Molecular Routes to Metal Carbides, Nitrides, and Oxides. 2. Studies

1590 cm-1 is consistent with δ(H−N−H).8 The observation of new bands at around 1000−1100 cm-1 could be due to NH2- and N3- groups.9 Reactions ...
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Chem. Mater. 1996, 8, 1222-1228

Molecular Routes to Metal Carbides, Nitrides, and Oxides. 2. Studies of the Ammonolysis of Metal Dialkylamides and Hexamethyldisilylamides David V. Baxter, Malcolm H. Chisholm,* Gennaro J. Gama, and Vincent F. DiStasi Departments of Physics and Chemistry, Indiana University, Bloomington, Indiana 47405

Andrew L. Hector and Ivan P. Parkin Department of Chemistry, University College, London University, 20 Gordon Street, London WC1H OAJ, England Received October 18, 1995. Revised Manuscript Received March 19, 1996X

Ammonolysis of metal amides M(NR2)n, where M ) a group 4, 5, or 6 transition metal, M2(NMe2)6, where M ) Mo, W, and metal hexamethyldisilylamides M(N(SiMe3)2)n, where M ) Ti, V, Cr, Mn, Co, Cu, La, Y, and Sn have been carried out in hydrocarbon solvents. The initially formed products are hydrocarbon-insoluble powders wherein the metal retains its original oxidation state. Elemental analysis of these powders reveals a dramatic loss of NR2 ligands as evidenced by relatively low C and H content. Thermogravimetric analysis coupled with mass spectroscopy indicates that these powders readily eliminate amine (HNR2) and ammonia to form metal nitrides upon heating, typically in the temperature range 200400 °C. Upon further heating, redox reactions may occur with the evolution of N2 gas leading to more thermodynamically stable nitrides or the metal, i.e., where M ) Fe, Co, Cu. The nitrogen content of the nitrides has been shown to be derived from ammonia and not from the parent amide. Ammonolyses of mixtures of metal amides have been studied, and the resultant powders have been shown to give, upon heating, either a mixture of the metal nitrides, nitride solid solutions, e.g., (Nb,Ta)N and (Ti,V)N or, for the later transition elements, metal alloys. The metal nitrides and metal alloys were characterized by XRD and SEM/EDAX.

Introduction Transition-metal nitrides, TMNs, find several industrial applications because of their high wear resistance, high melting or decomposition temperatures, high microhardness, and wide range of magnetic and electrical properties.1 The past decade has seen a significant increase in the number of patents related to their production and applications.2 Traditional routes to TMNs involve high-temperature (800-3000 °C) and high-pressure reactions between a metal source, typically the pure metal, or a metal oxide, hydride, or halide, and a nitrogen source such as N2, N2/H2, NH3, or N2H4 or a mixture thereof. These methods may lead to relatively impure materials with the original metal source being a common contaminant. Industrial use of bulk TMNs generally require small particle size and typical ceramic syntheses employing “heat and grind” techniques can lead to agglomerization of particles, along with the incorporation of impurities.3 The potential advantage of molecular routes, such as control of stoichiometry and intimacy of mixing, are Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Toth, L. E. Transition Metal Nitrides and Carbides; Academic Press: New York, 1971; Chapters 1-3. (2) See for example: (a) Hiroshi, K.; Kitabataka, M.; Yamazaki, O. U.S. Patent 4,877,677. (b) deZaldivar, J. S. Eur. Patent 0,416,707. (c) Brown, G. M.; Maya, L. U.S. Patent 4,758,539. (d) Walther, J.; Wasowicz, Int. Patent WO 91/18740. (e) Haddix, G. W.; Bell, A. T.; Reimer, J. A. J. Chem. Phys. 1989, 93, 5859. (3) Robinson, A. L. Science 1986, 223, 25. X

S0897-4756(95)00499-6 CCC: $12.00

often espoused by chemists in the literature. Studies of the thermolyses of Ti(NR2)4 compounds revealed that carbonitrides of titanium were formed with carbon atom percent ranging from 30 to 50%, this being somewhat dependent upon alkyl chain and the availability of βand γ-C-H groups that are presumably responsible for the formation of the TiC bonds during the decomposition reaction.4 Hoffman and Gordon5 have studied the atmospheric pressure use of M(NR2)4 (M ) Ti, V, Sn; R ) Me) for the production of high-purity films of metal nitrides by the addition of NH3 as a coreactant. The presence of NH3 suppresses the incorporation of carbon into the thin films and metal nitride thin films prepared in this manner are of use in the microelectronics industry. This work has been complemented by mechanistic studies of the film growth employing Ti(NMe2)4 and NH3 by DuBois at AT & T.6 Brown and Maya were, in 1988, the first to study the reaction between Ti4+, Zr4+, and Nb5+ dialkylamides and ammonia.7 Thermolyses of the products of these reactions yielded high-purity bulk metal nitrides in the (4) (a) Bu¨rger, H.; Neese, H. J. J. Organomet. Chem. 1970, 21, 381. (b) Dyagilva, L. M. Zh. Obsch. Khim. 1984, 54, 609. (5) (a) Hoffman, D. M. Polyhedron 1994, 13, 1169. (b) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. Chem. Mater. 1991, 3, 1138. (c) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. Mater. Res. Soc. Proc. 1990, 168, 357. (d) Gordon, R. G.; Hoffman, D. M.; Riaz, U. Chem. Mater. 1992, 4, 68. (e) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. J. Am. Chem. Soc. 1990, 112, 7833. (6) Dubois, L. H. Polyhedron 1994, 13, 1329. (7) Brown, G. M.; Maya, L. J. Am. Ceram. Soc. 1988, 71, 78.

© 1996 American Chemical Society

Ammonolysis of Metal Amides

Chem. Mater., Vol. 8, No. 6, 1996 1223

temperature range 850-900 °C. In most cases the metal was in the +3 oxidation state in the resultant nitride. Since that time, no comprehensive study has been undertaken on the ammonolysis of metal dialkylamides. Herein we report such a study, along with the ammonolysis of metal hexamethyldisilylamides and some mixed-metal dialkylamides. The questions that we specifically wish to address are (i) what are the nature of the initial products of ammonolysis? (ii) At what stage in the reaction does redox occur? (iii) Can new mixed metal ternary nitrides be prepared by these methods? (iv) Is ammonolysis of metal dialkylamides/ hexamethyldisilylamides a viable low-temperature route for the synthesis of high-purity metal nitrides? Results and Discussion Synthesis and Studies of Reactions: Metal Amides. Ammonolyses were performed in modified Fischer-Porter bottles. The ideal ammonolysis reaction was taken to have the form of eq 1.

M(NR2)n + (n/3)NH3 f MNn/3 + nR2NH

(1)

In reaction 1 each NH proton is used to liberate an equivalent of amine, R2NH, and no redox reaction occurs. To ensure sufficient ammonia was present, a 100-250% excess amount of NH3 was added, i.e., 2-5 times that required based on eq 1. The ammonia was condensed into the reaction vessel from a vacuum manifold with the metal amide M(NR2)n being dissolved in pentanes or hexanes (R ) SiMe3, Me, Et, Pr) or THF (M ) Cu, R ) SiMe3). Upon addition of NH3 at l-N2 temperature (-196 °C), the flask was allowed to warm to room temperature and the solution was stirred with the aid of a magnetic spin bar. The ammonolyses reactions typically proceeded rapidly to give brightly colored precipitates, with reaction rates being significant at -30 °C. Notable exceptions to this general statement were the ammonolyses of the M2(NMe2)6 compounds (M ) Mo and W) which required a few days at room temperature to go to completion. An attempt was made to study the reaction between ammonia and M(NMe2)n compounds, where M ) Ta, n ) 5 and M ) Ti, n ) 4, by 1H NMR spectroscopy in toluened8. However, at ca. -30 °C a rapid reaction occurred leading to copious precipitates and HNMe2 as the only detectable species present in solution. When stoichiometric amounts of NH3 were employed, the reaction was incomplete, with excess N(NR2)4 still in solution as shown by 1H NMR. The finely divided precipitates so formed were collected by filtration, washed with the pure solvent employed in the reaction, and dried at 45-55 °C under a dynamic vacuum, 10-2 Torr, for 2 h. The colorless filtrate was also collected and analyzed by GC/MS. The latter revealed only ammonia and amine R2NH. From reactions employing ND3 and 15NH /14NH mixtures, we found no evidence by GC/MS 3 3 of nitrogen exchange by the amine. The only reaction determined in these studies was that ammonolysis involved the transfer of the NH protons (deuterons) from ammonia to the amide nitrogen with the liberation of R2NH (or R2ND). The brightly colored precipitates were collected and submitted for microelemental analysis for carbon, hy-

Table 1. Amides and Their Ammonolysis Products amidesa

color and composition of the precipitateb

Ti(NMe2)4 Ti(NMeEt)4 Zr(NMe2)4 Zr(NMeEt)4 Hf(NMe2)4 V(NMe2)4 Nb(NMe2)5 Ta(NMe2)5 Cr(NEt2)4 Cr2(NEt2)6 Cr(NiPr2)3 Mo2(NMe2)6 W2(NMe2)6 W(NMe2)6 Ti(N(TMS)2)3 V(N(TMS)2)3 Y(N(TMS)2)3 La(N(TMS)2)3 Cr(N(TMS)2)3 Mn(N(TMS)2)2 Fe(N(TMS)2)3 Co(N(TMS)2)2 CuN(TMS)2 Ti(NMe2)4 + V(NMe2)4 Ta(NMe2)5 + Nb(NMe2)5 Cr(N(TMS)2)3 + Fe(N(TMS)2)3 Fe(N(TMS)2)3 + 18CuN(TMS)2 16.7 CuN(TMS)2 + Sn(N(TMS)2)2 3.7 CuN(TMS)2 + Sn(N(TMS2)2 3Mn(N(TMS)2)2 + 7Co(N(TMS)2)2

orange: TiC1.2N1.2H4 red-orange: TiC1.5N2.4H6.1 white: ZrC1.1N1.3H4 pale-yellow: ZrC1.3N1.9H4.9 white: HfC1.1N1.9H4 black: VC1.1N2.1H4.1 orange: NbC0.8N1.7H4.2 yellow: TaC0.8N2.4H4.3 red-purple: CrC2.4N2H6.8 pink: CrCN2.5H6.1 light-pink: CrC0.7N2.3H6 brown: MoCNH2.5 brown: WC0.8NH4.1 red-brown: WCN1.1H5.1 bluish-gray: TiC3H8.1N1.1Si light-brown: VC3.1H8.3N1.2Si white: YC3H7.8N2.0Si white pink: CrC1.6H7.3N1.4Si0.6 light-pink black: FeC2.8H7.7NSi1.5 navy-blue: CoC0.7H3.5N0.5Si0.3 copper-like: CuC0.6H3.1N0.3Si0.1 violet: VTiC1.3N3H6 red-orange: NbTaC1.8N2.7H6 black dark-brown copper-like light-orange greyish-blue

a TMS ) SiMe . b Composition estimated from C, H, N micro3 elemental analysis.

drogen, and nitrogen content. These results are listed in Table 1. The estimated composition of the precipitate reveals a dramatic decrease in carbon content. Typically the C:N ratio is less than 1 and the N-to-M ratio ca. 2:1 with the H-to-C or -N ratio in the range 6:1 to 4:1. It is, however, worth noting that the microanalytical data that form the basis of information reported in Table 1 are not necessarily very reliable inasmuch as the powders may be viewed as preceramic materials and combustion analysis may not reveal total nitrogen or carbon content. For example, microelemental analysis of a commercial sample of CrN (99% purity) yielded a 15.5% nitrogen content, whereas that calculated is 21.2%. Furthermore, the precipitates are labile to loss of amine and ammonia as determined by TGA/MS studies at temperatures in the range 70-120 °C. Thus during the sample shipping (sealed vials under vacuum) and handling period (up to 2 weeks) some further mass loss may have occurred from some samples and not from others. It shall also be taken into account that the molecular formula of a ceramic does not necessarily reflect the actual composition of the material. Instead, it represents a possible composition range. For instance, TiN represents several compounds with formula TiN1(x (x < 0.33). Magnetic susceptibility measurements for the powders obtained from Ti4+, Zr4+, Nb5+, and Ta5+ amides showed these to be diamagnetic, thus indicating the conservation of the oxidation state of the metal centers, while those obtained for the powders from V4+ and Cr4+ amides were paramagnetic. A valency determination on the ammonolysis product of Cr(NEt2)4 gave an average oxidation state of Cr as 3.7, i.e., close to 4.0, consistent with our assertion that ammonolysis does not involve redox reactions as evidenced by only the presence of R2NH and ammonia in the filtrates.

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Figure 1. TGA curve (top) and emission profile (bottom) for the derivative of the ammonolysis of Ti(NMe2)4.

The precipitates formed by ammonolysis, including those employing ND3 and 90% HN3 + 10% 15NH3, were studied by FTIR. The reaction with NH3 generates a product with two bands at ca. 3300 cm-1 which may be taken as consistent with the presence of NHx groups in the precipitate (cf. νas + νs of NH3). Also absorption at ca. 1590 cm-1 is consistent with δ(H-N-H).8 The observation of new bands at around 1000-1100 cm-1 could be due to NH2and N3- groups.9 Reactions employing ND3 caused the higher energy N-H absorptions (ν and δ) to shift to 2400 (8) Nakamoto, K. Infrared and Raman Spectra of Inorganicc and Organic Compounds, 3rd ed.; J. Wiley: New York, 1978. (9) (a) Chong, A. O.; Oshima, K.; Sharpless, K. B. J. Am. Chem. Soc. 1977, 99, 3420. (b) Chatt, J.; Dilworth, J. R.; Leigh, G. J. J. Chem. Soc. A 1970, 2239.

and 1460 cm-1. The band potentially assignable to ν(M-N) at 630 cm-1 shifted to 586 cm-1 upon reaction with ND3. The reactions employing a 9:1 mixture of 14NH :15NH caused a reduction in the intensity of the 3 3 band at 1000 cm-1 and the appearance of a new absorption at 877 cm-1. Attempts to interrogate the nature of the precipitates by CPMAS 15N NMR spectroscopy were uninformative, indicating that the ammonia nitrogen is present in the ppt as H-poor fragments such as N3- and NH2-. The precipitates are completely insoluble in hydrocarbon solvents and even in donor solvents such as pyridine. However, in pyridine-d5, while the precipitate derived from Ti(NR2)4 + NH3 remained insoluble, some amine (HNEt2 or HNMeEt) was observed in solution by 1H NMR spectroscopy. This suggests that the pyridine-

Ammonolysis of Metal Amides

d5 displaces some coordinated amine but otherwise fails to dissolve the oligomeric species formed upon ammonolysis. When the pyridine-d5 solution was evaporated to dryness, some red solid was obtained, which implies that while most of the precipitate is highly oligomeric and insoluble in pyridine-d5, some smaller oligomers are soluble. Desorption mass spectroscopy (EI and CI) on the pyridine-soluble extract of the Ti(NMeEt)4 ammonolysis product gave three significant ions of high mass, namely, m/z ) 923, 940, and 1295, each corresponding to a different product identified in the chromatogram. The mass spectra do not display any identifiable fragmentation pattern. Collectively these results imply that ammonolysis yields a mixture of polymeric/oligomeric products containing relatively small amounts of amine, along with NH3, NH2-, NH2-, and probably N3- groups. Bearing in mind that the pKa of NH3 is lower (more acidic) than R2NH, the displacement of R2N and formation of NH2 is readily understood. The reaction is directly analogous to hydrolysis of alkoxides, and the conversion of the amido group NH2- to NH2and ultimately N3- by proton transfer parallels that of the conversion of hydroxyl groups to oxo groups.10 Thermogravimetric Analysis, TGA, of the Precipitates. The finely divided powders obtained upon ammonolysis were examined by a TGA/MS system installed inside a purified, O2-free He drybox assembly. A He flow of 40-50 sccm was used for all samples. The TGA curve for the thermolysis of the product from the ammonolysis of Ti(NMe2)4 is shown in Figure 1 and serves as a general example of this group. The initial decomposition starts at around 70 °C and ends at ca. 400 °C and is due to two distinct processes as shown by MS. The first is the elimination of ammonia and the second is the elimination of HNMe2. These two steps are distinct being separated by approximately 50 °C. Following the first decomposition phase, a plateau is observed. The residual mass in the plateau, when compared to the composition of the initially formed precipitate, suggests the formation of a nitride containing the metal in its original oxidation state (Ti3N4, Zr3N4) for group 4 metals. No equilibrium crystalline compound of this composition exist in these systems; however, this stoichiometry has been proposed as an intermediate in the CVD of TiN films using Ti(NMe2)4 as a precursor and a He/NH3 carrier/reagent gas mixture.5a,6 X-ray diffraction of the material after this first plateau reveals no crystalline material. Analysis of the residue from this stage, after further baking for 45 min at 650 °C, by SEM/EDAX and microanalysis using Sn as a combustion aid revealed that the blue residual solid is composed mainly of Ti and N (N:Ti ) 1.2:1) with a trace of carbon. Following the first plateau, a second decomposition step is observed at higher temperature. This second step involves the elimination of N2 as determined by MS (m/z(N2+) ) 28, I ) 100%; m/z(N22+) ) 14, I ) 50% as unique emissions). X-ray diffraction studies of the material at this stage reveals cubic TiN (a ) 4.230(2) Å) as the dominant crystalline phase in the Ti system. For the Zr system, cubic ZrN (a ) 4.544(7) Å) is seen (10) As in the hydrolysis of metal alkoxides to yield metal oxides: Brinker, C. J.; Scherer G. W. Sol. Gel SciencesThe Physics and Chemistry of Sol-Gel Processing, Academic Press: New York, 1990; Chapters 1-3, 14.

Chem. Mater., Vol. 8, No. 6, 1996 1225 Table 2. TGA Data for the Derivatives of Groups 4-6 Metal Amides

amide Ti(NMe2)4 Ti(NMeEt)4 Zr(NMe2)4 Zr(NMeEt)4 Hf(NMe2)4 V(NMe2)4 Nb(NEt2)4 Nb(NMe2)5 Ta(NMe2)5 Cr(NiPr2)3 Cr2(NEt2)6 Cr(NEt2)4 Mo2(NMe2)6 W2(NMe2)6 W(NMe2)6

residue after the residue after the first plateau 2nd mass loss to 1st mass loss (starts) - form MN (calc) (calc)a - found (%) ends (°C) found (%) M3N4 (80) - 76 M3N4 (63) - 65 M3N4 (87) - 80 M3N4 (79) - 83 M3N4 (89) - 86 M3N4 (72) - 80 M3N4 (79) - 82 M3N4 (85) - 72b Ta3N5 (89) - 90 MN (66) - 66 MN (62) - 58 MN (57) - 61b MN (88) - 77 MN (93) - 88 MN (91) - 88b

(350) - 705 (400) - 690 (370) - 712 (418) - 759 (400) - 715 (350) - 460 (365) - 708 (400) - 690 (400) - 795 (325) - 581 (331) - 584 (335) - 585 (400) - 440c (380) - 663 (355) - 675

MN (74) - 67 MN (57) - 49 MN (81) - 80 MN (76) - 79 MN (87) - 81 MN (67) - 70 MN (75) - 70 MN (75) - 54 MN (85) - 84 M2N (59) - 61 M2N (56) - 50 M2N (51) - 48 M2N (82) - 66 M2N (90) - 83 M2N (88) - 85

a Calculated from the composition of the precipitate formed upon amonolysis given in Table 1. b Emission of N2 is observed at low temperatures, following the elimination of amine. c The plateau for this derivative is ill-defined due to the almost immediate transition from MoN to Mo2N, with concomitant loss of N2.

along with a second cubic phase (a ) 5.098(3) Å). An analysis of the variation of peak width with peak position reveals that the grains in both these materials to be smaller than 200 Å with the root-mean-square in homogeneous strains being less than 0.7%.11 The ammonolysis of the group 5 metal amides M(NMe2)n also proceeded in a two-step process. In each case the TGA indicated a mass loss between 70 and 400 °C wherein Me2NH, NH3, and some N2 were eliminated. The size of the mass loss at 400 °C suggests that the intermediate phase was M3N4 or M3N5, but again the intermediate product showed no crystalline products in an XRD scan. Upon further heating (through 690 °C for M ) Nb and 795 °C for M ) Ta) N2 was evolved and cubic MN phases formed (NbN a ) 4.390(5) Å, L ) 220(20) Å and TaN a ) 4.365(5), L ) 230(40) Å, where a is the lattice constant and L is the estimated grain size). In some cases small traces of unidentified impurity phases could also be seen and, of course, XRD studies have very limited sensitivity to the presence of amorphous impurity phases. The ammonolysis products of Cr(NPri2)3, [Cr(NEt2)3]2 and Cr(NEt2)4 upon heating yielded in each case CrN. In the case of Cr(NEt2)4, N2 was eliminated as one of the volatile components upon heating (∼200 °C). CrN was identified by XRD as the dominant crystalline product of decomposition of the ammonolysis precipitate from Cr(NEt2)4 precursor. The ammonolyses of M2(NMe2)6(MtM) and W(NMe2)6 were slow in solution, requiring several hours. Again the products upon heating were MN and in the case of W(NMe2)6, N2 was identified by mass spectroscopy during the initial heating process. Upon further heating (380 °C, M ) Cr; 450 °C, M ) Mo; 670 °C, M ) W) an additional loss of N2 is observed as the MN compounds are converted to M2N. Metal Hexamethyldisilylamides. Ammonolysis of M(NSi2Me6)n compounds allows us to study related compounds that are not known for NR2 ligands. In this (11) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1974.

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Figure 2. TGA curve (top) and emission profile/MS (bottom) for the derivative from the ammonolysis of Ti(N(TMS)2)3.

way we compared the ammonolysis of a series of M3+ complexes where M ) Y, La, Ti, V, Cr, and Fe. The studies paralleled those of the amides discussed earlier. Heating the insoluble precipitates led to loss of NH3 and some HNSi2Me6 leading to MN (M ) Y, La, Ti, V, Cr) or Fe3N and upon further heating by loss of N2 to Cr2N or Fe(m). The data are summarized in Table 3. The TGA and emission (MS) profile for the ammonolysis product of Ti(NSi2Me6)3 are shown in Figure 2. Ammonolysis of the later transition metal hexamethyldisilylamides (Mn2+, Co2+, Cu+) yielded thermally unstable precipitates that readily evolved NH3 and HNSi2Me6 followed by a short-lived plateau in the TGA prior to loss of N2 and formation of the pure metal. Again these data are summarized in Table 3. Cu3N is detected as an intermediate in the formation of Cu(m). For manganese the expected decomposition temperature of Mn3N2(η-phase) is below 300 °C (P(N2) , 1 atm) leading to Mn4N(-phase) which then decomposes at ca. 470 °C to yield Mn(m) and N2.11 (12) Massalski, T. B., Ed. Binary Alloy Phase Diagrams; ASM International: New York, 1990.

Table 3. TGA Data for the Derivatives of M(NSi2Me6)x Compounds

compound

first plateau (starts) - ends (°C)

Y(N(TMS)2)3 La(N(TMS)2)3 Ti(N(TMS)2)3 V(N(TMS)2)3 Cr(N(TMS)2)3 Fe(N(TMS)2)3 Mn(N(TMS)2)2 Co(N(TMS)2)2 CuN(TMS)2

(438) - 1000 (390) - 1000 (300) - 1000 (270) - 1000 (400) - 580 (250) - 720a (350) - 460 no low-T plateau (190) - 231

a

phase present in the first plateau YN LaN TiN VN CrN Fe3N Mn4N Cu3N

second plateau (starts) ends (°C)

end product of TGA

(590) - 1000 730 - 1000 (883) - 1000 (415) - 1000 (455) - 1000

YN LaN TiN VN Cr2N Fe(metal) Mn(metal) Co(metal) Cu(metal)

Loss of N2 occurs during the first weight loss (below 250 °C).

Cobalt nitrides are very unstable when the oxidation state of the metal is below +3. Upon heating, elimination of HNSi2Me6, NH3, and N2 is observed at low temperatures, 100-190 °C, from the precipitates formed upon ammonolysis. A substance of composition Co3N2, based on TGA mass balance and mass conservation, is first reduced to Co3N which then decomposes at 400 °C to Co(m).11

Ammonolysis of Metal Amides

Chem. Mater., Vol. 8, No. 6, 1996 1227 Table 4. TGA Results in the Formation of Mixed Metal Nitrides and Alloys

mixed nitride

temp of formation of the nitride (°C)

Ti, Nb, Tab Cr, Tib Nb, Tib Nb, Crb Cr, Fed

Ti0.32V0.68Ne (Nb, Ta)Ne

1085 850

Cr2N, TiN NbN, TiN NbN, Cr2N (Cr, Fe)Nx

371

Cu, Mnb (19:1) Cu, Feb Cu, Sn (3.7:1)c Cu, Sn (16.7:1)c Mn, Cob

a a a a a

413 260 350 260 346

parent metals Vd

Figure 3. X-ray diffraction patterns from ammonolysis of mixed M(NMe2)x complexes. The upper curve shows a case (M ) Nb, Ta; x ) 5) where a solid solution of NbTaN was formed (all peaks index to the cubic nitride phase except for the two small peaks at 3.88 and 3.15 Å, which correspond to a trace of TaO as an impurity phase). In the lower curve (M ) Ti, V; x ) 4) the ammonolysis leads to a physical mixture of TiN and VN.

Mixed-Metal Nitrides and Alloys. The ammonolysis of a mixture of two-metal dimethylamido complexes can lead to two limiting situations. First, each dimethylamido complex could undergo ammonolysis independently, leading to the formation of a physical mixture of the products derived from the individual ammonolyses. Second, an ammonolysis product containing both metal species within the same phase (a mixed-metal product) could be formed. X-ray diffraction measurements can distinguish between these two cases since the mixed-metal product would appear as either a different lattice constant for a given phase (for a solid solution) or as a new set of diffraction peaks (for a distinct compound), whereas the former case naturally would show a superposition of individual diffraction patterns. These two limiting cases are exhibited by the system (Nb,Ta)N and (Ti,V)N, respectively, as demonstrated in Figure 3. In the Nb-Ta system reduction of the metal centers leads directly to a (Nb,Ta)N solid solution (a ) 4.395(4) Å). Interestingly this cell constant is larger than that of NbN (the larger of the two mononitrides). This indicates the presence of some impurities in the material (or a deviation from ideal stoichiometry) and makes it impossible to estimate the metal ratio on the basis of X-rays. In contrast to this, reduction of the Ti/V mixture leads to a physical mixture of TiN (a ) 4.233(3) Å) and VN (a ) 4.128(1) Å) which yields a solid solution V0.62Ti0.38N (a ) 4.168(2) Å) through solid-state reaction only upon prolonged (1.5 h) annealing above 1100 °C. The composition given for the VTiN solid solution is derived from the given lattice constant and assuming that Vegard’s law holds between the measured lattice constants for the individual (nonstoichiometric) nitrides and thus has an uncertainty of at least 10%. The disparate metal radii in the Ti/V system (10% in contrast to the mere 2% difference in the Nb/Ta system) could account for the greater kinetic barrier for

alloy none none none none none bcc (R- or δ-) Fe or (R-CrFe)f R-Cu(Cu95Mn5) Cu0.96Fe0.04 Sn21Cu79 Cu17Sn Mn3Co7

temp of formation of the alloy (°C)

820 606 663 522 662

a A N-rich phase exists, as shown by TGA and MS (loss of N ). 2 This phase is amorphous and cannot be characterized by XRD. b Maximum TGA temperature set to 1000 °C. c Maximum TGA temperature set to 680 °C. d Maximum TGA tempeature set to 1100 °C. e New phase. f R-CrFe represents an alloy, rather than a phase, with composition Cr1.16Fe. Due to the large solubility of Fe in Cr, no systematic nomenclature exists for the several possible alloys.

the formation of the mixed metal nitride in the former case. We note with interest that, despite this contrasting behavior, both the Ti/V and Nb/Ta systems show TGA curves that are simple superpositions of the curves found for the individual metal systems. Thermolysis of a mixture of late TM silylamides (Cu/ Fe-N and Mn/Co-N) leads to metallic phases. In the case of Cu/Fe (which exhibits very limited terminal solubility at both ends of the phase diagram), the diffraction pattern of the final product shows fcc Cu (a ) 3.6167(3) Å, consistent with no more than 3% Fe in solution). In this case the precursors were mixed in a roughly 9:1 (Cu:Fe) ratio, and no direct evidence for the Fe was seen. In contrast to this, the primary final product in the thermolysis of the Co/Mn system (which exhibits considerable solubility at each end of the phase diagram) is a solution of 36 ( 5% Mn in fcc Co (a ) 3.5756(3) Å). This corresponds quite closely to the roughly 2:1 ratio in which the precursors were mixed. Concluding Remarks Ammonolysis of transition-metal amides and silylamides along with tin, yttrium, and lanthanum silylamides in hydrocarbon solvents yields insoluble precipitates which upon heating yield metal nitrides or, in the case of the later transition elements and tin, the metal. For the early transition metals, group 4 and 5, and for Y, La this procedure provides a relatively lowtemperature route for the bulk production of an fine grained (∼30 nm or less) metal nitrides. Care must be employed to exclude H2O and O2 during the handling of the metal amides/silylamides since this can lead to contamination by metal oxide. The nitrogen of the nitrides arises from the ammonia as was found in the production of TiN thin films by CVD from Ti(NMe2)4 and ammonia.6 Ammonolysis of mixtures of different metal amides/ silylamides gives rise to a variety of products including mixtures of the individual metal nitrides and either nitride or metallic solutions depending upon the selection of the metals. Evidence for kinetic control is seen in the formation of cubic-NbN and cubic-TaN rather than the thermo-

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dynamically more stable hexagonal phases. Cubic TaN (δ-TaN) is normally prepared by a high temperature (ca. 2000 °C) and high pressure (5 to 100 000 atm) modification of the more stable hexagonal TaN. Even though less drastic conditions are required to convert hexagonal NbN to cubic-NbN (δ-NbN), classical methods lead almost exclusively to the formation of the hexagonal form.13 In the ammonolysis of (tBuCH2)3TadCHtBu laddered “M2N2” units were observed.14 It may be that similar structural motifs are favored in the ammonolysis of M(NMe2)5 and that upon heating these favor the kinetic formation of fcc-MN. Further speculation is, however, not warranted based on the present data at hand. The original oxidation state of the metal center in the precursor is a significant factor regulating the temperature at which a nitride is formed. For instance, TiN is only formed at ca. 700 °C when a Ti4+ precursor is used, while this temperature drops to ca. 300 °C when a Ti3+ precursor is formed. Experimental Section All reactions were performed in oven-dry (150 °C) glassware by using Schlenk or drybox techniques under a dry N2 atmosphere. Solvents were distilled from Na/benzophenone ketyl and stored over 4 Å sieves or CaH2. Pyridine was distilled from CaH2 under N2. Electronic grade NH3 was used for all ammonolysis reactions. Transition-metal and other metal amides or silylamides were made according to published procedures15 and were purified by sublimation/distillation and/or crystallization. They were also characterized by 1H and 13C{1H} NMR spectroscopy (where appropriate) and MS and IR spectroscopy. FTIR spectra were obtained on a HP-FTIR spectrometer (4000-400 cm-1) with the sample in pressed KBr pellets. GC/ MS were performed on a HP 5890 chromatograph with a Supelco SPB5 column, coupled in tandem to a HP5971 massselective detector. NMR spectra were recorded on a Varian XL 300 spectrometer. Thermograms were obtained by use of a duPont 951 Thermobalance under a He flow of 40-50 sccm. Volatile products of thermolysis were analyzed by realtime sampling of the TGA’s hot chambers atmosphere through a gas side-bleeding connected to a VG Microgram PC-quadrapole mass spectrometer. MS data were analyzed by employing Microsoft Exel 5.0 for Windows. Elemental analysis were obtained from Oneida. Volatile byproducts of thermolysis obtained in the 40-450 °C range were also collected over frozen (-196 °C) deuterated

Baxter et al. solvents for analysis by NMR and GC/MS. X-ray powder diffraction (XRD) were collected from samples on acrylic holders in a Scintag XDS-2000 diffractometer employing Cu KR1 radiation. XRD data were treated by using the Scintag GRAPHICS software. Peak positions were calculated by line fitting of a background-corrected diffractogram and were compared with the JCPDS files.16 Particle sizes were determined from XRD line widths by employing the Scherer equation. Ammonolysis Reactions. These were performed in a modified Fischer-Porter flask. The flask is built with heavywall glass and is divided into two parts connected by a flat O-ring joint. The top part is connected to a Kontes valve through which solvent or gases can be transferred into the flask. An excess of NH3 was condensed over the frozen solutions (-196 °C) of the metal amides/silylamides in hexanes, pentanes, or aromatic solvents. Ammonolysis of Cu(N(SiMe3)2) was carried out in tetrahydrofuran. The systems were kept sealed at -20 °C in a freezer for 24 h prior to warming to room temperature. A typical procedure is given below. Ti(NMe2)4 + Excess NH3. Ti(NMe2)4 (0.2 g, 0.86 mmol) was dissolved in n-pentane (45 mL) in a 100 mL modified Fischer-Porter bottle. The solution was frozen at -196 °C and the flask evacuated until the pressure was less than 5 mmHg. NH3 (97 cm3, 4.33 mmol) was then condensed into the flask and the Kontes valve closed. The flask was placed in the freezer for 24 h at -20 °C before warming to room temperature. A bright orange precipitate was formed above which a clear supernatant mother liquor was present. The precipitate was collected by filtration, washed with pentane and dried at 45-55 °C for 2 h under a dynamic vacuum (10-2 Torr). Samples were then taken for microelemental analysis and TGA. The supernatant liquid was examined by GC/MS revealing the presence of only Me2NH and NH3. Reaction employing ND3 and 15NH3/NH3 (isotopic NH3 being purchased from Cambridge Isotopes) were carried out similarly. A summary of the characteristics of the precipitate is given in Table 1.

Acknowledgment. We thank the National Science Foundation for financial support and the Indiana University Office of Graduate Research and Development for the purchase of the Scintag XDS-200 diffractometer. G.J.G. thanks the Conselho Nacional de Desenvolvimento Cientifico e TechnologicosCNPq, Brazil, for a graduate fellowship. CM950499+

(13) Cullity, B. D. Elements of X-Ray Diffraction; Addison-Wesley: Reading, MA, 1967. (14) The reaction between TaCl5 and LiNH2 at 300 °C followed by heating to 700 °C also produces cubic TaN: Parkin, I. P.; Rowley, A. T. Adv. Mater. 1994, 6, 780. (15) Holl, M. M. B.; Wolczanski, P. T.; Van-Duyne, G. D. J. Am. Chem. Soc. 1990, 112, 7989.

(16) Chisholm, M. H.; Rothwell, I. P. Comp. Coord. Chem. 1987, 2, 161 and references therein. (17) Joint Committee for Powder Diffraction Standards, International Center for Diffraction Data, Swarthmore, PA, 1992, CD-Rom version.