JOURNAL OF MATERIALS SCIENCE LETTERS 14 (1995) 536-538

On the/~-03N4 search

P. H. FANG F.S. Lab, 156 Common Street, Belmont, Massachusetts 02178, USA

The seminal paper [1] on a carbon-nitrogen com- pound (C,N) with a theoretical prediction of hard- ness superior to diamond, has attracted active search in recent years. Since diamond is the hardest known material today, a superior d~amond would be tanta- mount to a material revolution. The present status of this search is the content of the present communica- tion. We ask:

A1. Does C3N 4 exist? Yes. Chemists over 70 years ago may already have made this material [2, 3].

A2. Has crystalline ~-C3N4 been made? Not yet, according to our comprehensive analysis.

A3. Can ~-C3N4 be made? It is very difficult to say something is impossible, but may not be from the currently prevailing approaches.

A paraphrase of these three points is the content of the present letter.

Since the starting material for carbon in [2] and [3] is from a hydrocarbon source, there is possible hydrogen contamination. The effects are: (i) part of the nitrogen could combine with hydrogen instead of with carbon, and (ii) termination of carbon bonds to prevent carbon-nitrogen bonding. Therefore, estab- lishment of C3N 4 based on a stoichiometric determi- nation may not be reliable. This point will be discussed again later.

We have compiled a list of available publications pertaining to the stoichiometry of carbon nitride. Since some publications do not make distinctions, we will include both crystalline and amorphous material especially because of possible ambiguity between fine crystallinity and amorphosity. Some publications gave the N/C ratio or N/(N + C) ratio. For uniformity, we will represent the material by the formula C3Nx.

We have collected published data on the (C,N) compound and the results are shown in Fig. 1 [4-12]. Data (&), which fall on one vertical line, are from the reference source. The reference number is given in brackets. A conspicous manifestation of this figure is that except for the work of Fujimoto and Ogata [4] (the vertical dashed line in the figure), all other data are situated rar below the x = 4 region, yet x -- 4 is the objective in all these works. To deal with x < 4, a tacit interpretation seems to be that the material is a nitrogen deficient C3N4 . Numerous attempts have been made to augment the nitrogen portion of the (C,N) source to amend this defi- ciency. This strategy could be valid if, in the (C,N) system, ~-C3N 4 is the only viable structure [10]. The work of Marton et al. [12] presents at least one different compound corresponding to x = 2. In

536

5.0

4.0

3.0 -x z G c-

2.0

1.5

1.o

x=15

ù1. •

i C3N2.4 (05N4)

[41 • [61

[51

C3N 2 (C2N)

C3N1. 5 (C2N)

[91

[111 [121

"[71 C3N6(CsN ) [101

[81

Figure 1 Compilation of data pertaining to C3N 4. Actual data are presented as C3Nx.

hindsight, it seems strange that prior to these results, no other work on fl-C3N 4 research has considered materials with x deviating significantly from 4 as a separate phase. The existence of multi-phases where many data are distributed along several horizontal lines far away from the x = 4 line in Fig. 1, in addition to the elemental carbon phase might very weil be a reason for the difficulty in realizing a single-phase fl-C3N 4.

In the following, several regions will be listed:

x = 2.4 region. The corresponding compound is CsN 4. Two structures are known; rhombohedral [14] (carbon cyanide) and monoclinic (tetracyano- methane) [15].

x = 2 region. The work of Morton et al. [12], probably for the first time in fl-C3N 4 history, reported this composition, C3N2 as a stable com- pound.

There are two compounds which are iso- stoichiometric to C3N2: tetracyanoethylene (CóN4) and hexaazatriphenylenehexacarbonitride (C18N12). Both of them, and many other carbon nitrides, have ~ N , C-C, and C = C , and the consequent infrared active mode, including the 2210 cm -1 absorption

0261-8028 © 1995 Chapman & Hall

band. Therefore, using the presence of the 2210 band as evidence of C3N 4, as suggested in [6-8], evidently is insufficient.

Structurally, sirnilar to/3-C3N4, a hexagonal C6N 4 has been reported but no structural data is available [131.

x = 1.5 region. Fig. 1 shows considerable data situated near the x = 1.5 line. An iso-stoichiometric compound is dicyanoacetylene (C4N2) . There are two structures, monoclinic and cubic [14].

x = 12 region. This compound, cyanuric triazide, with a molecular formula C3N3(N3) 3 -- C3N12, has a melting point of 93.5 °C, and, therefore, is stable at ambient temperature [15]. Crystallographically, the structure is hexagonal, but with a much larger d-spacing than that of the hypothetical/J-C3N 4.

x -- region. For a complete compilation of the (C,N) compound, we include C3N 3. The correspond- ing iso-stoichiometric compound, cyanogen (C2N2), has a melting point below room temperature and is therefore not expected in the high temperature processing (C,N) compounds.

The above list shows a large number of (C,N) compounds which could occur in the synthesis of carbon nitrogen complex. Therefore, C3N 4 is not the only viable product, contrary to the suggestion of Niu et al. [10].

The case of C3N 4 seems to be the most elusive. An exception is the work of Fujimato and Ogato [4]. Their value of x can be varied continuously from 1.5 to 15 or beyond, including x = 4. A possible reason is that their specimen has a high amorphosity and this amorphosity could be induced by their high energy bombardment of 0.2 to 20 keV nitrogen ions. The ion energy used by other researchers is less than 100 eV [9, 12], albeit with different methods to produce elemental carbon. An interesting result [4] is that in their data for hardness versus value of x, a distinct maximum occurs near x -- 4. Their material could be either amorphous or very fine crystallites, but the prediction of Liu and Cohen [1] is partially confirmed.

A datum with a marginal closeness to x = 4 is the point x -- 3.6 from [7]. However, because of evident hydrogen contamination in their work, this x value may not be meaningful. Hydrogen seems to persist in the pyrolysis of (C,N,H) precursors.

A basis used by Niu et al. [10] and Haller et al. [16] to confirm the /J-C3N3 is a close matching between theoretical and experimental d-spacings. The data of Niu et al. is analysed in Table I with the following parameters:

a = 0.6533 nm, c = 0.2400 nm (1)

dhk 1 = {[(h - k )2 /aV '3] 2 + [(h + k ) / a ] 2 + (1/c2} 1/2

(2) In Table I, the data in the top six rows are from the experimental data of Niu et al. [10]. A comparison with the h k l assigned by these authors with their experiment is quite impressive. However, some important lines which should have high diffraction

TABLE I d-spacing of/3-C3N4

d (experiment) d (calculation) I (fl-Si3N4) h k l (nm) (nm) (%)

1 0 1 0.217 0.221 100 2 1 0 0.210 0.214 100 3 2 0 0.130 0.124 35 0 0 2 0.118 0.120 35 4 1 1 0.107 0.110 85 6 1 1 0.081 0.081 - 2 0 0 - 0.2829 85 3 2 1 - 0.1142 140 2 1 2 - 0.1046 85 3 0 1 - 0.1403 70

intensity, and therefore should be distinctly observ- able, are missing in their report. The normalized intensity I , based on/J-Si3N4, is listed in Table I for reference. In Table I, in the lower four rows, there are four strong lines absent in their list, on the other hand, two much weaker lines are listed.

Haller et al. [16] have attributed the missing lines to the effect of substrate crystal orientation. In both cases, no effort seems to have been made to use different substrates to elucidate the orientational degeneration.

In [10] the specimen chemical composition is reported as 60%C and 40%N (see the caption in [10, Fig. 4]), and in [16] as CI.»N1.0; both of these are suggestive of the compound C3N2 of [12]. However, at present, no crystallographical data for C3N 2 is known.

A way to maintain a / ~ - C 3 N 4 interpretation of [10] and [16] is that their specimens actually represent C3 N 4 with a C contamination. An equivalent for- mula would be:

2C3N2 = C3 N 4 4- 3 C (3)

The above decomposition is by no means unique because instead of the element C, compounds of (C,N) such as C»N could be a component. In this case, the relative concentration of the contaminated material will be even higher.

In addition to the work of [10], there are three other papers on the binding energy of/3-C3N 4 based C-ls and N-ls core level spectra from X-ray photo- emission spectroscopy [17-19]. There are discrepan- cies in the energy value assignment among these references. In addition, their spectral line shape implies multiple components [18, 20]. Therefore, an indirect evaluation, even based on a direct measure- ment, is arbitrary and the conclusion of high N/C concentration ratio may be tentative.

The above comment applies also to be Rutherford backscattering spectroscopy of these works: when the material is not single phase, backscattering data cannot be related directly to individual phases.

In searching for a possible route to realize/3-C3N4, Liu and Cohen [1] suggested a polymeric amorphous carbon nitride as precursor. An induetion of a phase transition to a structure with tetrahedral bonding by the application of high pressures and temperatures might be expected.

537

An experiment has been made by Sekine et al. [21] with high pressure pyrolysis of two materials: t e t r a c y a n o e t h y l e n e (C6N4) and triazine (C3H3N3).

With a pressure of 5 GPa and a temperature of 1400 °C for 30 min, a graphite-like material with composition closer to C»N was obtained in the case of C6N4 starting material. In the case of triazine, all nitrogen was decomposed and vanished from the compound. Therefore, the final nitrogen content is apparently dependent on the precursor in a complex fashion, and in general, the value is lower than the starting material. In this respect, a starting material with higher N content than that of C3N 4 could be interesting.

There is another complexity: non-equivalencies have been found in post-treatment at a high temp- erature versus in situ treatment at high temperature [18]. Therefore, in the design of the experiment, this factor has to be taken into account.

Based on the above analysis, we have to conclude that today there is no credible evidence of this incredible material with chemical composition C3N 4

and an iso-structure of/3-Si3N 4.

Acknowledgements The author thanks Drs R. Roth and A. D. Mighell of National Institute of Science and Technology for the crystallographical information. A possible struc- ture discrepancy of [10] was pointed out by Dr R. R. Reeber of the US Army Research Office, North Carolina. Teresa Fang has been most helpful in the preparation of this paper. This work is supported by

the Material Science Division of the US Army Research Office.

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Received 25 August and accepted 2 November 1994

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