Acoustic shock wave-induced formation of an undefined high-pressure carbon allotrope

Carbon is nothing less than a rock star in the world of research on allotropes which has a tremendous ability to form several simple and complex allotropic structures of various sp, sp² and sp³ hybridizations, thanks to its flexible chemical structure. By high-pressure experimental and theoretical approaches, new carbon forms were synthesized from known carbon structures. Herein, we report the phase transition from amorphous carbon nanoparticles to a novel thermodynamically stable carbon allotropic structure, denominated as SD carbon (Sivakumar-Dai carbon), obtained through the impact of acoustic shock waves with a transient pressure of 16.5 MPa. The SD carbon exhibits a couple of Raman bands at 482 and 2431 cm^-1 with mixed sp² and sp³ hybridizations belonging to the non-layered structural type and the formation of the SD carbon mechanism has been proposed based on the thermal conductivity driven super-heating approach. The structure will offer platforms that may pave the way for several novel carbon structures at low shock pressures compared to conventional static pressure compression and other techniques.