Polymorphic phases of sp3-hybridized superhard CN

Area of Science:

• Materials Science
• Solid-State Chemistry
• Computational Materials Science

Background:

• Carbon nitride (CN) is a promising superhard material due to its short bond lengths compared to diamond.
• The precise crystalline structure of superhard carbon nitride remains experimentally undetermined.

Purpose of the Study:

• To conduct an extensive structural search for high-pressure crystalline phases of carbon nitride.
• To identify energetically favorable and potentially synthesizable superhard carbon nitride structures.

Main Methods:

• Utilized particle swarm optimization for an unbiased structural search.
• Employed density-functional theory calculations to assess structural stability and properties.
• Investigated seven low-energy sp(3)-hybridized carbon nitride polymorphs.

Main Results:

• Identified seven low-energy polymorphic structures of sp(3)-hybridized carbon nitride.
• The Pnnm carbon nitride structure (8 atoms/cell) was found to be energetically more favorable than previously reported structures.
• The Pnnm structure exhibits exceptional hardness of 62.3 GPa.
• Calculated formation enthalpies indicate synthesis is possible at 10.9 GPa, lower than for β-C(3)N(4).

Conclusions:

• The Pnnm carbon nitride structure represents a significant advancement in the search for superhard materials.
• This computationally discovered material offers a potentially lower synthesis pressure, making it more accessible.
• Further experimental validation is warranted to confirm the existence and properties of the Pnnm carbon nitride phase.