We report depressurization amorphization of single-crystal boron carbide (B4C) investigated by in situ high-pressure Raman spectroscopy. It was found that localized amorphization of B4C takes place during unloading from high pressures, and nonhydrostatic stresses play a critical role in the high-pressure phase transition. First-principles molecular dynamics simulations reveal that the depressurization amorphization results from pressure-induced irreversible bending of C-B-C atomic chains cross-linking 12 atom icosahedra at the rhombohedral vertices.

We report low- and high-temperature Raman spectroscopy of pressure-induced amorphous boron carbide (a-B4C). Coarsening of carbon clusters in a-B4C was characterized during heating, whereas unusual temperature coefficients of a-B4C Raman peak shifts were observed during cooling. These results experimentally evidence that the amorphization of B4C is associated with the destruction of the C–B–C chains and a-B4C is composed of sp2 carbon aromatic rings and boron clusters. This disordered structure with relatively weak carbon sp2 bonds is believed to be responsible for the loss of B4C shear strength at high pressures.

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