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 Pna 2 1 and stable P 2 1 / n ) of 3-hydroxybenzoic acid (MHBA). The activation energy E a (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.
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Clevers et al. (2013) studied this question.
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