Understanding configurational entropy and polymorphism: a computational study of lithium alanate and molecular crystals

This thesis investigates complex energy landscapes and phase transitions of two different classes of materials; alanates and molecular crystals. With the use of the Minima Hopping structure search method, the effects of a structurally tolerant phase are studied in the case of lithium alanate and the molecular crystal consisting of N-(4'Methylbenzylidene)-4-methylalanine. A structurally tolerant phase allows for many variations of its basic structural motif that change the energy only marginally.
Firstly, a new implementation of Minima Hopping method is discussed. The Minima Hopping method combines short molecular dynamics trajectories with local geometry optimizations to efficiently explore the potential energy surface and locate the global minimum. With the new implementation in the Python language, different existing and new features have been brought together into one software package. Furthermore, the code is interfaced to the atomic simulation environment which offers a broad variety of energy and force evaluation routines. The method's effectiveness is demonstrated through its application to broad range of materials, highlighting its capability to overcome the challenges posed by the vast and high-dimensional search spaces typical of potential energy surfaces.
Combined with a machine learned potential, the Minima Hopping method is applied to lithium alanate, a promising candidate for hydrogen storage applications, to explore the configurational density of states. Lithium alanate exhibits an ionic phase which, unlike its polymeric counterpart, demonstrates high structural tolerance. The configurational density of states derived from Minima Hopping runs reveals that the ionic phase is stabilized through configurational entropy. A detailed analysis shows that despite the polymeric form being lower in energy, the ionic form remains prevalent due to its higher configurational entropy, explaining the absence of the polymeric phase in experiments.
Finally, the polymorphism in molecular crystals is studied, with a particular focus on N-(4'-Methylbenzylidene)-4-methylalanine. The configurational density of states, derived from Minima Hopping runs, indicates that Form II is the most structurally tolerant despite being higher in energy compared to Form III, the form lowest in energy. The phenomenon of disappearing polymorphism, where a stable polymorph transforms into another, rendering the original form challenging to reproduce, is explored for the case of Form I of N-(4'-Methylbenzylidene)-4-methylalanine. Such transformations had significant implications in pharmaceutical applications, where the stability of the polymorphic form directly impacts drug efficacy. The Boltzmann probabilities of different polymorphic forms are calculated to understand their thermodynamic stability, revealing that structural tolerance plays a critical role in the persistence of certain phases.