Novel Group IVA (A) equiatomic hexagonal nitrides A4N4 (A = C, Si, Ge) with characteristic {A2N2} layered tetrahedra: crystal chemistry and first-principles calculations
- Superhard materials (Vickers hardness >40 GPa) are the key to demanding materials processing in industry scale and synthesis via high pressure routes like in diamond anvil cells. Despite extensive research both by experiments and computational chemistry, synthetic diamond (Vickers hardness HV ≈ 90 GPa) is still widely used. Sharing metastability and structure with diamond, softer cubic boron nitride cBN (HV ≈ 60 GPa) found application as e.g. refractory material where diamond lacks chemical resistance. This is most prominent in contact with iron containing compounds, ruling out diamond-coated machining tools for steels. While there are improvements of those materials like grain and grain boundary optimizations, none has reached wider industrial application due to processing difficulty and costs. Based on the tetragonal coordination of diamond and cBN, the present work addresses the question of novel superhard materials from the class of carbonitrides CxNy and its analogues from GroupSuperhard materials (Vickers hardness >40 GPa) are the key to demanding materials processing in industry scale and synthesis via high pressure routes like in diamond anvil cells. Despite extensive research both by experiments and computational chemistry, synthetic diamond (Vickers hardness HV ≈ 90 GPa) is still widely used. Sharing metastability and structure with diamond, softer cubic boron nitride cBN (HV ≈ 60 GPa) found application as e.g. refractory material where diamond lacks chemical resistance. This is most prominent in contact with iron containing compounds, ruling out diamond-coated machining tools for steels. While there are improvements of those materials like grain and grain boundary optimizations, none has reached wider industrial application due to processing difficulty and costs. Based on the tetragonal coordination of diamond and cBN, the present work addresses the question of novel superhard materials from the class of carbonitrides CxNy and its analogues from Group IVA. A novel family of hexagonal A4N4 structures with A = C, Si, Ge is proposed based on crystal chemistry considerations and structure optimizations using Density Functional Theory with the Generalized Gradient Approximation. The structures are characterized by {A2N2} bilayers of AN polyhedra separated by repulsive N–N interactions. Due to short covalent bonds, the carbonitride is predicted to classify as superhard material. Ab initio calculations indicate thermodynamically and dynamically stable C4N4 at ambient conditions, laying the foundation for successful synthesis at p > 40 GPa in the future.…
