M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

4 4 H I G H L I G H T S 2 0 2 4 I

Synthesis and X-ray characterization of three superhard carbon nitrides

Three new carbon nitrides, expected to be nearly as hard as diamond, have been successfully synthesized under extremely high pressures exceeding 72 GPa. The characterization of these materials, mainly achieved with X-ray diffraction, reveals that these materials are not only ultra-incompressible and superhard, but also exhibit high energy density and piezoelectricity, and remain stable at ambient conditions.

The synthesis of novel carbon nitrides has been the focus of intense research since 1989, when Liu and Cohen predicted that a fully saturated polymeric C3N4 solid isostructural to b-Si3N4, and thereby comprised of corner- sharing CN4 units, could be formed and would have exceptional mechanical properties due to the short and stiff C-N covalent bonds [1]. This material was calculated to have a bulk modulus of 427 GPa, close to that of diamond (446 GPa), and thought to rival its hardness. In addition to its outstanding mechanical properties, such a carbon nitride is also expected to feature other useful characteristics, such as high thermal conductivity, wide bandgap, and exotic electronic properties, emphasizing its multifunctional potential. Over the past three decades, a multitude of experimental approaches have aimed to synthesize such a solid, without success.

This work attempted to produce the elusive carbon nitrides by loading diamond anvil cells with a variety of carbon- nitrogen precursors, such as tetracyanoethylene (C6N4) and cyanuric triazide (C3N12). These were compressed to extreme pressures between 72 and 135 GPa, and laser- heated to temperatures above 2000 K. Single-crystal X-ray diffraction (SCXRD) of the transformed polycrystalline samples was carried out at beamlines ID27 and ID11 to investigate the nature of the reaction products.

Analysis of the X-ray diffraction data revealed the formation of three novel carbon nitrides, tI14-C3N4, hP126-C3N4, and tI24-CN2. While both C3N4 polymorphs are exclusively composed of a three-dimensional arrangement comprised of corner-sharing CN4 tetrahedra, the tI24-CN2 solid produces a framework of corner- sharing CN4 tetrahedra interlinked through N2 dimers, with nitrogen atoms making two C-N bonds and one N–N bond (Figure 31). In all three compounds, carbon is fourfold coordinated and sp3-hybridized. However, the nitrogen atoms are sp2-hybridized in hP126-C3N4 and tI24-CN2, but sp3-hybridized in tI14-C3N4.

Following the synthesis and structural characterization of the novel carbon nitrides, their pressure stability range and compressibility was assessed. Remarkably, all three materials were found to be fully recoverable to ambient conditions and stable in air. Transmission electron microscopy (TEM) measurements were performed on a small crystallite of tI14-C3N4 extracted from a recovered sample and prepared by focussed ion beam (FIB), as seen in Figure 32. Selected area electron diffraction (SAED) found reflections consistent with the unit cell determined from SCXRD, while electron energy loss spectroscopy (EELS) revealed the presence of sp3-carbon and sp3-nitrogen in a ratio of 3:4 – all in agreement with the properties already identified for tI14-C3N4.

From the pressure-volume data collected upon decompression, the incompressibility of the three carbon nitrides was determined. They were found to have a bulk modulus of 407(26) GPa, 397(16) GPa and 429(28) GPa, respectively for tI14-C3N4, hP126-C3N4, and tI24-CN2, nearing that of diamond (446 GPa) and even above that of cubic boron nitride (395 GPa). According to density functional theory calculations, all three C-N compounds are superhard, with hardness values of 80.4 GPa, 80.8 GPa and 86.8 GPa for tI14-C3N4, hP126-C3N4, and tI24-CN2. These values are significantly

Fig. 31: Crystal structure of the (a) tI14-C3N4, (b) hP126-C3N4, and (c) tI24-CN2 compounds at ambient conditions. The white and blue spheres represent carbon and nitrogen atoms, respectively.