2~/' (CvK~)
R. MARCHAND
4. Quaternary and higher oxynitrides
In the rare-earth-¢ontaining quaternary oxynitrides, the rare-earth element R is associated to another element M in the cationic network. Such R-M-O-N compounds, regarded as intermediate between pure oxides and nitrides, can be easily compared with R-M-O ternary oxides, because nitrogen and oxygen, generally speaking, play the same role in the anionic network. A quaternary R - M - O - N oxynitride may thus be considered a temary oxide in which part of divalent oxygen has been replaced by trivalent nitrogen, while the trivalent R elements have been replaced by divalent or univalent cations in order to maintain electroneutrality, as illustrated by the following equations:
R 3+ + N 3- : Ca 2+ + 02-, R 3+ + 2N 3- = Na + + 202-.
This is the cross-substitution principle, which allows the same stoichiometry to be kept, and possibly also the same structure type, if other parameters, such as a compatible size of the substituting cations, have been taken into account. This parallel drawn with oxides, the corresponding structures of which are well-known, has led to a comprehensive structural study of nitride-type compounds (see for example Marchand et al. 199 la).
As for the corresponding iono-covalent ternary oxides, the structure of the quaternary R - M - O - N and higher oxynitrides will be advantageously described from the coordination polyhedra of M atoms, the tetrahedra for example in silicates, the larger R atoms being located within the holes formed by the different arrangements of these structural units.
Such a description is particularly well adapted to the case of tetrahedra and octahedra.
Three types of environment for M atoms can be pointed out in the rare-earth-containing oxynitrides, namely tetrahedral, octahedral and "cubic".
4.1. Tetrahedral environment 4.1.1. Scheelite-type structure
If calcium in scheelite CaWO4 is replaced by a lanthanide atom R = Nd, Sm, Gd or Dy, taking into consideration the cross-substitution
R llI+ + N IIl- = Ca n+ + O n-,
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 71 this leads to the oxynitride series with the formulation RWO3N which has the same structure as the calcium tungstate (Antoine et al. 1987). The tetragonal (I41/a) structure is made up of [WO3N] isolated tetrahedra which are linked to each other by R cations, the coordination number of which is equal to 8. These nitrido-tungstates are prepared, as brown non-hygroscopic powders, by heating the corresponding tungstates R2W209 at 700-750°C in flowing ammonia. Their insulating behavior is consistent with tungsten W vI+ which is thus stabilized in a tetrahedral nitrogen environment. Yet, the reducing character of the NH3 atmosphere was a priori unfavorable to keep this high oxidation state, as was the fact that no corresponding nitride (i.e. WN2) exists in the W-N binary system, where only WN and W2N are known.
4.1.2. Silicate and alumino-silicate structures
This subsection covers R-Si-O-N and R-Si-A1-O-N oxynitrides where Si(O,N)4 and (Si,A1)(O,N)4 tetrahedra are the structural units.
The different crystalline and glassy phases of the R-Si-O-N and R-Si-A1-O-N systems have given rise to many studies, because they form during sintering of silicon nitride and related nitrogen ceramics called "Sialons", the acronym of the four elements Si, A1, O, N. This important class of ceramic materials has rauch attracted interest for high-temperature engineering applications owing to their excellent properties, firstly the mechanical ones. The sintering is achieved by means of oxide additives such as rare- earth oxides, mainly Y203, with formation of phases mentioned above, the composition of which is located in the quinary R-Si-A1-O-N system.
The R-Si-A1-O-N system is represented by the so-called Jänecke's triangular prism (Jänecke 1907) in which all edges are equal. Figure 8 outlines this representation in the case of yttrium. It is based on a Si3N4-A14N4-A1406-Si306 square in which concentrations are expressed in equivalent units, with yttrium also in equivalent units, along a third dimension. This forms two additional squares Si3N4-Y4N4-Y406-Si306 and A14N4-Y4N4-Y406-A1406. The front triangular face of the prism thus represents oxides and the rear face nitrides. So, any point of the prism is a combination of 12 +ve and 12 -ve valencies, the compounds being regarded in ionic terms, regardless of the real character of the interatomic bonding (Gauckler et al. 1975, Jack 1976, Gauckler and Petzow 1977).
4.1.2.1. Apatites. Apatites constitute a large family of compounds, the most representative member of which is fluorapatite CaI0(PO4)6F2, corresponding to various cationic and anionic substitutions with, in addition, possibility of vacancies in both subnetworks.
Although their formulation is comparable to that of fluorapatite, the rare-earth silicon oxynitrides RIoSi6024N2 (R = La, Ce, Nd, Sm, Gd, Y) and R10_xR~xSi6024N2 (Gaudé et al.
1975a, Hamon et al. 1975, Wills et al. 1976a, Mitomo et al. 1978, Guha 1980a) and the chromium CrHLsubstituted compounds RsCr2Si6024N2 (R = La-Dy) (Hamon et al. 1975) are somewhat different because the two nitrogen atoms are not located in the same position as the two fluorine atoms in the hexagonal structure. Whereas F atoms occupy the trigonal
72 R. MARCHAND Y4N4
Si3N 4
AI406 Si306
Fig. 8. Outline representation of the Y-Si-A1-O-N system. Glass-forming region within the yttrium-sialon triangular prism on cooling from 1700°C (Drew et al. 198t).
site 2a, the so-called "tunnel" position, N atoms are part of the silicon environment, thus forming mixed [SiO4_xNx] individual tetrahedra (Gaudé et al. 1975b, 1977, Maunaye et al.
1976, Morgan 1979a). As shown by Morgan (1979a), such an occupancy agrees woll with Pauling's second crystal rule (PSCR), which is essentially a local charge/bond strength balancing rule (McKie and McKie 1974):
N Vcation Vanio n ~
Ccation ' 1
where V is the valence and C the coordination of the N cations.
Figure 9 shows the Sm~0(Si6022N2)O2 structure (space group P63), The nitrogen apatites are thought to exist over a range of compositions extending to silicate phases with defect apatite structure R9.33(SIO4)602 and R8[~2(SiO4)61212 ([] = vacancy).
Presence of nitrogen in the silicon tetrahedral environment means that there is the possibility to introduce more than two nitrogen atoms per unit cell (Lang et al. 1975):
this was demonstrated by the preparation of RsMävSi6N4022 (M w = Ti or Ge) nitrogen apatites, the maximum enrichment reported so rar being in a vanadium VV-containing composition, i.e. Sms.6»Vj.3»SiöN4.TN21.3 (Guyader et al. 1975, 1978).
4.1.2.2. Melilites. Whereas apatites contain isolated tetrahedral units, melilites, which are natural alumino-silicates having the general formula (Ca,Na)2(Mg,A1)(Si,AI)2OT, belong to the sorosilicate family, This family is characterized by [Si207] disilicate groupings formed by two tetrahedra sharing one corner. Typical examples of melilites are given by akermanite Ca2Mg[Si2OT] and gehlenite Ca2AI[SiAIO7]. In the tetragonal
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 73
B~~~ o , .... ... ~ ~ q ~ # ° °»~,'~ ,
Fig. 9. Projection of the hexagonal apatite structure of Smr0SiöN2024 along the c-axis (Gaudé et al. 1975b).
(P421m) structure, the coordination number of calcium is 8 whereas magnesium (or aluminum) occupies a tetrahedral site. Introduction of rare-earth atoms into the calcium site (R 3+ instead of Ca 2÷) coupled with N/O substitution leads to several possibilities to form isostructural rare-earth-containing melilite-type oxynitrides (N-melilites), and a maximum nitrogen enrichment was obtained in the R2Si303N4 series (R=La-Yb, Y) by simultaneous replacement of Ca by R and Mg by Si in akermanite (Rae et al.
1975, Marchand et al. 1976, Wills et al. 1976b). These N-melilites were prepared by heating pressed pellets of 1R203-1Si3N4 stoichiometric mixtures at 1500°C in nitrogen atmosphere. As silicon atoms occupy now all the tetrahedral (Si + Mg) sites, their structure may be described, considering the arrangement of silicon tetrahedra, as a sheet structure formed by an infinite linkage of Si(O,N)4 tetrahedra lying in the plane perpendicular to the [001] direction, stacked one on top of the other and held together by layers of larger R 3+ ions sandwiched between them (Jack 1976, Horiuchi and Mitomo 1979) (fig. 10). According to a preliminary neutron diffraction study of the Y2Si303N4 phase, Roult et al. (1984) concluded that the [Si203N4] grouping was arranged as two [SiO2N2]
tetrahedra with the third Si in a pure nitrogen [SiN4] environment. However, more recent 29Si MAS-NMR results (Dupree et al. 1988) favor another nitrogen distribution, i.e. a structure with all silicon atoms in [SiOzN2] tetrahedra, according to the presence of a single peak at -56.7 ppm.
In rare-earth-Si-Al~)-N systems, the R2Si303N4 melilites are the highest nitrogen- containing compounds, apart from «-sialons, and they are characterized by a high melting point: -1900°C for Y2Si303N4 (Jack 1986).
74 R. MARCHAND
Fig. 10. Projection onto the (001) plane of the
R2Si303N 4 melilite structure in which both T~ and T 2 tetrahedra are occupied by silicon atoms (after Lejus et al. 1994).
N-melilites, e.g. Y2Si303N4, form a continuous series of solid solutions with other members of the melilite series of silicates, e.g. akermanite and gehlenite (Jack 1976, Thompson 1989), and, recently, a significant solubility of A1 in R2Si303N4 was observed, in particular for R = S m by Cheng and Thompson (1994a) and for R = Y by Chee et al. (1994), resulting in the formation of a melilite solid solution
R2Si3_xAlxO3+xN4_x
(0 <~ x ~< 1), with A1 atoms in the Mg sites of the akermanite structure.
This substitution of A1-O for Si-N bonds results in an improved oxidation resistance of the material, compared to the original N-melilite phases (Chee et al. 1994). Such a kind of composition appears as an intermediate phase during sintering of «-sialon ceramics and also forms in SiA1ON materials as a grain boundary phase with good refractory properties (Cheng and Thompson 1994a,b).
4.1.2.3.
Cuspidines.
The structure of the mineral cuspidine Ca4Si2OTF2 (monoclinic, P 2 / c ) is made up of [Si207] groupings (Saburi et al. 1977). This structure is also that of rare-earth aluminates R4A1209 (Brandle and Steinfink 1969). Starting from such R4MIIIO9 oxide compositions, the cross-substitutionSi lv+ + N lIl- = MIII+ + 0 I1-
leads to the oxynitride compositions R4Si207N 2 which were characterized as nitrogen cuspidines for R = N d - Y b and Y (Marchand et al. 1976, Wills et al. 1976a,b, Mah et al. 1979, Morgan 1979b, Guha 1980b). They were prepared by solid-state firing of stoichiometric mixtures of R203, SiO2 and Si3N4 powders at high temperatures (1550- 1700°C). Large single crystals of La4Si2OTN2 were obtained by Ii et al. (1980) by the ftoating zone method. It was of interest to know whether the nitrogen atoms bond
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 75 to silicon or whether they replace fluorine atoms. Morgan (1986) developed crystal chemistry arguments using Panling's second crystal rule (PSCR) for nitrogen bonding to silicon. On the other hand, neutron diffraction showed that oxygen and nitrogen atoms are crystallographically ordered with the only presence of [SiO3N] tetrahedra, which means that the bridging atom in the Si20»N2 disilicate groupings is an oxygen atom: O2NSi- O-SiO2N (Roult et al. 1984, Marchand et al. 1985). This result is in agreement with the 29Si MAS-NMR spectrum which shows a single resonance at -74.4ppm (Dupree et al. 1988). Note finally that intermediate cuspidine members between P,4Si2OTN2 and R4A1209 compositions such as pseudo-tetragonal Y4SiA1QN are also known (Thompson 1986).
4.1.2.4. Pyroxene-type. The yttrium compound MgYSi2OsN, which can be regarded as being derived from diopside [CaMg(SiO3)2]n with repläcement of Ca by Y and of one O by N is the only known example so far of nitrided silicate with chain structure of pyroxene-type (Patel and Thompson 1988).
4.1.2.5. Wollastonites. The structure of «-CaSiO3 or pseudo-wollastonite, one of the CaSiO3 polymorphs, is characterized by polytypism. In the predominant polytype, the structure is formed by four layers, one of which is composed of [Si309] rings of three tetrahedra (Yamanaka and Mori 1981). The structure thus consists of alternate layers of 3-membered rings and close-packed metal cations stacked perpendicular to the c-axis.
Isostructural RSiO2N oxynitrides (R=La, Ce, Y) result from the rare-earth/calcium- oxygen/nitrogen substitution, with [SiO2N2] tetrahedra forming [Si306N6] rings (Morgan and Caroll 1977, Morgan et al. 1977, Roult et al. 1984). Each tetrahedron is connected via nitrogen to the other two tetrahedra (Morgan et al. 1977) and the corresponding 29 Si NMR resonance peak, studied in the case of YSiO2N, is situated at -65.3 ppm (Dupree et al.
1988). Similar structures have also been observed along solid solutions between RSiOzN and RA103. In the nitrogen wollastonites, the number of layers is determined by the nature of the R cation and by the Si/A1 ratio: YSiO2N has a 4-layer structure, CeSiO2N a 6-layer structure and Y2SiA1OsN a 2-layer structure (Korgul and Thompson 1989).
4.1.2.6. a-Sialons. The general composition for ct-sialon is MxSil2-(m+n)Alm+nOnNl6-n - or more simply Mx(Si,A1)12(O,N)I6 - with x ~< 2, where M is a modifying cation such as Li +, Mg 2+, Ca 2+ or R 3+ (Hampshire et al. 1978, Park et al. 1980; review article: Cao and Metselaar 1991); or a multication (Ekström et al. 1991, Hwang et al. 1995). As the name indicates, the structure of ct-sialons is derived from c~-Si3N4 (trigonal, P31c) (Marchand et al. 1969). In a-Si3N4, the lower half of the hexagonal unit cell is the same as in [3-Si3N4 (hexagonal, P63/m), and the upper part is related to the lower half by a c-glide plane.
In this way, a large interstitial site is formed (two per unit c e l l ) - instead of the large channel observed in the [3-Si3N4 structure - in which M cations can be introduced, without structural change. They insure the charge balance after partial replacement of Si by A1 and N by O. In the case of rare-earth metal ions Rx, x is equal to m/3 (if v is the valency of the metal M, electroneutrality requires x = ra~v) and the upper limiting compositiõn of
76 R. MARCHAND
a purely nitrided R «-sialon is R2Si6A16N16. The value x=2 has not been achieved, and x decreases as the size of the modifying R 3+ cation becomes larger, from ytterbium to neodymium (Huang et al. 1986a,b).
Y-Si-A1-O-N has been studied the most extensively of the ct-sialon systems (Huang et al. 1983, Sturz et al. 1986, Slasor and Thompson 1987). In this case 0.33 ~<x ~< 1 (Y) and 0.5<~n < 1.5 (O). A Y0.5Si9.75A12.2500.75N15.25 (m = 1.5, n=0.75) yttrium ct-sialon composition, for example, was prepared by hot-pressing a powder mixture of Y203, Si3N4 and A1N at 1750-1900°C. A structure refinement of this Y-sialon composition from X-ray powder profile data showed that the structure is built up of [(Si,A1)(N,O)4] tetrahedra, each Y atom being surrounded by seven (N,O) neighbors in the interstitial sites (Izumi et al.
1982, 1984, Sturz et al. 1986). Cao et al. (1993) showed from neutron diffraction results that A1 and Si atoms are ordered.
Cao and Metselaar (1991) have deseribed the properties of ~-sialons. These ceramics offer good high-temperature mechanical properties, in particular excellent hardness and thermal shock resistance. Two-phase yttrium ~@-sialon composite materials (~-sialons:
Si6_~AlxOxNs_x with 0 ~<x~<4.2) are also of great interest (Cao et al. t992, Van den Heuvel et al. 1996). The [3-phase forms elongated grains in an isomorphic ~-matrix, and because of the analogy to whisker-toughened materials, they are called self-reinforced
Si3N4 (Metselaar t994). Such materials show high fracture toughness, Kic > 8 MPam I/2, and high flexture strengths, > 1000 MPa (Pyzik and Beaman 1993).
The parameters affecting pressureless sintering of a-sialons with rare-earth modifying cations have been specified by O'Reilly et al. (1993). Hwang and Chen (1994) have studied the reaction hot-pressing mechanism of yttrium ~- and Œ-[~-sialons and, recently, Menon and Chen (1995a,b) have reported the densification behavior during the reaction hot-pressing of different c~-Si3N4, A1203, A1N and M oxide powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er and Yb), forming ~-sialon ceramics.
4.1.2.7.
U-phases.
The so-called "U-phase" occurs in rare-earth sialon ceramics as a crystalline grain-boundary phase (Spacie et al. 1988). Its composition corresponds to R3A13+~Si3_xOl>xN2-x with R--La, Ce, Nd, Sm, Dy and Y, and 0 ~<x <~ 1. The structure of the neodymium phase, determined first from X-ray powder data (Käll et al. 1991b), then from Nd3A13.»Si2.5012 »N1.5 single crystals obtained by heating mixtures of Si3N4, SiQ, A1203 and Nd203 at 1550 K under nitrogen (Käll et al. 1991a), is of the trigonal (P321) La3Ga~GeO14 structure type (Kaminskii et al. 1983). It exhibits layers of corner-sharing (Si,At)(O,N)4 tetrahedra intereonnected with AI(O,N)6 octahedra. The larger Nd atoms are located between the tetrahedral layers and coordinate to eight (O,N) atoms forming a distorted cubic antiprism, as in the melilite structure (Belokoneva and Belov 1981).A partially ordered distribution of A1 and Si atoms was deduced from the M-(O,N) bond distanee values.
4.1.2.8.
[3-K2SO4-type oxynitrides.
From a crystallographical point of view, silicate minerals of the olivine group, sometimes called "hexagonal spinels", and compounds crystallizing with the ~-K2SO4 type structure can easily be confused due to similaritiesTERNARY AND HIGHER ORDER NITRIDE MATERIALS 77 in crystal symmetry and unit cell dimensions (orthorhombic, Pnma). The REunSiO3N oxynitrides (R = La, Nd, Sm) (Marchand 1976b) are isotypic with the high-temperature modification of Eu~lSiO4 which is stabilized at room temperature, probably by the presence of(Eu 3+ + N 3-) traces (Marchand et al. 1978). Their structure is of~-K2SO4 type, with isolated [SiO3N] tetrahedra held together by the two kinds of crystallographically independent lanthanide ions. The method used to prepare these divalent europium compounds is quite original. The oxidation-reduction reaction:
3Eu 3+ + N3---+3Eu 2+ + ~N 2,
which was carried out "in situ" enables the use of Eu203 rather than EuO. So, an excess of Si3N4 is introduced in the reaction mixture in order to obtain the following reaction:
3Eu203 + 3R203 + 2Si3N4 135°°c 6REuSiO3N + N2;.
With respect to REuSiO3N compositions, a second substitution N/O leading to isostruc- tural R2Si202N2 oxynitrides, with isolated [SiO2N2] tetrahedra, has not been reported so far.
4.1.2.9. Oxynitride glasses and glass ceramics. Oxynitride glasses, included those containing rare-earth elements as modifier cations, were first observed in the 1970s when an intensive effbrt of research was made on the sintering of Si3N4-based ceramics, either silieon nitride itself or sialon ceramics (Jack 1976).
It was found that addition of a metal oxide, such as MgO or Y203, induces a liquid- phase densification process which results in the formation of a grain-boundary oxynitride glass. The mechanical properties of the nitrogen ceramics at elevated temperatures depend markedly on the amount and characteristics of these intergranular glassy phases which need to be eliminated by crystallization. In particular, they ean deteriorate the high- temperature strength, and the creep and oxidation resistance of the ceramic.
Later, it was realized that oxynitride glasses, which revealed themselves to be more resistant to high temperatures than the eorresponding non-nitrided glasses, were of interest in their own right. Various systems, in particular the Y-Si-A1-O-N quinary system, were extensively investigated with the preparation and study of properties of a wide variety of bulk glass compositions.
The R-Si-AI-O-N rare-earth oxynitride glasses have been generally prepared by melting, then quenching, mixed powder batches of R203, SiO2, A1203 and A1N. Si3N4 was also used as nitrogen souree. High temperatures up to 1700°C are necessary for melting and homogenization which require also a very low oxygen partial pressure to avoid oxidizing the glass. Nitrogen overpressures have been used in some cases (Makishima et al. 1980, 1983, Mittl et al. 1985). Note that the choiee of non-reactive crucibte materials is somewhat limited: molybdenum and boron nitride are the most used materials.
Above all, the characteristies of the glasses are closely related to the amount of nitrogen incorporäted. First of all is the extent of the vitreous domains which, at the
78 R. MARCHAND
Fig.
.~ 950
" I I
g
u. t
x
~ ~ & Y " C a
/ x
$
8 0 0 ~ J X Il Nd X Mg
| . . . , , , ,
i 1 I I
5 I0 15 20
Nitrogen (equivalent % )
11. Variation of Tg with nitrogen content for M-sialon glasses with the same cation composition M:Si:AI=28:56:16 (Hampshire et al. 1985).
same temperature, is smaller in the oxynitride systems than in the corresponding oxide systems, for example in the Y203-SiO2-A1N cut as compared to the Y203-SiQ-A1203 cut.
For oxynitride glasses of the same cation composition but of varying O/N ratios, a wide range of glass properties, e.g. density, glass transition temperature Tg, viscosity, microhardness, Young's elastic modulus, fracture toughness, refractive index, ehemical durability, all increase or improve with increasing nitrogen content, while the linear thermal expansion coefficient decreases. As an illustration, figs. 11 and 12 present the variation of glass transition temperature for different M-sialon glasses (M = Y, Nd, Ca, Mg), and the change in viscosity at two different temperatures for yttrium sialon glasses with the same cation ratios. All the observed changes point out that the replacement of O by N strengthens the glass structure. This is evidence for the structural role played by nitrogen which substitutes for oxygen in the aluminosilicate glass network to produce a more tightly bonded and highly cross-linked structure. The general improvement of the glass properties has been mainly attributed to the nitrogen coordinating to three Si (A1) atoms as compared with only two for oxygen.
Among the rare-earth oxynitride glass systems, yttrium sialon glasses have been the most extensively investigated, certainly because of the interest in Y203 as a sintering aid for sialon ceramics. After the first reports of Mulfinger et al. (1973) and Jack (1976, 1978), extensive compositional studies have been carried out on the entire five component Y-Si-A1-O-N system by Loehman (1979, 1980) and Drew et al. (1981, 1983). Shillito et al. (1978), Milberg and Miller (1978), Messier (1982), Messier and Broz (1982) and