  The compressive properties of metal-organic framework (MOF) crystals are not
only crucial for their densification but also key in determining their
performance in many applications. We herein investigated the mechanical
responses of a classic crystalline MOF, HKUST-1 by using in situ compression
tests. A serrated flow accompanied by the unique strain avalanches was found in
individual and contacting crystals before their final flattening or fracture
with splitting cracks. The plastic flow with serrations is ascribed to the
dynamic phase mixing due to the progressive and irreversible local phase
transition in HKUST-1 crystals, as revealed by molecular dynamics and finite
element simulations. Such pressure-induced phase coexistence in HKUST-1
crystals also induces a significant loading-history dependence of their Young's
modulus. The observation of plastic avalanches in HKUST-1 crystals here not
only expands our current understanding of the plasticity of MOF crystals but
also unveils a novel mechanism for the avalanches and plastic flow in crystal
plasticity.
