Polymorphic phase transitions in carbamazepine and 10,11-dihydrocarbamazepine

Temperature-induced phase transitions in carbamazepine (CBZ) and 10,11-dihydrocarbamazepine (DHC) were studied by simultaneous differential scanning calorimetry–X-ray diffraction in this work. The transitions generally involve a transitional melt phase which is quickly followed by recrystallisation. The expansions of the unit cell as a function of temperature could be quantified and allow us to determine a directional order of stability in relation to the lattice constants. Dihydrocarbamazepine form II undergoes a conversion to form I by a localised melt phase. Carbamazepine (CBZ) form IV converts to form I at 182 °C, again by a localised intermediate melt phase. CBZ form II converted to form I at 119 °C by a pathway that appears to have included some melting, and form III underwent a part melt-recrystallisation and a part sublimation-recrystallisation to form I