Crystal phase transition of urea: what governs the reaction kinetics in molecular crystal phase transitions

Because of their weak intermolecular forces and flexible molecular geometry, molecular crystals are renowned for their structural versatility (polymorphism) and the great difficulty in controlling the crystal form during synthesis. Despite its great importance in determining the final solid form (e.g. single crystal, polycrystal or amorphous), the kinetics of the crystal-to-crystal transformation between structures with different molecular packing has long been a fundamental challenge in both measurement and simulation. Here we report the first global potential energy surface (PES) for urea crystals obtained by stochastic surface walking global PES exploration. With the big data from thousands of crystal/amorphous forms, we, using exhaustive reaction pathway sampling, resolve the solid-to-solid transformation pathways between urea crystals from first principles. We demonstrate that the strong tendency to grow a large single crystal of urea can be attributed to the flat PES between major crystal forms that share the same hydrogen-bonding network pattern, where one crystal can transform to another facilely via crystal-to-crystal transition. Other crystal forms with distinct hydrogen-bonding network patterns can be excluded in crystallization due to their poor thermodynamic stability and high barrier of solid-to-solid transition. A general theory for predicting molecular solid transformation is proposed and illustrated in a simplified one-dimensional global PES, which is now obtainable from computational techniques established here.