A new, low-temperature polymorph of O′-SiAlON

Article abstract of DOI:10.1111/j.1151-2916.1998.tb02605.x

A new phase in the Si-Al-O-N system has been identified, following syntheses based on the nitridation of silicon/clay mixtures at low temperatures (<1350°C). The structure of the new phase was determined using a combination of diffraction and high-resolution imaging techniques, and this new phase possessed the same sheet structure as O′-SiAlON (Si_2-xAl_xO_1+xN_2-x) but with a different stacking arrangement. It is considered to be a low-temperature polymorph of O′-SiAlON and transforms to conventional O′-SiAlON at temperatures greater than ～1350°C.

Full text of DOI:10.1111/j.1151-2916.1998.tb02605.x

J. Am. Ceram. Soc., 81 [8] 2188–90 (1998)
ournal
J
A New, Low-Temperature Polymorph of O؅-SiAlON
Mark E. Bowden, Glen C. Barris, and Ian W. M. Brown
Industrial Research Limited, Lower Hutt, New Zealand
David A. Jefferson
Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom
A new phase in the Si-Al-O-N system has been identified,
following syntheses based on the nitridation of silicon/clay
mixtures at low temperatures (<1350°C). The structure of
the new phase was determined using a combination of dif-
fraction and high-resolution imaging techniques, and this
new phase possessed the same sheet structure as O؅-SiAlON
(Si2−xAlxO1+xN2−x) but with a different stacking arrange-
ment. It is considered to be a low-temperature polymorph
of O؅-SiAlON and transforms to conventional O؅-SiAlON
at temperatures greater than 1350°C.
copy (HREM) utilized a 200 keV instrument (Model 200CX,
JEOL, Tokyo, Japan) equipped with a high-resolution (Cs ס
0.41 mm) pole piece. Simulated HREM images were calculated
with the EMS software system,5 using values of 0.5 mrad for
beam divergence and 100 Å for focal spread.
III. Results and Discussion
The first indications of the formation of a new phase arose
when the XRD patterns of reacted mixtures could not be
matched to any known compounds. The unmatched XRD
peaks were particularly apparent in samples reacted at tempera-
tures <1300°C, which is a temperature considerably lower than
those normally used for solid-state syntheses of OЈ-SiAlON
(e.g., 1800°C6). The ability to prepare SiAlON phases at low
temperature was a consequence of the synthesis method used
and, in particular, the use of clay and silicon as reactants.
Adding small amounts of iron (1.5% as Fe2O3) improved the
reaction rate, as has been observed for the nitridation of silicon
to Si3N4.7 The results obtained here do not show whether the
iron or other impurities derived from the clay have any role in
stabilizing the new phase, in a manner similar to the additional
cations used to stabilize ␣-SiAlON. However, we do note that
the level of impurities in the present material is considerably
lower than that required to stabilize ␣-SiAlON.
The composition of the new phase approximated that of
OЈ-SiAlON, as reacted mixtures with this composition pro-
duced the highest yields. In addition, the new phase trans-
formed to OЈ-SiAlON at 1350°C without a significant weight
change and without the appearance or disappearance of sec-
ondary phases. All these factors have led us to believe that the
low-temperature conditions used here permitted the synthesis
of a new, low-temperature polymorph of OЈ-SiAlON.
Although the crystallites prepared were too small for single-
crystal analysis, the structure was solved by using a combina-
tion of HREM and selected-area electron diffraction. Electron
diffraction patterns taken from different zone axes suggested an
orthorhombic unit cell, and preliminary structural coordinates
were proposed on the basis of the HREM images. Simulation
of these images and the XRD pattern confirmed the proposed
structure, which was subsequently refined from the observed
XRD data using the Rietveld technique. Some small discrep-
ancies between the observed and calculated XRD patterns re-
mained after the refinement; these discrepancies were ascribed
to the presence of planar defects in the specimen (this point will
be discussed later in the paper). However, the overall agree-
ment (Bragg factor (R) of 5.37%, goodness of fit is 10.3) was
sufficient to give us considerable confidence in the validity of
the model. Structural coordinates that resulted from these re-
finements are given in Table I, and tabulated XRD pattern data
are provided in Table II.
I. Introduction
HE Si-Al-O-N system has become the subject of widespread
T
research since the initial discovery1,2 that aluminum and
oxygen can be substituted into the Si3N4 lattice. One reason for
the intensity of the research effort is the many different
SiAlON phases that can be prepared, each of which presents
different physical properties. The Si-Al-O-N phase diagram
was explored in the early history of SiAlON research, and by
the time Jack performed his review in 1976,3 all the presently
known phases had been discovered. In this communication, we
describe the structure of the first new SiAlON to be found since
that times: a low-temperature polymorph of OЈ-SiAlON
(Si2−xAlxO1+xN2−x).4
II. Experimental Procedure
Mixtures of clay (light kaolin, BDH Chemicals Pty., Ltd.,
Poole, U.K.), silicon (grade 4D, Permascand AB, Ljungaverk,
Sweden), and silica (Superfine Quartz, Commercial Minerals
Ltd., Auckland, New Zealand) were blended for 20 h in a ball
mill using Si3N4 media and hexane solvent. Major impurities
(>0.05%) in this clay are 0.75% K2O, 0.45% Fe2O3, 0.35%
MgO, 0.21% P2O5, and 0.14% Na2O. The stoichiometry for
each mixture was calculated according to the following reac-
tion scheme, illustrated for the x ס 0.2 OЈ-SiAlON composition:
0.1Si2Al2O5(OH)4 + 1.35Si + 0.25SiO2 + 0.9N2
→ Si1.8Al0.2O1.2N1.8 ם 0.2H2O
After drying, the powder was lightly pressed (8 MPa) into disks
15 mm in diameter and heated under flowing H2 (10%) in N2
for 8 h at 1270°C.
X-ray diffractometry (XRD) was performed using a system
(Model PW1700, Philips Research Laboratories, Eindhoven,
The Netherlands) with a post-diffraction graphite monochro-
mator and CoK␣ radiation. High-resolution electron micros-
The structure is best described by comparing it with OЈ-
SiAlON,6 and projections of both structures are shown in Fig.
1. For simplicity, we will consider the x ס 0 composition
(Si2N2O), although the actual specimen that was examined
contained 10 mol% of aluminum and oxygen, substituted on
T. Ekstro¨m—contributing editor
Manuscript No. 190375. Received February 16, 1998; approved April 27, 1998.
2188

August 1998
Communications of the American Ceramic Society
2189
Table I. Positional Coordinates from Rietveld Refinement†
Position
Atom
x
y
z
Si (Al)
N (O)
O
0.3226 (4)
0.2847 (9)
0
0.0865 (9)
0.3857 (19)
0.5
0.1275 (15)
0
0.085 (4)
†Space group is Bba2; cell parameters are a ס 8.8488(13) Å, b ס 5.3752(6) Å, and
c ס 4.8355(5) Å. There is 10 mol% substitution of Al on the Si sites and of O on the
N sites.
Table II. XRD Pattern Data†
hkl
dcalc
dobs
Intensity ratio, I/I0
200
210
111
020
002
311
121
400
202
410
212
022
230
511
412
113
600
521
430
313
123
4.424
4.433
86.5
38.1
100.0
20.5
42.7
15.3
16.4
2.3
3.416
3.418
3.331
3.337
2.688
2.690
2.418
2.419
2.280
2.282
2.270
2.274
2.212
2.210
2.122
2.124
3.5
2.046
2.047
8.1
1.9735
1.7975
1.6607
1.5878
1.5617
1.5209
1.4748
1.4135
1.3923
1.3679
1.3657
1.9747
1.7982
1.6607
1.5879
1.5608
1.5220
1.4749
1.4133
1.3919
4.5
6.9
7.1
4.3
2.6
5.3
4.5
Fig. 1. Polyhedral representations of (A) a single sheet of OЈ-
SiAlON, (B) a projection of OЈ-SiAlON on the (001), and (C) an
equivalent projection of the new compound. Each silicon atom is
tetrahedrally coordinated to one oxygen atom (at x ס 0, 0.5) and three
nitrogen atoms.
2.4
6.7
1.3678
11.4
†CoK␣1 radiation, wavelength of 1.78896 Å.
Although the layer structure of the new material facilitates a
new stacking arrangement, it does not preclude the arrange-
ment found in conventional Si2N2O. HREM images revealed
several planar faults parallel to (100) that we believe to be
isolated occurrences of just such behavior. An example is
shown in Fig. 2, which includes a simulation of the image
expected from a single planar fault. Above the fault, the simu-
lation is best compared with the experimental image immedi-
ately to the right (Fig. 2(B)); however, below the fault, the area
to the left (Fig. 2(A)) provides the best match. The simulation
reproduces the main features of the observed image correctly,
including the overall appearance of the planar fault as well as
the relative displacement along the b-direction of the structure
above and below the fault. Stacking faults have also been ob-
served by HREM in X-phase SiAlON,8 although, given the
scarcity of structural data concerning X-phase, it is unclear
how they relate to the present observations. It seems likely that
the planar faults in the new compound have an important role
in its transformation to OЈ-SiAlON, because they represent
local regions of the conventional OЈ-SiAlON structure. The
faults may be either the initial stages of a transformation al-
ready underway or intrinsic defects that act as nucleation sites
for subsequent transformation at a higher temperature. Such
transformations, which involve shear along a crystallographic
plane, have been shown to occur between the polymorphs of
Mg2SiO4 spinel.9
the Si and N sites, respectively. Conventional Si2N2O is
composed of layers of SiN3O tetrahedra, parallel to (100), that
are linked through a corner sharing of their nitrogen atoms
(Fig. 1(A)). Tetrahedra that are adjacent in the b-direction have
the oxygen atoms at their apices alternately directed upward
and downward in a regular sequence. Tetrahedra that are simi-
larly oriented (i.e., either upward or downward) form zig-zag
chains that are oriented parallel to the c-axis. Each upward-
pointing chain in one layer meets a downward-pointing chain
in the layer above it to complete the three-dimensional arrange-
ment (Fig. 1(B)). This arrangement leads to two different dis-
tances between the apical oxygen atoms in a single layer: a
shorter distance between oxygen atoms within a chain, and a
longer distance between those in neighboring chains. The pro-
jection shown in Fig. 1(B) shows that the tetrahedra are tilted
slightly so that their apices are not oriented exactly along the
a-axis. This skewed orientation has the effect of adjusting the
Si−O−Si bond angles so that they are not in an unfavorable 180°
configuration.
The new structure (Fig. 1(C)) is composed of similar layers
of SiN3O tetrahedra; however, this structure has a more pro-
nounced tilting of the tetrahedra around the central axis of
the zig-zag chains. The effect of this greater tilting is to
make the O−O distances within and between the zig-zag
chains in each layer equivalent. This arrangement, in turn, al-
lows the layers to be stacked in a new configuration so that
each chain in one layer bridges two chains in the adjoining
layer, rather than being directly above and below the adjoin-
ing layers, as in the original structure. The bond angles and
lengths of the two structures are comparable, although the
smaller unit cell of the new material increases its density from
2.80 g/cm3 to 2.90 g/cm3, which is consistent with the general
rule that low-temperature polymorphs have higher-density
structures.
The discovery of this new phase is of interest to SiAlON
research, not simply because it is a new material but also for
its polymorphic relationship to OЈ-SiAlON. The polymorphism
of other phases in the Si-Al-O-N system, notably Si3N4, has
been shown to offer many practical benefits. For example,
when fabricating ceramic bodies from Si3N4, it is advantageous
to begin with ␣-Si3N4 powder, because, although it transforms
to the ␤-phase modification when hot-pressed, the final body
exhibits superior mechanical properties than if it were fabri-

2190
Communications of the American Ceramic Society
Vol. 81, No. 8
Fig. 2. (A) HREM image of a planar defect within the new material; a simulated image (thickness of 58 Å, defocus of −475 Å) is shown in the
inset. (B) Structural model used for the simulation.
cated directly from ␤-Si3N4 powder.10 In addition, the sub-
stituted analogs of Si3N4—␣Ј- and ␤Ј-SiAlON—each possess
different physical properties and, thereby, lend themselves to
different applications: ␣Ј-SiAlON is intrinsically a harder
material, whereas ␤Ј-SiAlON forms an acicular microstruc-
ture more readily, which imparts a higher fracture toughness.11
Whether similar behavior can be found in the relationship
between the present material and OЈ-SiAlON remains to be
determined.
Acknowledgment: The authors wish to acknowledge the assistance of
Profs. K. H. Jack and D. P. Thompson in the preparation of this manuscript.
References
1K. H. Jack and W. I. Wilson, ‘‘Ceramics Based on the Si-Al-O-N and Re-
lated Systems,’’ Nature (London), Phys. Sci., 238, 28–29 (1972).
2Y. Oyama and O. Kamigaito, ‘‘Solid Solubility of Some Oxides in Si3N4,’’
Jpn. J. Appl. Phys., 10, 1637 (1971).
3K. H. Jack, ‘‘Review: Sialons and Related Nitrogen Ceramics,’’ J. Mater.
Sci., 11, 1135–58 (1976).
4K. H. Jack, ‘‘Nitrogen Ceramics,’’ Trans. J. Br. Ceram. Soc., 72, 376–84
(1973).
5P. W. Stadelmann, ‘‘EMS—A Software Package for Electron Diffraction
Analysis and HREM Image Simulation in Materials Science,’’ Ultramicrosco-
py, 21 [2] 131–46 (1987).
IV. Summary
6O. Lindqvist, J. Sjo¨berg, S. Hull, and R. Pompe, ‘‘Structural Changes in
OЈ-SiAlONs, Si2−xAlxN2−xO1+x, 0.04 Յ x Յ 0.40,’’ Acta Crystallogr., Sect. B:
Struct. Sci., B47, 672–78 (1991).
By using a synthesis route based on the nitridation of
mixtures of silicon and clay, a low-temperature polymorph
of OЈ-SiAlON has been prepared. This new polymorph
contains the same structural elements as conventional OЈ-
SiAlON, with sheets of corner-shared (Si,Al)(O,N)3O tetra-
hedra linked through their apical oxygen atoms. The position-
ing of one sheet above another is different in the new mate-
rial, which has a smaller unit cell and, consequently, a higher
density. Stacking faults observed via high-resolution elec-
tron microscopy have been identified as being individual
sheets of conventional OЈ-SiAlON, which leads to the possi-
bility of a solid-state shear transformation between the two
polymorphs.
7P. Popper and S. N. Ruddlesden, ‘‘The Preparation, Properties and Structure
of Silicon Nitride,’’ Trans. Br. Ceram. Soc., 60, 603–26 (1961).
8D. P. Thompson and P. Korgul, ‘‘Sialon X-Phase’’; pp. 375–80 in Progress
in Nitrogen Ceramics. Edited by F. L. Riley. Martinus Nijhoff, The Hague,
1983.
9D. C. Rubie and A. J. Brearly, ‘‘Mechanism of the ␥–␤ Phase Transforma-
tion of Mg2SiO4 at High Temperature and Pressure,’’ Nature (London), 348,
628–31 (1990).
10F. F. Lange, ‘‘Relation between Strength, Fracture Energy, and Microstruc-
ture of Hot-Pressed Si3N4,’’ J. Am. Ceram. Soc., 56, 518–22 (1973).
11E. M. Knutson-Wedel, L. K. L. Falk, H. Bjo¨rklund, and T. Ekstro¨m,
‘‘Si3N4 Ceramics Formed by HIP Using Different Oxide Additions—Relation
between Microstructure and Properties,’’ J. Mater. Sci., 26, 5575–84 (1991).ᮀ