2~/' (CvK~)
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5. Nitride halides and nitride sulfides
5.1. Ternary and higher nitride halides 5.1.1. Nitride fluorides
Nitride fluoride compounds correspond to the idea, expressed by Andersson (1967), to substitute in an anionic oxide network the couple (N 3- + F-) for two 02- in order to make pseudo-oxides. Lanthanide nitride fluorides are known for R = La, Ce, Pr and Gd (Tanguy et al. 1971, 1972, Pezat et al. 1976, Vogt et al. 1989), but with a N/F ratio different from unity. They are cubic solid solutions of general formula RNxF3 3x (0.33 ~< x ~< ~0.5) except for R = Gd where x is strictly equal to 0.33 corresponding to Gd3NF6. They were prepared either by reaction between nitride RN and fluoride RF3, or by heating the fluoride in ftowing ammonia. Attempts to obtain Eu or Tm compounds, the latter being representative of a lanthanide with a small ionic radius, were unsuccessful. Also, to date no one has been able to obtain a Ce w nitride fluoride CeNF, like ThNF or UNF, and similar to ceria CeO2. A neutron diffraction investigation of CeNxF3_3x, which is very sensitive to air, and of PrNxF3-3x with x ~ 0.33 (Vogt et al. 1989) confirmed an anion-excess-fluorite-related structure and revealed that the tetrahedral holes within the fluorite structure are occupied by nitrogen and fluorine, and that additional interstitials are fluorine atoms occupying the position 32f: x,x,x (x=0.41) in the Fm3m space group.
5.1.2. Nitride chlorides, bromides and iodides
Rare-earth nitride chlorides have been reported essentially for the early lanthanides and for yttrium with the three main stoichiometries R2NC13 (R=La, Ce, Pr, Nd, Gd, Y), R3NC16 (R = La, Ce, Gd) and RNC1 where R = Ce, however a great richness of struetural arrangements has already been found.
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 89
Whereas CeNF was not obtained as a cerium ~v nitride fluoride, CeNC1 was shõwn by Ehrlich et al. (1994) to crystallize with the layered tetragonal (P4/nmm) BiOCl-type structure, like ThNC1 (Juza and Sievers 1968).
All the crystal structures of the nitride chlorides have as a common feature that they contain N3--centered R] + tetrahedra, the electroneutrality of the compound being assured by the chlorine ligands. This dominating structural feature, underlined recently by Schleid (1996), is common to (oxy)nitride bromides and iodides and also to nitride sulfides and nitride sulfide chlorides.
Gd3NC16 (Simon and Koehler 1986) contains isolated pairs of Gd4N tetrahedra that share a common edge. A similar cerium compound, formulated Ce6Cl12N2 (Ehrlich et al.
1994), also contains these [RöN2] 12+ edge-sharing units formed by two cerium tetrahedra centered by nitrogen, but the arrangement of chlorine ligand atoms about the tetrahedra is different. La3NCI6 also exists in the same series (Meyer, Lissner and Schleid, unpublished results).
In orthorhombic (Pbcn) c~-Gd2NC13, which was the first reported nitride chloride (Schwanitz-Schüller and Simon 1985), all the nitrogen-containing gadolinium tetrahedra are connected via shared opposite edges to form one-dimensional infinite chains
1 3+
~[R4/2N] , as well as in the isostructural compounds c~-Y2NC13 (Meyer et al. 1989) and Nd2NC13 (Uhrlandt and Meyer 1995). The corresponding nitride chlorides with La, Ce and Pr contain also these anti-SiS2 analogous chains but their symmetry (orthorhombic, Ibam) is somewhat different (Meyer and Uhrlandt 1993, Uhrlandt and Meyer 1995).
Meyer et al. (1989) described a [3-form for Gd2NC13 and Y2NC13 in which the pairs of R4N tetrahedra, thus forming [R2NR2NR2] units, are connected by sharing all four
1 3+
terminal vertices to generate double infinite chains according to oc[(R1)4/4(R2)2/2N] . Save for the presence of nitrogen, [3-Y2C13N is isostructural with the binary yttrium sesquichloride Y2C13.
All the R3NC16 or R2NC13 nitride chlorides are formally R(III) compounds, therefore with empty R d bands and negligible metal-metal bonding. On the other hand, they present strong R-N interactions. However, as noted by Meyer et al. (1989), the borderline between a cluster and a valence compound is not so obvious, in particular in the case of the [3-phases. [3-Y2C13N: which is not colorless as c~-Y2C13N but black; which looks tike Y2C13 in color and in its fibrous nature; and which requires a small amount ofmetal in its synthesis, contrary to the c-form; could be somewhat deficient in nitrogen, i.e. Y2C13NI-»
Thus, a normal phase transition between «- and 13-Y2NC13 does not seem likely.
The borderline between cluster and valence compounds is clearly crossed with the scandium nitride chlorides which are unambiguously cluster compounds. Scandium shows here its difference in behavior, with compounds such as Sc4C16N, Sc7CI12N (Hwu and Corbett 1986) or ScsC18N (Hwu et al. 1987), where nitrogen is an interstitial element.
The nitride chlorides »vere generally obtained from non-stoichiometric proportions of RC13, R metal and NaN3, but nitrides RN were also used as a source of nitrogen. Meyer et al. (1989) have noted the need for metal in the synthesis of [3-Gd2NC13 and [3-Y2NC13 and suggested a sma!l nitrogen deficiency.
90 R. MARCHAND
Mattausch et al. (1996) have recently reported the nitride iodide Ce15N7124 that they obtained as red and transparent needles by reaction at 1050 K of a mixture 8CeI3 + 7CeN in sealed Ta tube. The structure contains two crystallographically different types of nitrogen atoms: not only N atoms tetrahedrally coordinated by Ce atoms, but also N atoms in a triangular environment of Ce atoms. The Ce4N tetrahedra are condensed via opposite edges to form chains. This air and moisture sensitive compound Ce~»NTI24 is paramagnetic (/~eff = 2.55/~ß)-
Mattausch et al. (1996) have mentioned the preparation of other new ternary nitride halides containing cerium and/or bromine with the formula Ce3C16N, Ce3BröN, Ce2Br3N and Gd2Br3N.
The orthorhombic (Fddd) structure of the compound CsxNat-xLagI16N4 (Lulei and Corbett 1996) contains two kinds of lanthanum atoms: La atoms forming infinite zigzag
~[La4/2N] chains of edge-sharing La4 tetrahedra centered by nitrogen atoms (as in ct-Gd2NC13), and "isolated" La atoms that interconnect the chains via common iodine atoms. The alkali-metal atoms are statistically distributed within channels in the structure.
Isostructural compositions ALa9116N4 (A=Na, Rb or Cs) have also been prepared.
Finally, CsPr9NbBrsN6 (Lulei and Corbett 1997) is a recently reported quinary nitride bromide in which a partial substitution of niobium for praseodymium has been achieved, resulting in mixed Pr3Nb(N) tetrahedra.
5.2. Ternary nitride sulfides
Lissner and Schleid (1993a,b, 1994b) have prepared the lanthanide nitride sulfides R2NS3 (R=La, Ce, Pr, Nd, Sm) and Sm4N2S3 from mixtures of metal R, sulfur, and NaN 3 as a nitrogen source (an also SmC13 in the case of Sm4N2S3), in the presence of NaC1 as a flux at 850°C in evacuated sealed silica tubes. In addition, Gd2NS3, Tb2NS3 and Dy2NS3 compositions have also been recently reported by Meyer et al. (1997). These nitride sulfides are not water sensitive. In all these compounds, nitrogen atoms are tetrahedrally surrounded by the lanthanide atoms. In the orthorhombic (Pnma) R2NS3 nitride sulfides (R = L a N d ) , these [(N3-)(R3+)4 ] tetrahedra are connected via two corners fbrming linear chains l[N(R~)l/l(R2)l/l(R3)2/2] o r /[NR4] 6+. In the monoclinic (C2/m) Sm4N2S3 compound they share two cis-oriented edges to form chains ~[N(SmL)3/3(Sm2)I/1]3+ or 1 N R 4 3+
~ [ ] . The lanthanide atoms are sixfold or sevenfold coordinated by nitrogen and sulfur, the role of the S 2- anions in the structural arrangement being to assume the charge neutrality and the three-dimensional interconnection.
5.3. Quaternary nitride sulfide chlorides
Several compositions with the early lanthanides are known: R4NS3C13 (R=La, Ce, Pr, Nd, Gd) (Lissner and Schleid 1994a, Schleid and Meyer 1996), RöN3S4C1 (R = La, Ce, Pr, Nd) (Lissner et al. 1996), Pr5N3S2C12 (Lissner and Schleid 1997) and RIoNS13C1 (Meyer and Schleid, unpublished results).
TERNARY AND HIGHER ORDER NITRIDE MATERIALS 91 The R4NS3C13 compounds, like the lanthanide nitride sulfides, are not sensitive to hydrolysis. They were obtained from appropriate molar ratlos ofR metal, sulfur and NaN3, in the presence ofNaC1. Their symmetry is hexagonal (P63mc) and they are isostructural with oxychloride Ba4OC16 (Frit et al. 1970) or oxide sulfide chlorides R4OS4C12 (R = La- Nd) (Schleid 1991, Schleid and Lissner 1994).
The structure is built up from isolated °oo[NR419+ tetrahedra which are interconnected via S:- and C1- anions which assume the charge neutrality.
The chlorine-poor isotypic series of nitride sulfide chlorides R6N3S4C1 (R=La-Nd) were prepared by Lissner et al. (1996) from appropriate molar ratios of metal R, sulfur, NaN3 and RC13 chloride at 850°C. Their orthorhombic (Pnma) crystal structure, determined from single crystal data of the lanthanum compound, exhibits two different chains of connected [NR4] tetrahedra, which are held together by the X-ray diffraction indistinguishable anions S 2- and CI-.
In addition, two new compositions PrsN3 $2C12 and R10NS 13 C1 (R = La-P0, which were only briefly mentioned by Uhrlandt and Meyer (1995) and also recently by Schleid (1996), will be discussed in full detail by Schleid and coworkers. Both contain [R4N] tetrahedra, however, whereas in the first one the [Pr4N] tetrahedra are connected to form chains, isolated tetrahedral units can be found in the second ones. These R10NS~3C1 nitride sulfide chlorides can be structurally compared to the oxysulfide Prl0OS14 (Schleid and Lissner
1991) and to the new quaternary nitride sulfide NaPr~0NS~4 (Schleid 1996).
5.4. Quaternary and higher oxynitride bromides
Quaternary and higher oxynitride bromides are illustrated by the praseodymium(III) compounds Na2Pr4Br9NO (monoclinic, P21/m) and PrsBr3N30 (monoclinic, C2/c), which are structurally and electronically similar to each other (Lulei et al. 1995), and also related to the nitride chlorides «-R2NC13 (R=Gd, Y, L a N d ) reported earlier. Both structures can be described as being built up of infinite zigzag chains of trans-edge- sharing N- and O-centered Pr4 tetrahedra. These chains ~ [Pr4/2 (O,N)] are interconnected by Br atoms. It may be noticed that the mixed N and O interstitials, which play the same role in the structure, are necessary to fulfill Pr(III) valence requirements. Thus, both compounds appear to be normal Pr HI valence compounds. However, Na2Pr4Br9NO is isotypic (hut not isoelectronic) with Na2Pr4CI902 (Mattfeld and Meyer 1994). This shows that these "interstitially stabilized" Pr(III) compounds have structures which lie on the border between salts and clusters.
5.5. Quaternary carbide nitride haIides
Mattausch et al. (1994, 1995) have recently reported several stoichiometries for this new class of compounds: R4XoCN (R=Gd, X=Br; R=La, Gd, X=I), R7|I2C2N (R=Y, Ho) and Y619C2N. All of these compounds are air and moisture sensitive. Gd416CN exists with two modifications.
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The tetragonal (P42/mnm) «-Gd416CN structure, which is also that of La4IöCN and Gd4Br6CN, contains R6 octahedra centered by C~- anionic groups and double tetrahedra centered by N atoms (N3-). The units are alternately connected via common edges to form linear chains [R2tLI/2C2] [R2/2R2/2N]2. In the hexagonal (P6) ~-Gd416CN, which is obtained at 1300 K from the « form, the chains are more densely packed.
The same units, C2-centered octahedra and N-centered double tetrahedra are found in the two isotypic triclinic (P1) compounds Y7It2C2N and Ho7112CN. They exhibit a semiconducting behavior.
Lastly, the structure of Y6IgC2N is composed of chains of pairs of Y-octahedra and Y-tetrahedra, respectively. The octahedra are centered by C2 groups, the tetrahedra by N atoms.