AhO groups. A problem in studies of twins in α-Al O arises from the fact
6
2
3
2–
that a unidirectional shear involving successive displacements of pairs of O planes does not give a twin
37]. Chen and Howitt [37] proposed successive motion of partial dislocations, while Bilde-Sorensen
et al. [38] included cross-slip of the partials to model twin formation in α-Al O . Kaplan et al. studied
[
2
3
HREM images with energy and grain boundary structure calculations with an ionic model [18]. The twin
boundary assumes one of two different configurations, which arise in anisotropic grain growth during twin
formation.
3
+
2–
The Al holes and O ions surrounding a hole in an AhO site appear to be stable at 1475 K.
6
They become effectively mobile at a temperature as high as 1900 K and thus locally react with nearby
oxygen-deficient AlO6–δ groups to cure in regular AlO groups. This is shown in Fig. 13 with δ = 2 in
6
AlO group. As in θ-Al O [13], the four O atoms in AlO are in three different Al–O bond lengths,
4
2
3
4
0.1710, 0.1745, and 0.1811 nm. They assume enhanced 0.1969 and 0.1857 nm values [13] in cured AlO6.
This is well reflected in 1.1 % increase in V in the lattice (Fig. 8e) relative to that in the sample processed
at 1700 K (Fig. 8c).
3
.5 IR spectrum in vacancy-controlled α-Al O structure
2 3
IR spectroscopy is a very sensitive analytical tool for analyzing local structure in molecular species and
their reordering in a specific structure [39–43]. This is applied here by analyzing whether our metastable
α-Al O phase (obtained at 1475 K) has really a new local struc-
2
3
3
+
ture of AlO groups, or defects of Al vacancies (in AhO sites)
6
6
and AlO6 – δ architects, or just an intermediate structure in the two
phases α- and θ-Al O . To resolve it, we studied the spectra in
2
3
(
a) θ-Al O , (b) metastable α-Al O , (c) α-Al O , and (d) a 1:1
2 3 2 3 2 3
mixture of (a) and (c). The results compared in Fig. 15 reveal the
–1
spectrum in two primary band groups of (i) 300–525 cm and (ii)
–1
4
00–1000 cm as marked by the letters A, B, and C. The shape,
size, and internal structure of bands differ markedly in the four
–1
samples. For example, band group (i), which extends to 400 cm in
–1
–1
(
(
a), marked by point B, shifts to 475 cm in (b) and to 525 cm in
c) or (d). The intensity profile in band group (ii) varies in a near V-
shape in (a) and (b), while no such distinct structure persists
Fig. 15 IR spectra in nanocrystals in (a) θ-Al O , b) metastable
2
3
α-Al O , (c) usual α-Al O , and d) 1:1 mixture in a) and c). The three
2
3
2
3
polymorphs have been annealed for 2 h at 1275 K, 1475 K, and
1525 K, respectively.
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
441
Table 4 Infrared characteristic bands in nanoparticles in the metastable α-Al O and usual θ-Al O and
2
3
2
3
α-Al O polymorphs.
2
3
–
1
bands (cm )
assignment
θ-Al O
metastable α-Al O3
α-Al O3
2
2
3
2
3
3
30 (vw)
63 (w)
335 (vw)
366 (w)
ν
2
(AlO
), deformation
4
ν (AlO ), deformation
4
4
3
83 (vw)
ν2 (AhO6 – δ), deformation
ν (AlO ), deformation
4
60 (vw)
448 (ms)
450 (s)
490 (vw)
585 (vs) 
640 (s)
2
6
5
35 (s)
ν4 (AhO6 – δ), deformation
ν (AlO ), deformation
5
6
7
7
8
8
50 (vs)
10 (s)
04 (ms)
72 (s)
20 (vs)
75 (s)
580 (vs)
640 (s)
712 (ms)
785 (s)
820 (s)
820 (s)


4
1
6
4
710 (ms)
ν
(AlO
)/(AlO
6
), stretching



805 (ms)
ν (AlO ), stretching
3
6
ν
3
(AlO
), stretching
4
Relative band intensities are given in parentheses: vs, very strong; s, strong; ms, medium strong; w, weak;
and vw, very weak.
–
1
in other samples. Furthermore, in comparison to (a), a new band (bandwidth ∆ν = 30 cm ) has developed
1/2
–1
–1
–1
at 448 cm together with an improved energy as well as intensity in the band at 580 cm (550 cm in (a)) in
b). A rather broad band, ∆ν = 50 cm , at 450 cm appears with two satellite components at 383 and
–1
–1
(
1/2
–1
490 cm in (c).
The metastable α-Al O is thus characterized by an unambiguously different IR spectrum than in θ-
2
3
Al O or α-Al O . It does not represent their mixed phase. A mixed α-Al O phase, if it exists at all with
2
3
2
3
2
3
significant θ-Al O (under 50% as per I in the X-ray diffraction peaks in Fig. 7), results in an altogether
2
3
p
different and complex spectrum, as shown in Fig. 15d. Assignments of all observed bands in terms of
possible modes of vibration in AlO and/or AlO groups are given in Table 4. As can be derived through
4
6
group theory [39], there are a total of four vibration groups ν , i = 0 → 4, in AlO whereas there are six
i
4
(
i = 0 → 6) in AlO . Their distributions and symmetry species in T (AlO ) and O (AlO ) point groups
6
d
4
h
6
are given in Tables 5 and 6. The occurrence of most of them in the IR spectra in Table 4 infers lowered
site symmetry C in AlO and D or C in AlO in the presumed sublattice distortion. Otherwise, only
1
4
2d
1
6
the ν and ν bands occur in IR in both AlO and AlO . This is obeyed in γ-Al O with four bands at 357,
3
4
4
6
2
3
–1
5
36, 744, and 807 cm [41].
The doublet band at average 350 cm , intrinsic to ν and ν AlO deformation vibrations [39], in
–
1
2
4
4
spectra (a) and (b) demonstrates AlO groups to be present in metastable α-Al O as well. The ratio (F )
4
2
3
t/0
of AlO /AlO is 1:2 in θ-Al O [13]. While heating at 1475 K, part of the AlO in it converts to AlO
6
4
6
2
3
4
with the phase transformation to the metastable α-Al O . This reflects a drastic increase in intensity in
2
3
Table 5 Distribution of four fundamental molecular vibration groups in AlO structural unit in Al O .
4
2
3
mode
symmetry species
point group (Td)
site symmetry (C1)
(R, IR)
factor group (D2d)
ν
A
E (R)
1
(R)
A
1
(R)
1
ν
2
A
1
B
B
1
(R)
(IR, R)
ν
3
F
F
2
(R, IR)
(R, IR)
2
ν
4
2
E (IR, R)
R, Raman-active; and IR, infrared-active vibrations. Vibration ν is doubly degenerate while ν and ν are
2
3
4
triply degenerate in the total of nine vibrations.
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

442
S. Rana and S. Ram: Self-controlled growth in highly stable α-Al O nanoparticles
2 3
Table 6 Distribution of six fundamental molecular vibration groups in AlO structural unit in O , T and
6
h
d
C point groups.
1
mode
species
point group (Oh)
C1
Td
ν
A
1g (R)
(R)
A
E (R)
1
(R)
1
ν
2
E
g
ν3,ν4
ν
F
F
F
1u (IR)
2g (R)
2u (IA)
A
1
(R, IR)
F
2
F
2
F
1
(R, IR)
(R, IR)
(IR)
5
ν
6
R, Raman-active; IR, infrared-active; and IA, inactive vibrations. Vibration ν is doubly degenerate while
2
ν to ν are triply degenerate in the total of 15 vibrations.
3
6
–
1
–1
the characteristic band ν at 448 cm (or ν at 580 cm ) at the expense of that in the ν (AlO ) band at
8
2
4
3
4
–
1
75 cm . A fractional value of AlO groups of
4
It
k
F = κ
~18%
(5)
t
It + I0
–
1
is obtained from relative intensities I and I in the 366 and 448 cm bands in two sites in metastable
t
0
k
1
α-Al O . The constant κ = 0.475, derived with F = with I and I in the θ-Al O bands, is used. An-
other band at 535 cm , which lies as a shoulder of ν (AlO ) band at 580 cm in metastable α-
Al O , is ascribed to the ν band in Al holes in AhO sites. It disappears on raising the temperature to
2
3
t
t
0
2
3
3
–1
–1
4
6
3
+
2
3
4
6
1
525 K (Fig. 15c) as the holes become mobile at this temperature, presumably recombining to AlO4
k
architect defects, F ~ 18%, to cure into regular AlO groups. This is accompanied with a concomitant
decrease in I in the 800–900 cm region (marked by the downward arrow in Fig. 15) with an increase in
t
6
–1
t
–
1
I in the band at 585 cm . Moreover, two new bands developed with an extremely weak intensity at 383
0
–
1
and 490 cm (in ν and ν vibrations in Table 4) in a modified force field in trace of AhO sites in
coupling with neighboring Al cations. Other AhO vibrations do not appear in distinct bands in over-
2
4
6 – δ
3
+
6
lapping with AlO bands.
6
4
Conclusions
3
+
Thermodynamically stable α-Al O nanoparticles in support with high-energy surface and Al vacancies
2
3
form at a controlled porosity, 10–25%, in a mesoporous structure. The structure is analyzed and modeled
with X-ray diffraction, microstructure, and IR spectrum. The synthesis carried out with a monolithic
AlO(OH) x H O precursor powder (mesoporous) precludes the possibility of other structural inclusions.
2
Unusually, the α-Al O nanoparticles arranged through immobile pores (5–15 nm diameter) behave as
2
3
highly stable in a confined size of 30–50 nm diameter. Otherwise, they readily grow as big as several
hundred nanometers in bulk sample [11, 12] at temperatures as high as 1900 K. The configurational
entropy predominates the enthalpy to inhibit their confined growth in a thermodynamically stable struc-
ture.
The specimen obtained at moderate temperature, ~1475 K, has copious structural defects of oxygen-
3+
deficient AlO6 – δ, δ ≤ 2, groups and Al vacancies (negatively charged holes) in octahedral AhO sites in
6
a metastable α-Al O structure. The like or unlike charges arrange themselves at a maximum or mini-
2
3
mum separation by neutralizing their total value. A hole thus reassumes a manifested AlO dimension. It
6
is modeled that an uneven distribution of holes in a specific (hkl) plane triggers it in a modified structure.
This accords with the X-ray diffraction observed with additional peaks relative to those in bulk α-Al O .
2
3
A similar twin structure has been observed in selected area electron diffraction and high-resolution elec-
tron microscopic images in bulk α-Al O [18, 20]. The average concentration in it is not sufficient to
2
3
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2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (a) 201, No. 3 (2004) / www.pss-a.com
443
show up in X-ray diffraction. The metastable α-Al O phase has 2.0% excess volume over the equilib-
2
3
rium value in nanoparticles after annealing (reorders holes and occupied sites by recovering AlO6 – δ
defects) at temperatures as high as 1700 K. It is characterized with a different vibrational structure than
in α-Al O .
2
3
3+
A mobile Al hole in a site neighboring an AlO6 – δ defect promotes a plane slipping or a twin struc-
ture formation. The dislocation moves into a region of the particle along the AlO defects and not
6
– δ
2
–
along the reverse direction. It is found that the O in the hole becomes mobile in metastable α-Al O at
2
3
1
525 K or above, and thus reacts with the defects to cure into a regular AlO polyhedron. A characteristic
6
–
1
–1
Ah–O deformation vibration ν observed at 535 cm (580 cm in AlO ) in the IR spectrum in metasta-
4
6
ble α-Al O characterizes the holes to be exclusively present in AhO sites. Its relative intensity with
2
3
6
3
+
respect to that in θ-Al O (F ~ 33%) determines a fractional Ft/0 ~ 18% value of Al in AlO6 – δ, δ ~ 2.
Two weak bands developed at the expense of this band in ν and ν vibrations at 383 and 490 cm in
2
3
t/0
–1
2
4
trace of AhO6 – δ in a modified force field in cured α-Al O at 1525 K.
2
3
The results demonstrate the possibilities of varying the microstructure and related properties of
α-Al O following controlled grain growth and self-induced pinnings of the local structures through
2
3
3
+
mesopores. The AlO6 – δ defects and Al holes provide sites for initiation and also arrest of self-confined
cracks, designing a strengthened structure. The pores act as local pinning barriers to a non-disrupted
crack propagation. Otherwise, it grows to be non-disrupted through several grains.
Acknowledgement This work has been financially supported by a research grant from the DRDO (Defence Re-
search & Development Organization) of the Government of India.
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