The phase diagram and hardness of carbon nitrides

Abstract

Novel superhard materials, especially those with superior thermal and chemical stability, are needed to replace diamond. Carbon nitrides (C-N), which are likely to possess these characteristics and have even been expected to be harder than diamond, are excellent candidates. Here we report three new superhard and thermodynamically stable carbon nitride phases. Based on a systematic evolutionary structure searches, we report a complete phase diagram of the C-N system at 0–300 GPa and analyze the hardest metastable structures. Surprisingly, we find that at zero pressure, the earlier proposed graphitic-C3N4 structure () is dynamically unstable, and we find the lowest-energy structure based on s-triazine unit and s-heptazine unit.

Pharaoh's Serpents: New Insights into a Classic Carbon Nitride Material

“…Although previous studies on carbon nitride systems would lead one to expect the structure to be layered, we cannot rule out the possible existence of three-dimensional framework components as suggested by theoretical calculations. [18,19,21] The formation reaction and presence of Hg in the resulting nanocomposite may hinder the practical implementation of these materials but related synthesis pathways can now be envisaged to create analogous semiconductor heterojunction combinations using more environmentally friendly elements. Analysis of this "classical" reaction and its products could give rise to more readily scalable ways to produce such nanoscale materials composites with useful properties for energy and sustainability applications.…”

“…[13,[15][16][17] Interestingly, it has also been predicted that open framework varieties containing mixed sp 2 as well as sp 3 bonded units could exist. [18][19][20][21] Most carbon nitride materials under current investigation are produced by thermolytic condensation reactions of organic molecular precursors such as dicyandiamide (DCDA, C 2 N 4 H 4 ) and melamine [C 3 N 3 (NH 2 ) 3 ], giving rise to a range of polymeric C x N y H z compounds. [22] However, Liebig's initial syntheses of "melon" formed part of a series of studies involving reactions of thiocyanate compounds, including NH 4 SCN and KSCN treated with Cl 2 .…”

Carbon nitride frameworks and dense crystalline polymorphs

“…(The Cm structure found in DFT searches by Dong et al 29 is not predicted to be stable at any pressure because our study has discovered new structures that are more stable. )…”

“…At low pressure a wide variety of corrugated layers or open framework structures based on linked heptazine or triazine motifs are predicted to occur and the number of potential polymorphs and structure types increases dramatically with system size 67 . These low-density materials typically have very large unit cell volumes and they rapidly become destabilized with increasing pressure 29 . The importance of such mesoporous phases and their relationship to layered and frameworkstructured carbon nitrides is discussed below.…”

“…First investigations began by considering known structures such as those of Si 3 N 4 ceramic phases, and more recently using structure searching methods 29 . However none of the predicted high density C 3 N 4 polymorphs has been demonstrated experimentally to date 1, 30 .…”