Flexural phonon instability defines intrinsic van der Waals elastic limits in the interlayer direction

Area of Science:

• Materials Science
• Solid-State Physics
• Nanotechnology

Background:

• Frenkel's cohesive model predicts a ~10% ideal elastic strain limit for strong solids, a limit generally observed in conventional materials.
• Van der Waals (vdW) solids, characterized by weak interlayer bonding, present a unique system to test established mechanical strength theories.

Purpose of the Study:

• To investigate the intrinsic elastic limits of vdW solids, specifically graphite and hexagonal boron nitride (h-BN).
• To identify the underlying mechanisms responsible for mechanical failure in vdW solids and compare them with conventional materials and other layered structures.

Main Methods:

• In situ tensile testing to observe mechanical behavior under controlled strain.
• First-principles calculations to analyze atomic interactions and failure mechanisms.
• Comparative analysis with other layered materials (MoS2, GaSe) and 3D crystals.

Main Results:

• Graphite and h-BN exhibit localized decohesion failure at significantly lower interlayer strain limits (~3% and ~2%, respectively) than predicted by conventional models.
• This premature failure is intrinsic and occurs well below defect-controlled limits.
• The phenomenon of strain localization and subsequent nanogap formation was observed and linked to flexural phonon instability.

Conclusions:

• Flexural phonon instability acts as a critical trigger for strain localization and decohesion in specific vdW solids.
• Established theories on solid strength require revision for vdW materials, necessitating new models for their intrinsic elastic limits.
• Understanding this dynamic instability opens avenues for strain engineering to tune the structural and transport properties of vdW crystals.