Acoustic shock wave-induced B1 to B2 phase transition of CdO nanoparticles: Structure-property relationship and its mechanism in thermodynamic aspects

The reversible pressure-induced B1 (NaCl) ↔ B2 (CsCl)-type phase transition is one of the classic models for understanding the general concepts of phase transition. Although several outstanding functional properties have been identified for B2 phase materials under high-pressure conditions, generating a stable B2 phase under ambient conditions is a critical task for device fabrication. Herein, we report a novel B1-B2-type phase transition in cadmium oxide nanoparticles (CdO NPs) by millisecond-low-pressure acoustic shock waves (16.5 MPa and 3173 K) that is highly stable under ambient conditions. Under static high-pressure conditions, the B1-B2 phase transition requires ∼102 GPa, which is reversible. A new thermodynamic mechanism is proposed to explain the B1-B2 transition considering the thermal conductivity, whereby materials with low thermal conductivity undergo significant structural phase transitions under acoustic shocked conditions. Herein, we consider several nanosized materials for structural stability, including B1 (CdO, MgO, NiO), Corundum (α-Fe₂O₃, α-Al₂O₃), layered (graphene, graphite) and nonlayered (TiO₂ (anatase), ZnO (B4) and amorphous carbon) materials, under acoustic shocked conditions, which fall within the proposed mechanism.