Complex Low Energy Tetrahedral Polymorphs of Group IV Elements from First Principles

Area of Science:

• Materials Science
• Computational Chemistry
• Condensed Matter Physics

Background:

• The carbon energy landscape is complex, featuring numerous metastable phases like diamond, fullerenes, nanotubes, and graphene.
• Discovering new carbon structures, especially those with large unit cells, presents significant computational challenges.

Purpose of the Study:

• To explore the complex energy landscape of carbon and identify novel, low-energy metastable structures.
• To investigate the stability and properties of newly predicted carbon allotropes and their analogues.

Main Methods:

• Employed a combined stochastic search strategy using ab initio random structure search and a random sampling strategy with graph theory and space group constraints.
• Applied connectivity constraints to unit cells containing up to 100 carbon atoms.
• Performed calculations for energetic, dynamical, mechanical, and electronic properties.

Main Results:

• Identified three new low-energy carbon polymorphs (Pbam-32, P6/mmm, I4[over ¯]3d) with unique topologies and 32, 36, and 94 atoms per primitive cell.
• These allotropes exhibit energies suggesting metastability relative to diamond (96, 131, 112 meV/atom) and possess mechanical and dynamical stability.
• The I4[over ¯]3d structure shows a direct band gap of 7.25 eV, the widest among carbon allotropes. Analogues in silicon, germanium, and tin also demonstrated stability.

Conclusions:

• The discovered carbon polymorphs are insulating, superhard materials with potential for advanced applications.
• Their electronic properties suggest suitability for semiconductor and photovoltaic devices.
• The study demonstrates the efficacy of the employed computational strategy for discovering novel materials.