Shock synthesis and characterization of new diamond-like carbon nitrides

New diamond-like carbon nitrides (C–N heterodiamonds), no less hard than diamond, have been synthesized for the first time in bulk form by shock compression of graphitic carbon nitride precursors in the dynamic pressure–temperature range around 30 GPa–3000 K. Typically, the C2N heterodiamond obtained from graphitic C3N2 consists of sp3-bonded carbon and nitrogen atoms, belongs to a diamond crystal system, has a lattice constant of 0.351 ± 0.001 nm, 1.6–1.9% smaller than that of diamond, is able to scratch the surface of a sintered-diamond and is very IR active due to the dipole moment of the C–N single bonds.

These properties suggest that it could be slightly harder than diamond.

From 1985 to 1996, very exciting model substances, namely β-C3N4and its allotropic cubic and pseudo-cubic phases with a bulk modulus calculated to be similar to that of diamond, were predicted by computational material scientists, on the basis of their C–N bond length being shorter than the C–C length of diamond.1–5

A great many carbon nitrides have been prepared through synthetic studies mainly using CVD and PVD methods, but, none of them have so far provided evidence for the presence of the above predicted phases (the expected β- and cubic forms of non-isolated deposits6–10 provide no evidence of chemical structure and hardness.) Both the low thermodynamic stability and metastable state of the predicted phases make the synthesis of the compounds very difficult.4

The N content of the known carbon nitrides6–10 prepared by the current CVD and PVD methods are appreciably less than the theoretical N/C = 1.333.

The methods also produced quantities too small to sufficiently purify and identify the product, hence were not necessary suited for the synthesis of the theoretical C3N4 phases.

Two essential problems remain unsolved: (1) whether it is possible to make the material harder than diamond and (2) whether the crystal and chemical structures of the material are the same as the predicted C3N4 structures.

And there are two indispensable requisites: that the shortest bond length of the material is to be shorter than the C–C length of diamond and that the material is expected to exist in a high-pressure phase in the PT phase diagram by analogy with diamond.

Under such criteria for exceeding the hardness of diamond, only diamond-like materials combined with hybrid C–C bonds11 and C–N covalent bonds will be allowed in theory.

The present study will report the shock synthesis of a new diamond-like carbon nitride.

This was done with the expectation that the shock compression would induce a hexagonal-to-cubic transformation of graphitic heterocompounds while preserving the composition.12

In the following typical experiment, heat-resisting graphitic C3N2 (Anal.

Found: C, 57.8; H, 0.2; N, 42.0%) was prepared by pyrolysis of 2,5,8-tricarbodiimide-tris-s-triazine copper salt at 1073 K for 1 h under a stream of N2 gas and used as a precursor.13

The obtained powders were mixed with spherical copper powders in a 6∶94 mass ratio, placed into a cylindrical apparatus12 with a TNT-mixed explosive charge and shock compressed.

The estimated shock pressure and temperature loaded on the sample were 30 GPa and 3000 K, respectively.

The recovered sample was machined and immersed in 70% HNO3 to remove the copper matrix, boiled in 40% KOH for 1 h, followed by boiling in a 8∶2 by volume mixture of 98% H2SO4 and 70% HNO3 for 4 h.

After the purification was repeated three times, highly purified dark gray fine powders were obtained in a 8% yield.

The obtained material was examined using authentic methods such as transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED), electron-energy-loss spectroscopy (EELS) and energy-dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), IR spectroscopy and CHN/S/O combustion elemental analysis.

The hardness of the material was evaluated by rubbing the sample powders between two sheets of sintered diamond.

The TEM observation of the shock product indicated a lattice image of a polycrystalline structure consisting of nanocrystals with the size of 4–5 nm (Fig. 1).

Micro SAED of the thin grains at several places yielded a narrow spot-pattern indexed with a cubic crystal system (Fig. 1).

A set of d-values reproducibly obtained from the SAED pattern were 0.2035 vs, 0.1245 s, 0.1055 s, 0.0880 m, 0.0808 m, 0.0717 w, 0.0674 w, 0.0619 w and 0.0593 nm w, which could be consistently indexed as (111) 0.2027, (220) 0.1241, (311) 0.1058, (400) 0.0878, (331) 0.0805, (422) 0.0717, (511, 333) 0.0676, (440) 0.0621 and (531) 0.0593 nm, respectively, assuming the cubic parameter a0 = 0.351 ± 0.001 nm.

Since the (200), (222) and (420) lines are absent, the pattern is assigned to a diamond structure.

The above lattice constant is 1.6–1.9% smaller than that of diamond (0.3567 nm) and 2.0–2.6% greater than the theoretical values (0.342–0.343 nm) of pseudo-cubic C3N4.3,5

The occurrence of a single phase of diamond-like material was confirmed by XRD, although the profile was of low quality due to bad measurement conditions.

This material having a diamond crystal system (a specific form in a face-centered cubic system) crystallographically differs from the predicted cubic and pseudo-cubic C3N4 phases of a body-centered cubic system.

Several lattice fringes shown in Fig. 1 allow us to estimate a lattice spacing of 0.203 nm and this value corresponds to the (111) value.

The EELS in the core-loss region indicates that the constituent C and N atoms have only a 1s → σ* transition at the K-edges (Fig. 2), that is, both atoms are combined with only sp3 bonds.

EDX measurements detected only C and N elements except some O and Cu due to the used sample-mounting membrane (a carbon-collodion membrane coated on a Cu-microgrid).

The EDX elemental composition corrected using the graphitic C3N2 standard was C∶N = 1∶0.5.

On the other hand, the exact chemical composition determined by the CHN/S/O combustion elemental analysis was C∶N = 1∶0.55 (Anal.

Found: C, 60.8; N, 39.2% the content of hydrogen and oxygen was below the detection limit of 0.1%).

The values are nearly the same and provide the approximate chemical formula C2N. The IR absorption spectrum shows a very broad and strong band centered at 1083 cm–1 due to the C–N single bonds (Fig. 3).

Since any peaks due to primary to tertiary amines, quaternary ammonium ion, NOx (x = 1–3), CO, CO2, and OH groups were not observed, the IR result indicates that each N atom in the C–N bonds neighbors three C atoms and each C atom neighbors four N atoms.

The strong IR activity is clearly due to a large dipole moment that originates from the anisotropic sp3 hybrid orbital of the covalent N atom, because the estimated 6% ionicity14 of the C–N bonds cannot be considered as the main origin of the IR activity.

This material that survived the harsh purification process is chemically very stable similarly to diamond and easily scratches a glass vessel.

Rubbing the sample powders between two plates of sintered diamond with polished surfaces produced grazes on the surfaces, which suggests that the material is no less hard than diamond.

The C2N heterodiamond, having a lattice constant 1.6–1.9% smaller than that of diamond, is expected to have a bulk modulus 5–6% higher than that of diamond, according to the empirical relation describing the bulk modulus inversely proportional to d3.5,1 where d is the bond length of hard materials.

The lattice constant vs. composition plot for the C2N heterodiamond agrees well with the ideal mixing value predicted from the Vegard rule when the material is assumed to be a solid solution of diamond and the predicted pseudo-cubic C3N4, which supports the validity of the computational prediction of the C3N4 phases.

This heterocompound, analogous to diamond regarding crystal structure and hardness but completely different in chemical structure, composition, lattice constant and optical properties, can be defined as a new binary diamond consisting of carbon and nitrogen, namely a C–N heterodiamond.

As a precedent, several diamond-like materials with small nitrogen content such as C5N and C10N have been obtained by shock-compression of other precursors.

The structure of these materials was, however, close to that of a shock diamond prepared from graphite.

Attempted shock synthesis of an expected C3N4 heterodiamond having a maximum stoichiometric ratio of C/N = 3/4 was difficult because of the low thermal resistance of the known carbon nitride precursors.

The C2N heterodiamond having an N content 3% smaller than that of the precursor probably occurred through a non-diffusive process, because the synthesis was carried out under mild PT conditions substantially lower than those for a diffusive transformation of disordered glassy carbon into diamond (100 GPa, 3000–4000 K).15

A probable mechanism explaining the slight N loss is: when the shockwaves predominantly compress the c-axis of the precursor to close the layers to a position where the covalent bonding is possible, some nitrogen atoms overlapped in the upper and lower layers would be easily eliminated as N2.

Probably, the best precursors of an expected C3N4 heterodiamond are the α- and rhombohedral C3N4 phases3,4 being more stable and more compressible in the a-axis direction than the β-type.

In addition to the superhardness of the C2N heterodiamond, a potential of wide bandgap semiconductivity due to one lone pair in the unused sp3 hybrid orbital of the N atom (carbon vacancy) should be expected.

The measurements of the bulk modulus and semiconductivity are currently under consideration.