First-order polyamorphic phase transition in boron nitride

A first-order polyamorphic (amorphous-amorphous) phase transition is often observed in amorphous materials that have a tetrahedral structure, such as ice, carbon, or silicon, but it has yet to be observed in amorphous boron nitride (a-BN). We investigated the existence of a first-order phase transition in a-BN by means of first-principles molecular dynamics (MD) simulations at a constant temperature and various pressures. At 300 K, the volume of a-BN gradually decreases at applied pressures of 0 to 18 GPa; it then rapidly decreases by about 3% at pressures of 18 to 24 GPa, providing evidence of a first-order phase transition. The structure remains amorphous throughout the transition. As the pressure is released from 24 to 0 GPa, the volume gradually increases with preservation of the difference at the phase transition, showing that the structure of the high-density phase is maintained at 0 GPa. An analysis of the coordination number revealed that a-BN consists mainly of sp2-hybridized BN bonds at 0 GPa. At the phase transition, sp2 bonds are rapidly converted into sp3 bonds, which subsequently account for about 30% of all bonds. This mechanism resembles that of the phase transition of amorphous carbon, in which the conversion rate is almost 100%. The low conversion rate in a-BN is probably attributable to the relative inflexibility of the structure.