Formation of molybdenum nitrides by ammonia nitridation of MoCl5

Journal of Crystal Growth 87 (1988) 295—298 North-Holland. Amsterdam

295

FORMATION OF MOLYBDENUM NITRIDES BY AMMONIA NITRIDATION OF MoCI5 Walter LENGAUER Institute for Chemical Technology of Inorganic Materials, Technical University of Vienna, Geireidemarkt 9, A-1060 Vienna, Austria

Received 11 August 1987; manuscript received in final form 28 September 1987

The formation of molybdenum nitride from MoCl5 and flowing NH3 was investigated at temperatures of 770—1060 K. Up to 910 K the reaction yields single-phase WC-type b-MoN with lattice parameters of a = 0.2851(2) nm and c = 0.2782(3) nm. At T 950 K. a reaction product consisting of the two phases b-MoN + y-MoN1 — was obtained, where the amount of y-phase increased with increasing reaction temperature. The y-MoN1 — lattice parameter is dependent on the reaction temperature, whereas the b-MoN lattice parameters remain unchanged. At reaction temperatures of 1060 K, a reaction product with a conspicuously different. coarse-grained facetted habit was formed — grown probably by a transport mechanism via the gas phase — with sharp diffraction lines and lattice parameters of a = 0.5740(2) nm and c = 0.5624(3) nm, which are significantly larger than the doubled cell parameters of the WC-type MoN. Debye—Scherrer photographs of this nitride suggest that it is isostructural with the recently observed MoN resulting from high-pressure high-temperature treatment of WC-type b-MoN.

I. Introduction Molybdenum mononitride (5-MoN) was first synthesized by Hagg [1] by the reaction of Mo metal powder with flowing ammonia. He indexed the diffraction pattern on the basis of the WC-type structure. Schonberg [2] also used this reaction, but he proposed a different structure model with slight displacements of certain metal atoms from the ideal position, resulting in an elementary cell of MoN with unit cell dimensions (a 0.5725 nm and c 0.5608 nm) twice those of Hagg’s simple hexagonal cell. In a study of the Zr—Mo—N systern Brauer and Leibbrandt [3] reported for a two-phase sample 8-MoN + y-MoN1 (prepared from Mo + NH3 at 1013 K) cell dimensions of a 0.5733 nm and c 0.5620 nm for 6-MoN and a 0.4187(5) nm for y-MoN~eIn several recent studies as well, 6-MoN unit cell dimensions somewhat larger than those given by ref. [1] (doubled) and ref. [2] were reported for MoN made by high-pressure N2 nitridation of Mo foil and powder [4] and NH3 reaction of Mo wire [5,6] or Mo powder [7]. However, none of these studies contain comments on these 8-MoN unit cell discrepancies. This is in so far remarkable, as 6-MoN =

=

-

= =

=

is known to be a compound with a very narrow homogeneity region (<1 at%) [1,2]. One possible explanation is the existence of structurally different MoN phases. In fact, Bezinge et al. [8] described a new modification obtained by high-pressure high-temperature treatment of MoN, which had been prepared by the reaction of MoC15 + NH1 at low temperatures and showed only the WC-type diffraction lines. Quite some time ago Troickaya and Pinsker [9] described several polytypes of molybdenum nitrides, but it is not clear whether these phases, which were observed as thin films and investigated by electron diffraction, are stable in bulk form and whether their appearance was influenced by oxygen. The preparation of MoN requires either very long reaction times (several hundred hours at 600—1000 K [1—3])if done with Mo metal or Mo oxides and NH3, or very high nitrogen pressures if molybdenum is reacted with N2. Since the rates of both reactions are diffusion-controlled, nitride formation takes place within reasonable times only with powdered (.~ 100 ~.tm)or thin film samples. In addition, nitridation of Mo metal to yield MoN is hampered by the extremely high nitrogen equilibrium pressure of 8-MoN. Because of this the

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diffusion rate cannot he accelerated by raising the

temperature. In this study the formation of 6—MoN (char— acterized h~ X-ray diffraction) was investigated using MoCI, and NH 3 as starting materials in order to substantially alter the reaction conditions. No literature data on this subject could he found. For detailed information about the rnol~hdenum nitrogen system the reader is referred to the studies of Ettmayer [10] and Jehn and Ettmayer 1111.

2. Experimental Ammonia nitridation is accompanied h~ the

decomposition of NH 1: 2 NH

-~

3 H + N-,.

~

H

=

80 kJ/inol.

(1)

The nitriding potential of flowing ammonia iS determined by the amount of ammonia that has undergone decomposition at the reaction temperature. If the flowing ammonia is used at atmospheric pressure and the portion x has undergone decomposition. the following relationship between the nitriding potential RT In p(N2) and the ternperature can he applied: RT In p(N7

)

—

~G

3RT In p(

+ 2 R T In p (1

.~

A

A

~

~ 3

-

Fig. I.

(_

ro~ ,ection of the central part of the reaction

urnacc

for preparing MoN: (I) mullite tube: (2) ,ilica tube ~~ith ~lit~: (1 Mo boat u ith the Mo(l bed: (4) thermocouple ga~

has a lower temperature than that in the furnace. the actual sample temperature is differenl from the furnace temperature. There is also a temperature gradient from the bottom plate of the Mo boat to the walls since the edges of the boat are in direct contact with the heated mullite tube. 1~hus the exact temperature distribution is not kno~~ ii. Blank tests showed that at 1000 K the temperature at the center of the bottom plate of the Mo boat is 50 K higher than the temperature measured with the thermocouple as shown in fig. I In order to a~oid contact between the hsgroscopic MoCI and air, the reaction tube ~as icmoved from the lines and placed in a glove ho~ with a dry Ar atmosphere. After loading the starting material, the ends of the reaction tube were closed with stopcocks and reconnected to the NH lines.

2-~/

x ).

2

(2)

where ~G is the free energy of the above decornposition reaction (1) (~G= ill T~S). If is kept small, very high values of 1(N2) can he obtained. Because of the increased rate of decomposition (the equilibrium concentration of NH1 in an NH3--H2 N2 mixture at 100 kPa and 873 K is as low as 0.05% [12]). the reaction temperatures are restricted to values below 1300 K. To meet these special requirements a tube furnace was constructed (fig. 1). NH3 is passed first over NaOH to absorb water vapor and then into a quartz tube with an inside diameter of 4 mm and several slits at the end. This end is positioned directly above a Mo boat containing the MoCl~. The entire set-up is contained in a mullite tube which can he heated by a wire-wound furnace, Since the ammonia gas flowing onto the sample

After adjusting the N H feed to about 0.1 1/mm, which gave a flow rate in the central qLiart/ tube of 13 cm/s. the temperature was raised within 30 mm to the final reaction temperature. After the reaction time the heating current was switched off and the product was allowed to cool to room temperature in flowing NH ~. The resulting products were characterized h\ optical microscopy and by taking goniometer patterns and Debye Seherrer filtered (‘u radiation).

photographs

(Ni-

3. Results and discussion The onset of the reaction of MoCI, with NH. was observed at 520- 570 K, accompanied first b~ the emission of brown vapors and then by the

W. Lengauer

/ Formation

297

of molybdenum nitrides

Table I Reaction conditions and results of the nitridation of MoCI

5

Run No.

Reaction tempera-

A-800

Time of reaction

Phase analysis (b-MoN:

ture(K)

(h)

WC-type)

1060

3

b-MoN: y—MoN1

-

-~

Estimated

a)

.

Contained the deposited product shown in fig. 4.

Fig. 2. Microphotograph ~amplc bedshaped after reaction at temperatures of upthc to top 910ofK.theSpherically WC-type MoN particles are visible. The bar represents 500 pm.

formation of a white precipitate. The brown gas probably consisted of molybdenum chlorides and amidochiorides. The white substance was identified as NH4C1. All runs yielded a finely dispersed black nitride powder as the main product. The applied process yields MoN within substantially shorter reaction times than with the nitridation of Mo metal. Obviously the slow diffusion reaction is replaced by a “chemical reaction”. The reaction conditions as well as the X-ray results of the main product are given in table 1. It can be seen from table 1 that the relative amounts of the phases change between 910 and 950 K. Up to 910 K the product was the singlephase WC-type 6-MoN. On the to of th b d spherical nitride particles were observed (fig. 2). This shape suggests that MoCl5 melts before the reaction to 8-MoN is completed. At T 950 K the main product also contained y-MoN~ The amount of this phase increased with increasing temperature and its lattice parameter decreased from a 0.4188(1) nm at 970 K to a 0.4170(1) nm at 1060 K. Based on the extrapolation of the relationship between lattice parameter and nitrogen content [10], these compositions would be MoN064 and MoN056 respectively. On the other hand, the 6-MoN cell dimensions of a 0.5701(3) nm and c 0.5563(5) nm remained constant independent of reaction temperature. Since no superlattice lines were observed the

primitive WC-type setting of a 0.2851(2) nm and c 0.2782(3) nm is applicable. The surface of samples prepared at T 950 K exhibited a facetted crystal-like character (fig. 3). This morphological difference between runs at T 950 K and T 910 K is most probably due to the evaporation of MoCI5 (Bp 901 K) and the sublimation of “MoCiJNH3)~” intermediate compounds before nitridation has been completed. The deposition of these compounds may also cause the appearance of y-MoN1 ~: Since the diffusion of nitrogen in the solid phase is slow, the =

=

=

* *

________

~.

=

=

_____

-

1” ,

=

=

-

_______________ _____

__________

.,p~

.

___________

-

_______ ______

.

~

.‘. .

Fig. 3. \licrophiitograph of ihc top ul ilie sample bed afier reaction at T 950 K. The c~stal-like WC-type MoN + yMoN1 two-phase product is visible. The bar represents 500

It leji goner

29S

-

‘S

F urination of mo/i I,Jenim: in irides

grown on the bottom plate is isostructLtraf with the sample described h~ Bezinge et al. [s]. Ap-

~

parentl\ this modification can also he obtained :tt

a 4 I nec mS—MoN ~ixst.il~ i limin run

\—5O)i aroun on

the

Mo surface, The sample has sharp dii friction lines and d~lcix structurall~ from the WC-t~peMoN Note the increase of grain size with increasing temperature from the top to the bottom, corresponding to the temperature gradient inherent in the experimental set—up. The bar represents 500 pm.

probably the equilibrium form of hexagonal MoN. The lattice parameters of the low-temperature an ambient pressure growth process. It is therefore WC-type MoN could he considered in line with high s~aeanc~concentrations in both the metal and the non-metal suhiattices. A similar phenomenon was encountered in TtO with high vacanc\ concentrations in the Ti and the C) sublattices. If the vacancies are ren~o~ ed either h~annealin° and or h~high static pressure. the metal atoms are shifted — sliohtly from their ideal positions in order to make room for the nitrogen atoms, thus lowering the cell s~mmetr~..Apparentl\ this process resLilts in a clustering of Mo atoms [~. 13]. Acknowledgements

product is not completel~ nitrided. However, the decrease of the y-MoN 1 , lattice parameters with increasing temperature points to a lower nitrogen potential at higher temperatures due to the acedcrated decomposition of NH i. At temperatures T 950 K a further product consisting of hexagonal MoN with trace,s of yMoNt was obtained. Fig. 4 show’s this sample. which grew on the bottom plate of the molyhdenutii boat. The grain size of this product increases in the direction of increasing temperalure. Dehye Scherrer patterns of this sample showed sharp diffraction lines. The unit cell di— mensions were found to he a = 0.5740(2) nm and o = 0.5624(3) nm. thus substantially deviating from those of the WC-type sample material (doubled lattice parameters) and those given by Schonherg [2]. hut agreeing more closely with results reported by several other authors [3 71.

Recently Bezinge et al. [8] obtained similar cell dimensions on samples which were annealed at high static pressure. with the WC-type 8-MoN as

starting material prepared in the present study, They proposed an MoN structure in the space group

P61mc, which differs from that suggested

h~Schonherg [2] (P6~/rnnic). The agreement of the observed diffraction pattern with the calcu[ated pattern. i.e. the presence of corresponding superstructure lines, indicates that the sample

Tile author wishes to thank Professor Dr. P. Ettma\er for his continued interest, many val~ uahle discussions and the critical reading of tile manuscript.

References Ill U. Hägg. Z. Physik. (‘hem t17 ) 193W 339. [21 N. Schonherg. Ac ta (‘hem. Scand. S ) 1954) 204. [31 U. Brauer and I-. I eihhrandt..J, I esx-( ommon \Iet,ils 12 1967) ~7. [~I B (‘endlexxska, A Mora~’,ki and ‘N ‘viisiiik I Phxs. I \iet:il Phx’~.)17 it 957) 1 71, 5] H. Bauer. L Saur and I). Schechtinger. in: Pror, 14th Intern. (‘on). on L on Temperature Ph~sics. 1975, \‘ol. 2.

0 0] F ..J. Saur. Fit), Schechtinger and L. Rinderer, I P1.1 1 rans. Magnetics MAG-l7 (1951) i029

171

NI. Kagawa. T. Ono, K, Fukuoka, Y. Ssono. Ni. Ikehe and

‘i’. Muto. Japan. .J, AppI. Ph’,s, 24 ) 1951) 1 74~ [5] A. Bezinge, K, \ son. J. Muller. W. Lengauer and P 1.ttnia3er. Solid State Commun. 6) (1957) 141. [9] NV - Troicka’,a and LU. Pinsker. cited iii: WI). I’e.irson, .N Handbook of Lattice Spacings and Structures of Metals and Allo~s Perganion. Oxford. 1967) p. 1437 [10] I’. Fttmaser, Monatsh. (‘hem, 101 )i970) 127 [It] I-i. .Jehn and P. Fttrna~er. .1. l.esx-( omnion Metals IS

(1975) 55. 1 21 ( ; mcliii s Handhuc Ii der ‘\norgani schen ( bernie. N ‘I - 4 herb. 1936) p 327

1131

,J.M. \‘andenherg and B. I i 1974) tOSS

-

Matthias, Mater Rex. Bull, 0