First-principles computational design and synthesis of hybrid carbon-silicon clathrates

Type I and Type II silicon clathrates (Si₄₆ and Si ₁₃₆), which can be considered as analogs of carbon fullerene materials, are composed with face-sharing Si₂₀, Si₂₄, and Si₂₈ cages linked through sp ³-covalent bonds. Besides silicon clathrates, theoretical computations have shown that both Type I carbon clathrate (C₄₆) and Type II carbon clathrate (C₁₃₆) may exist as metastable phases under high pressures. However, the energies of formation for the Type I and Type II carbon clathrates are extremely high and neither Type I nor Type II carbon clathrates have been synthesized. The objective of this investigation was to develop Type I hybrid carbon-silicon clathrates by substituting atoms on the silicon clathrate framework with C atoms. A first-principles computational approach was first utilized to design the framework structure and to identify appropriate guest atoms that are amenable to the formation of hybrid carbon-silicon clathrate compounds. A new class of Type I clathrates based on the carbon-silicon system was discovered as potential candidates. Some of the promising candidate clathrates were synthesized using an industrial arc-melting technique. The yield and stability of these newly discovered clathrates were evaluated. In addition, the electronic properties of selected clathrate materials were predicted using first-principles computations, which showed profound influences of the electronic properties by C atom substitution on the Si framework and insertion of guest atoms into the cage structure.