Original mechanism of transformation from soft metallic (sp2/sp3) C12 to ultra-dense and ultra-hard (sp3) semi-conducting C12: Crystal chemistry and DFT characterizations

Abstract

An original mechanism is proposed for a pressure-induced transformation of orthorhombic C₁₂ from ground state normal pressure (NP) sp²/sp³ allotrope to ultra-dense and ultra-hard high pressure HP sp³ form. Upon volume decrease, the trigonal CC parallel segments characterizing glitter-like tfi topology of NP C₁₂ change to crossing C–C segments with the loss of sp² character accompanied by a large densification with ρ = 3.64 g/cm³, larger than diamond, defining a novel orthorhombic HP C₁₂ with 4⁴T39 topology. The crystal chemistry engineering backed with quantum density functional theory DFT-based calculations let determine the ground state structures and energy derived physical properties. Furthering on that, the E(V) equations of states (EOS) let define the equilibrium NP(E₀,V₀) allotrope at lower energy and higher volume versus HP(E₀,V₀) allotrope at higher energy and smaller volume. A potential pressure induced transformation NP→HP was estimated at ∼100 GPa, reachable with a diamond anvil cell DAC. Both allotropes were found cohesive and mechanically stable with low and large Vickers hardness magnitudes: H_V(tfi C₁₂) = 24 GPa and H_V(4⁴T39 C₁₂) = 90 GPa; the latter being close to diamond hardness (H_V ∼95 GPa). Besides, both allotropes were found dynamically stable with positive phonon frequencies and a spectroscopic signature of CC high frequency bands in tfi C₁₂. The electronic band structures show a metallic behavior for NP tfi C₁₂ and a small band gap for HP 4⁴T39C₁₂ letting assign semiconducting properties. The work is meant to open further the scope of C (sp²)→C (sp³) transformation mechanisms that are fundamental in solid state physics and chemistry.