Local Structure of Molecular Materials via Neutron Total Scattering and Reverse Monte Carlo

Abstract

In this thesis the results of neutron total scattering experiments on a series of molecular materials are presented. Local uctuations in a material's structure may not be apparent from Bragg crystallography, but are captured within, and may emerge from re nement against, a pair distribution function. In molecular materials, the pair distribution function is dominated by intense low-r peaks corresponding to intramolecular correlations, making it more di cult to re ne the low-intensity high-r features corresponding to intermolecular correlations. Using reverse Monte Carlo re nement the local structure in three families of molecular materials have been studied. To reduce the problem posed by intramolecular correlations prior chemical knowledge of the molecular structure is incorporated into the re nement using empirical intramolecular potentials. One of the material families consists of three phases of ZIF-4, a metal-organic framework consisting of tetrahedrally coordinated Zn2+ ions connected by imidazolate rings. In this material, the molecular motion responsible for the greater exibility shown by the amorphous phase compared to the two crystalline phases was identi ed. The other two material families are organic crystals that consist of isolated molecules, the isomeric molecules para- and ortho-terphenyl and the other adamantane and its carboxylic acid derivative. Ortho-terphenyl is a paradigmatic glassformer, with a glass transition temperature of approximately 246 K and a critical cooling rate of less than 0.1 K min��1. An experimentally re ned structural model of the glass phase of this material was shown to be in good agreement with the primary structural predictions of glass theory. Para-terphenyl, on the other hand, is a crystalline material that undergoes a phase transition at 178 K. The presented results suggest that this phase transition is not as de nitively order-disorder as previously suggested but that there is in fact a very at energy surface between an order-disorder and a displace phase transition. Adamantane and adamantanecarboxylic acid are crystalline materials that undergo order-disorder phase transitions. The re ned structural models of both materials are consistent with a previously proposed structure for the high-temperature phases involving a disordered arrangement of molecules in two di erent orientations.