2~/' (CvK~)
3. Ternary and higher (oxy)nitrides
3.1. Cerium compounds
Among cerium-containing nitride-type compounds, ternary nitrides in which Ce is tetravalent are formed with electropositive metals such as lithium or barium. These hygroscopic compounds are essentially ionic in character. In addition, oxynitrides containing cerium and barium are also known. However, surprisingly, no ionic ternary nitride (or quaternary oxynitride) involving other R elements has been described.
3.1.1. Li2CeN2 and Ce:N20
Li2CeN2 is the only known rare-earth lithium ternary nitride. First reported by Halot and Flahaut (l 971), this red-orange, very moisture-sensitive Ce W nitride was prepared by heating, at 600-700°C under nitrogen atmosphere, an equimolecular mixture of the binary nitrides Li3N and CeN. Li2CeN2 is isostructural with the two ternary lithium nitrides of group IV transition metals Li2ZrN2 and Li2HfN2 (Palisaar and Juza 1971, Barker and Alexander 1974, Niewa and Jacobs 1995) (fig. 2). The anti-La203 trigonal (P3ml) structure contains hexagonal close-packed nitrogen atoms with cerium in octahedral coordination forming filled layers of edge-sharing octahedra ~[CeN6/3], as in the CdI2 structure type. Lithium atoms, which occupy all edge-sharing tetrahedral holes, form alternate layers.
The anti-type structure is illustrated by another Ce Iv nitride-type compound, the green cerium oxynitride Ce2N20, analogous to La203, which was obtained by Barker and Alexander (1974) on heating a 3:1 mixture of cerium mononitride and cerium dioxide
A
B
A
B
Fig. 2. Structure of Li2CeN 2 which is isostructural with Li2Zr(Hf)N 2. The solid, open and shaded spheres represent Ce or Zr(Hf), Li and N, respectively (after Niewa and Jacobs 1995).
60 R. MARCHAND
at 1100°C under nitrogen. Th2N20 and Th2N2(NH) are isostructural thorium compounds (Benz and Zachariasen 1966, Juza and Gerke 1968), just as the cerium oxysulfide Ce202S but where Ce is formally trivalent.
3.1.2. BaCeN2
Besides Li2CeN2, Ce(IV) is associated with an electropositive element and nitrogen in BaCeN2. This ternary nitride was prepared by Seeger and Strähle (1994), either from the reaction of Ba3N2 with CeN in the stoichiometric ratio 1:3 at 850°C under N2 atmosphere, or directly from elements. This moisture sensitive product crystallizes hexagonally (P63/mmc) with the anti-TiP structure, like [3-RbScO2 or some other ternary oxides of lanthanides (CsRO2), but the zirconium compound BaZrN2 is structuraIly different (Seeger et al. 1994).
The stacking sequence along [001] can be described as A~AyBaB,/ with an AABB arrangement of nitrogen atoms. The Ce atoms occupy octahedral holes y while the Ba atoms in « and [3 are in trigonal prismatic coordination. N atoms are thus octahedrally surrounded by 3 Ce+3 Bä. MnTa3N4 (Schönberg 1954), Li3TaN4 (Brokamp and Jacobs 1992) and Li3NbN4_« (Tessier et al. 1997) have the same structure but with a partially disordered metal arrangement due to the 3:1 proportion. Let us note that ScTaNI_« and ScNbN~_~ which have also an anti-TiP type structure are nitrided alloys and they have been described as a nitrogen insertion between metal atom layers (Lengauer and Ettmayer
1988, Lengauer 1989).
The distances Ce-N = 242.4 pm in BaCeN2 are shorter than in the Li2CeN2 structure (253 pro) which contains also edge-sharing CeN6 oetahedra. They correspond to the sum of covalent radii while the Ba-N distances are essentially ionic in character: BaCeN2 could thus be considered as a barium nitrido-cerate, formulated Ba2+[CeN2] 2-.
The hypothesis of a second modification of BaCeN2 crystallizing with the «-NaFeO2 structure has been envisaged by Seeger and Strähle (1994).
3.1.3. BaCeR(O,N)4 oxynitrides
Heating BaCeO3 perovskite with RN nitride (R = La, Ce) at 800°C leads to the formation of reddish-colored hygroscopic BaCeR(O,N)4 oxynitride powders, with cerium ions in a mixed-valence state (Liu and Eick 1990). These phases have the same orthorhombic (Pnma) CaFe204-type structure (Wyckoff 1965) as the purely oxygenated compound BaCe204 in which cerium is trivalent. The lanthanide ions are octahedrally surrounded while barium has eight close (O+N) neighbors.
3.2. Ternary and higher nitrides formed with non-metal elements'
The non-metals considered are boron and silicon. The corresponding binary nitrides, BN, either in the graphite-like or diamond-like form, and Si3N4 are well-known as high- performance materials owing to excellent thermal, electrical, mechanical and chemical properties. Their low reactivity explains that ternary nitrides with lanthanides are only
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 61 formed at elevated temperatures. Whereas the ternary silicon nitrides are unambiguously nitrido-silicates with SiN4 tetrahedra, the structure of which presents similarities to framework silicate structures, there are several types of ternary boron nitrides depending on whether they can be compared to borates or to borides.
It can be noted here, and this remark has some general character in this smdy of multinary rare-earth nitride-type compounds, that the tendency toward compound formation diminishes with increasing atomic number of the rare-earth elements.
Ternary rare-earth phosphorus nitrides are not known so far, probably because of the great difference of thermal stability between R-N and P-N bonds.
3.2.1. Ternary and quaternwy silicon nitrides
Ternary rare-earth silicon nitrides have been obtained only with the early lanthanides and with yttrium. Until recently, LaSi3N5 (Inoue et al. 1980, Holcombe and Kovach 1981, Inoue 1985) and Sm3Si6N~I (Gaudé et al. 1983) were the only described compounds.
They were prepared by reacting the corresponding binary nitrides at high temperature.
Single crystals of LaSi3N5 were isolated from a Si3N4/La203 mixture heated at 2000°C under 50bar N2. On the other hand, Thompson (1986) indicated the existence of three yttrium silicon nitrides, YSi3N5 (hexagonal), Y2Si3N6 (structure unknown) and Y6Si3N10 (orthorhombic, pseudo-hexagonal) as a result of firing YN-Si3N4 compositions in nitrogen at 1750°C.
Woike and Jeitschko (1995) have shown that the tetragonal (P4bm) R3 Si6Nll (R = La, Ce, Pr, Nd, Sm) and the orthorhombic (P2~212t) RSi3Ns (R=La, Ce, Pr, Nd) isotypic nitrides could be prepared by reaction of the binary lanthanide silicides RSi2 and the two- phase alloys "RSi3" with nitrogen at 1500°C. From the plot of the different cell volumes, it was concluded that cerium is trivalent in both series of compounds. Ce3Si6Nll was obtained at the same time from cerium metal and silicon diimide Si(NH)2 (Schlieper and Schnick 1995), as well as Pr3Si6Nll (Schlieper and Schnick 1996). Both structure types consist of corner-sharing SiN4 tetrahedra, thus forming covalent three-dimensional anionic networks in which nitrogen atoms determine voids occupied by the trivatent lanthanide ions (fig. 3). So, it is clear that such nitrido-silicates cannot be structurally compared to lanthanide silicon carbides (or silicide carbides) where the silicon atoms taust be considered as anionic.
All the ternary lanthanide silicon nitrides exhibit an excetlent thermal and chemical stability which predestines them for use as high-temperature materials.
Let us add that phase equilibria in three ternary systems R-Si-N (R = Sc, Ce, Ho) have been investigated at 1000°C by Weitzer et al. (1991) from appropriate arc-melted R-Si compositions mixed with Si3N4. No new ternary phase was found. The authm-s mention only a HosSi3Nl_x phase which is a nitrided D88-HosSi3 silicide, of CrB-type structure, with a slightly larger c hexagonal parameter.
Two isotypic quaternary nitrido-silicates, SrYbSi4N7 and BaYbSi4N7, have been recently synthesized by Huppertz and Schnick (1996, 1997) by reaction of silicon diimide with metallic strontium/barium and ytterbium at 1650°C. Their hexagonal structure
62 R. MARCHAND
Fig. 3. Layers of condensed four- and eight- membered rings of corner-sharing SiN 4 tetrahedra in Ce3SiöNI~ as viewed along the [001] direction.
The solid circles are Ce 3+, and the shortest Ce-Ce distances (372 pro) are line-drawn (Schlieper and Schnick 1995).
(P63mc) is also built up from a three-dimensional network of corner-sharing SiN4 tetrahedra in which, for the first time, N atoms connecting four Si atoms have been found.
3.2.2. Ternary boron nitrides
Boron behaves in rare-earth ternary nitrides as a particular element in the sense that it is able to form both BN3 units similar to BO3 units found in borates, and B-B pairs as in borides. Quaternary boro-nitrides analogous to boro-carbides will be discussed in section 3.4.3.4.
Many ternary metal-boron-nitrogen systems have been systematically investigated, just as M - S i - N systems, in order to provide basic information on the thermodynamic phase equilibria and compatibility in these systems. Concerning rare-earth metals, no ternary compound exists with the late lanthanides Tb to Lu and with Sc and Y. Klesnar and Rogl (1990) have established the corresponding thermodynamic phase equilibria at 1400°C and l bar of argon (in the absence of external nitrogen pressure), which are characterized by a stable three-phase equilibrium RN + RB4 + BN (for scandium ScN + ScB2 + BN).
Accordingly, there is no compatibility between boron nitride and the rare-earth metal.
Klesnar and Rogl (1992) have also studied phase relations and phase stabilities at T = 1400°C in the ternary systems R - B - N where R is Nd, Sm or Gd: in this oase ternary compounds are found to be stable. Three different stoichiometries were observed with these early lanthanides: RBN2, R3B2N4 and R15Bs(N,O)25. In addition, Kikkawa et al.
(1997) have recently reported the preparation under high pressure of another orthorhombic
" L a - B - N " phase. All these ternary lanthanide boron(oxy)nitrides are rather unstable under moist conditions.
3.2.2.1. PrBN2-type. The existence of ternary compounds associating nitrogen, boron and a lanthanide was first mentioned by Gaudé (1983) with the RBN2 nitrides (R=Nd, Sm),
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 63
prepared as gray and moisture-sensitive powders at T ~ 1550°C from RN-BN mixtures.
In a more recent study, Rogl and Klesnar (1992) determined the crystal structure of the isostructural PrBN2 compound from single crystals obtained by direct sintering of mixtures of BN and praseodymium metal under nitrogen at 1800°C. The gadolinium compound, which is only observed at temperatures above 1400°C, has also the same type of structure. The corresponding rhombohedral crystal structure (R3c) appears as a combination of planar B3N3 hexagons which are stacked to form infinite columns along the [001 ] direction of the hexagonal unit cell, as in hexagonal BN, and of irregular bipyramidal [Pr»B] units centered by other nitrogen atoms, as in the Ce3B2N4-type. Each boron atom is thus bonded to three N atoms to form a BN3 triangle. Such a nitrogen triangular coordination of boron has also been found in the Cet»B8N25-type.
3.2.2.2.
Ce3B2N4-type.
The R3B2N4 nitrides are the metal-rich lanthanide boron nitrides.They were synthesized with all the large lanthanides R = La, Ce, Pr, Nd (and mischmetal) as isotypic compounds (Rogl et al. 1990). The orthorhombic (Immm) crystal structure was determined for the cerium compound from X-ray and neutron powder diffraction data. As shown in fig. 4, boron atoms are in triangular prismatic coordination [CeöB], forming covalently bonded B-B pairs. In a tetrakaidecahedral surrounding [Ce6B]BN2, each boron atom forms single bonds with two adjacent N atoms. Nitrogen atoms thus are in rectangular pyramidal metal coordination [CesN] with one additional boron atom completing a distorted octahedron [CesBN]. It can be noted that the formation of such a distorted octahedron [Ce»B](N) is a typical structural feature of different lanthanide (or actinide) boron carbides. The structure of Ce3B2N4 reveals a close resemblance to those of the ternary borides CeCr2B6 (written as (CeCr2)B2B4) (Kuzma and Svarichevskaya
1973) and W2CoB2 (written a s ( W 2 C o ) B 2 [ ~ 4 ) (Rieger et al. 1966).
Fig. 4. Crystal structure of Ce3BaN 4 in a three- dimensional view (Rogl et al. 1990).
64 R. MARCHAND
3.2.2.3. Cel»BsN2s-type. The rhombohedral (R3c) crystal structure of the mixed-valence cerium nitride Ce15BsN25, which was determined from single crystal data by Gaudé et al. (1985), can be described as a three-dimensional arrangement of NCe6 metal octahedra linked together by trigonal planar (BN3) units. These planar BN3 units, similar to the (BO3) entities in orthoborates - [BN3] 6- anions are isoelectronic with [BO3] 3- anions - give to the structure its main originality. Isotypic lanthanum and praseodymium compounds are also known (Klesnar et al. 1989). In comparison, remember that the ternary alkali metal boron nitrides MfBN2 (Mr=Li, Na) and also Mg3BN3 are characterized by linear, symmetrical (NBN) 3- ions.
A partial oxygen/nitrogen substitution, with formation of RlsBsN25-xOx oxynitrides, was evidenced in the case of R = L a and Ce (I2Haridon and Gaudé 1985, Klesnar et al.
1989). In particular, in the Lal»BsN1906 (La~sB 8 Nl906 ) composition, O atoms were 3+ 3+ 3- 2- shown to statistically replace N atoms, but within the BN3 units rather than within the La6N octahedra (EHaridon and Gaudé 1985).
3.3. Chromium ternary nitrides
Broll and Jeitschko (1995) have reported the existence of the ternary nitrides Ce2CrN3 and R3Cr~0-xNtl (R = La, Ce, Pf) which are prepared by direct reaction of the corresponding binary nitrides RN and CrN at 900°C (Ce2CrN3) or 1160°C (R3Cr10-~Nll). Existence of a La-Cr-N lanthanum compound had been reported some time ago with the approximate composition La6Cr21N23, superconducting below 2.7 K, which could be also obtained by the reaction of calcium nitride with a mixture of the oxides La203 and Cr203 (Marchand and Lemarchand 198t). The crystal structures of Ce2CrN3 and La3Cr9.24N1L have been determined using single crystals obtained from a Li3N flux.
The orthorhombic (Immm) Ce2CrN3 nitride is isostructural with U2CrN3 and Th~CrN3 (Benz and Zachariasen 1970) and the structure may be considered either as a "filled"
U2IrC2 type structure (Bowman et al. 1971) or as a defect K2NiF4 structure (fig. 5). While the predominantly tetravalent cerium atoms are surrounded by seven nitrogen atoms, the chromium atoms are in a distorted square-planar nitrogen coordination. The CrN4 squares are linked via corner-sharing nitrogen atoms, thus forming infinite, straight -N-CrN2- N-CrN2- chains, corresponding to the formula (Ce4+)2(CrN3) 8-. An isotypic manganese compound Ce2MnN3 has been recently prepared (DiSalvo 1997).
In the face-centered cubic (Fm3m) structure of the Pauli paramagnetic La3Cr9.24NI1 nitride, the lanthanum atoms are coordinated by nine nitrogen atoms while the chromium and most nitrogen atoms form a three-dimensiõnally infinite polyanionic network of corner- and edge-sharing CrN4 tetrahedra.
3.4. Interstitial nitrides. Nitrided alloys 3.4.1. Scandium ternary nitrides
The two scandium ternary nitride phases ScTaNl_x and ScNbNI_« were commonly prepared by nitridation of Sc-Ta (Lengauer and Ettmayer 1988) or Sc-Nb alloys in
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 65
UzlrCz 14/ramm
Ce2CrN3 Immm
Cr
le
I
~K
K2NiF4 J41mmm
Fig. 5. The crystal structure of Ce2CrN~ and its relation to the structures of U2[rC z and KzNiF ~ (Broll and Jeitschko 1995).
a nitrogen atmosphere at temperatures up to 1770K (Lengauer 1989). In the case of tantalum, an impure product could also be obtained from a mixture of ScN and ~-TaN binary nitrides.
The hexagonal structure (P63/mmc) consists of a nearly close packed metal atom arrangement with a stacking sequence ABAC, ABAC, etc., along the c-axis, where A represents the layers of scandium atoms and B and C the layers of tantalum (or niobium) atoms (fig. 6). The nitrogen atoms are located in the interstitial octahedral sites N - 3 S c + 3Ta(Nb), with a random occupancy of 50%.
ScTaNl_x and ScNbNl_x have an anti-TiP structure (Wyckoff 1963) with an ordered arrangement of scandium and tantalum atoms at the phosphorus sites and nitrogen at the titanium sites. They are isostructural with the ternary barium and cerium nitride BaCeN2 (Ba ++ Ta or Nb, Ce +-+ Sc) (Seeger and Strähle 1994), in which all the nitrogen positions are occupied. However, whereas BaCeN2 is a iono-covalent compound, ScTaN~_x and ScNbN[~ contain a eombination of metallic bonds between transition metal and nitrogen and ionic bonds between lanthanide-like IIIB metal and nitrogen.
Besides these golden yellow hexagonal ternary nitrides, gray-blue cubic phases 6-(Ta,Sc)N and 6-(Nb,Sc)N have also been observed in the systems Sc-Ta(Nb)-N. They correspond to a partial solubility of ScN in the fcc high-temperature phases 6-TaN and 6-NbN (Lengauer 1989).
ScN also exhibits a limited solubility in TiN and VN (Aivazov and Rezchikova 1977).
66
I
I 1
r
i
)---(~
I I 1
)
I i I I I I iNb N Sc