High-pressure formation and characterization of a boron carbide polymorph featuring bent C–B–C chains

Boron carbide is a material of choice for multiple industries, e.g., aerospace, as a lightweight structural ceramic due to its high hardness, high melting temperature, and low density. However, its mechanical properties have been observed to radically degrade under shockwave compression, presumably as a consequence of stress-induced phase transitions resulting in its partial amorphization. So far, the physical mechanism underpinning this behavior remains unclear. Here, we report a pressure-induced phase transition in boron carbide occurring between 78 and 90 GPa during static compression in diamond anvil cells, both at room temperature and after quenching from high temperatures. The crystal structure of the new phase was solved and refined via synchrotron single-crystal x-ray diffraction measurements and further investigated by Raman spectroscopy as well as density functional theory calculations. The discovered high-pressure polymorph, 𝑚𝐶⁢60−B₁₃⁢C₂, has strong resemblance with the known ambient conditions phase, ℎ𝑅⁢45−B₁₃⁢C₂, with the important distinction that the linear C–B–C chain linking B₁₂ icosahedra in ℎ𝑅⁢45−B₁₃⁢C₂ is bent in 𝑚𝐶⁢60−B₁₃⁢C₂. Such bending of the C–B–C chain has been hypothesized as key to explain boron carbide's drop in strength, phase transitions, and amorphization. The observed reversibility of the phase transition, as well as the formation of covalent bonds between B12 icosahedra and the bent C–B–C chains, are assessed to decipher the C–B–C chain's bending importance on the amorphization and mechanical properties of boron carbide.