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.

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

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, NO_x (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 d^3.5,¹ 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.