Monotropic Transition Mechanism of <i>m</i>‑Hydroxybenzoic Acid Investigated by Temperature-Resolved Second Harmonic Generation

Temperature-resolved second harmonic generation (TR-SHG) and SHG microscopy were used to study under normal pressure the solid–solid transition mechanism occurring between the two monotropically related polymorphic forms (metastable Pna21 and stable P21/n) of 3-hydroxybenzoic acid (MHBA). The activation energy Ea (as a measure of the barrier energy) of the irreversible transition was determined via isothermal TR-SHG (137–144 kJ·mol–1). It fits well with that determined from differential scanning calorimetry (139 kJ·mol–1). Regarding the two crystal structures, optical microscopy observations, and kinetics parameters from TR-SHG, a destructive/reconstructive mechanism is proposed for the solid–solid transition. The present study clearly demonstrates that TR-SHG is a relevant and accurate technique for monitoring solid–solid phase transitions.