A near-ambient-pressure A7 phase achieved by high-pressure quenching of few-layered violet phosphorus and black phosphorus.

Stereochemical configuration in elemental crystals is critical for designing new phases and understanding the thermodynamic mechanism of allotropy, as exemplified by graphite and diamond. Phosphorus allotropes exhibit various crystal structures under high pressure; in particular, the A7 phase at intermediate pressure is spectacular, which hosts emergent quantum states such as superconductivity and large magnetoresistance. However, stabilizing the metastable phase near ambient conditions remains challenging. Here, starting from violet and black phosphorus, the application of quantum dimensional confinement modulates both the kinetic energy barrier and the thermodynamic stability under pressure, enabling stabilization of the A7 phase at low pressures (∼0.8-1.0 GPa) in few-layer samples while leading to its destabilization in ultrathin flakes. The kinetic relationship between the A7 structure and other allotropes underscores the pivotal role of surface energy in these quantum confinement effects.