Extended Data Fig. 10: Phase transition from onion-like BN to twisted-layer structure during the compression and heating.

a, Evolution of a two-shell onion-like BN structure during compression from 0 GPa to 6 GPa at 300 K. b, Structural evolution at 6 GPa during heating from 300 K to 1,500 K. In both a and b, the violet colored part represents graphene-like structure. The grey part represents an unidentified structure. Atoms and bonds are colored according to the polyhedral template matching algorithm. Arrows in the enlarged image mark unbonded atoms. The cutoff value for the bond is 1.52 Å. With the increase of temperature, the collapsed onion-like BN structure transformed into a layered structure. c, Top view of each layer formed at 6 GPa, 1,500 K. Twist angles of other layers relative to the layer 1 are shown. The results indicate that the onion-like structure underwent a series of changes under compression and heating, including distortion, collapse, fragmentation, and subsequent recrystallization, leading to the formation of the twisted-layer structure. Note that the size of onion-like BN structure in the theoretical simulation is significantly smaller than that used in the experiment. As a result, the pressure required for simulation is higher than the actual condition.