Phase-Diagram Rationalization of Thermally and Mechanically Triggered Multiple Mechanical Responses of an Organic Crystal

Abstract

Phase diagram mapping is a useful method for predicting and rationalizing unusual thermally and mechanically triggered crystal transition phenomena. Here we focus on crystals of pimelic acid, which is known to exist as at least three polymorphs: modifications I–III. Modification III exhibits twinning ferroelasticity and a polymorphic transition, involving superelastic behavior by shearing. Additionally, a phase transition from modification III to modification II by heating involves a rapid expansion of crystals. The reverse transition from the metastable modification II to the thermally stable modification III at ambient temperature by mechanical stimuli involves rapid shrinkage, causing crystals to jump or rapidly fragment. For the first time a comprehensive phase diagram has been presented that accounts for the three different mechanical responses observed for pimelic acid: jumping/fragmentation, superelasticity, and ferroelasticity. Both thermal and mechanical transitions have been rationally mapped on the phase diagram.

Smart molecular crystal switches

“…Notably, the stress‐induced phase transition can be “frozen” if the chemical driving force for the reverse transition is small, and the system could exhibit a pseudo‐ferroelastic behavior. [ 43–45 ]…”

“…Pimelic acid exhibits LTS phase III and an HTS phase II. Phase III→phase II transition can be induced by stress applied along [101] in an SE way [44] . Interestingly, when the applied stress is held for 3 s, the formed HTS phase II does not spontaneously reverse to LTS phase III anymore, and remains supercooled at room temperature (pseudo‐FE).…”

“…However, when the temperature is abruptly decreased from room temperature to 213 K, the high chemical driving force causes the supercooled phase II to revert back to LTS phase III, thereby completing the shape‐recovery process [75] . Note that phase III can also be deformed by shear‐stress‐induced twinning along [001] [44] . However, it cannot be reverted to the undeformed original shape in a similar way as TPTB does.…”