Abstract
Crystalline urea undergoes polymorphic phase transition induced by high pressure. Form I, which is the most stable form at normal conditions and Form IV, which is the most stable form at 3.10 GPa, not only crystallize in various crystal systems but also differ significantly in the unit cell dimensions. The aim of this study was to determine if it is possible to predict polymorphic phase transitions by optimizing Form I at high pressure and Form IV at low pressure. To achieve this aim, a large number of periodic density functional theory (DFT) calculations were performed using CASTEP. After geometry optimization of Form IV at 0 GPa Form I was obtained, performing energy minimization of Form I at high pressure did not result in Form IV. However, employing quantum molecular isothermal–isobaric (NPT) dynamics calculations enabled to accurately predict this high-pressure transformation. This study shows the potential of different approaches in predicting the polymorphic phase transition and points to the key factors that are necessary to achieve the success.
Highlights
Due to the fact that the high pressure studies are more demanding in terms of cost and time than experiments performed under normal pressure, it would be perfect if the density functional theory (DFT) calculations could be used to accurately predict the influence of the high pressure on the crystal structure and stability
For the more convenient assessment of the accuracy of calculations and influence of the tested parameters on the results, the obtained values were presented in tables together with the corresponding experimental ones
Using geometry optimization with no constraints resulting from the crystal space group applied it was possible to predict the Form IV to Form I transition at low pressure
Summary
Molecular Modeling of Pressure Induced Phase Transition. Polymorphism is a crucial phenomenon in many scientific disciplines, since the molecular packing determines the functional properties of organic solids. One of the methods that can be used to obtain new polymorphs is exposing the molecular crystals to high-pressure in order to induce the phase transition [1]. Many studies deal with the experimental pressure-induced polymorphic transformations in molecular solids [2,3,4], in some cases the high-pressure polymorph may have the same space group symmetry as the original ambient-pressure form and even be isostructural with it, which is defined as isosymmetric phase transition [5,6]. In most cases the polymorphic phase transition is accompanied with the change of cell dimensions and the crystal space group. Due to the fact that the high pressure studies are more demanding in terms of cost and time than experiments performed under normal pressure, it would be perfect if the DFT calculations could be used to accurately predict the influence of the high pressure on the crystal structure and stability
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