Structural and electron density changes in dense guest-host systems: Analysis of X-ray diffraction data by the Rietveld and Maximum Entropy Methods
Creators: name: Flacau, Roxana Ioana
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2007
Subjects: subject: Physics, Electricity and Magnetism.

description: When studying the high-pressure structural behavior of crystalline materials, it is highly desirable to determine structural changes accurately, preferably at electron density levels. The Maximum Entropy Method (MEM) has already proven to be a very powerful tool for extracting the most probable charge density distributions directly from X-ray diffraction data. This thesis presents high pressure X-ray diffraction studies on two distinct, but structurally similar, classes of guest-host materials: gas clathrate hydrates (M8(H2O)46, with M= Kr, Xe) and silicon clathrate (Ba8Si46). In order to characterize the change of crystalline structure and electron distribution resulting from the increase of density due to the application of high pressure, we have used a recently developed approach wherein the classical Rietveld analysis is complemented iteratively with MEM calculations. It is found that charge density distributions derived from probability maps obtained by MEM provide further, in-depth insights into the structural changes induced by pressure in guest-host compounds. Clathrate hydrates are inclusion compounds, in which guest atoms or molecules are trapped in cages formed by an ice-like host lattice of water molecules. In recent years, large deposits of methane hydrate (a clathrate hydrate) have been found on the oceanic floors, leading to a considerable interest in the physical properties of gas hydrates. In the present study the crystalline structure I of xenon and krypton hydrates was investigated by powder X-ray diffraction at room temperature, over the pressure ranges for which these compounds are stable. Structure I, which has a cubic symmetry with Pm3n space group, is formed by two types of polyhedron, also referred to as small and large cages. The pressure dependence of the structural parameters was determined by applying a Rietveld analysis to the X-ray diffraction data. To further explore the effect of pressure on the guest atoms and the water molecule framework, we used the combined Rietveld/MEM method to derive the most probable charge density distributions at each pressure. Our results show that the charge density distribution of the encaged atoms differs depending on the type of the host cage, small or large, at all pressures. Spherical density distributions were observed for the guest atoms in the small cages, while the atoms in the large cages showed longitudinal elongated electronic distributions. These findings are common to both Kr and Xe hydrates. Along with the observed cage deformations, this is a clear indication that the guest-host interaction differs significantly between the small and large cages at high pressures. A similar behavior has been previously reported in low-temperature studies of methane clathrate hydrate. The combined Rietveld/MEM method was also successfully applied to explore the subtle changes in the electronic density distribution induced in Ba 8Si46 clathrate by the application of high pressure. This compound has been the object of extensive studies since its superconductivity has been discovered. Previous X-ray diffraction, near-edge X-ray absorption, and Raman spectroscopy studies have revealed two iso-structural phase transitions occurring at 5 and 17 GPa in Ba8Si46; their physical origin, however, was still not clearly understood. In our study, the most probable electron density distributions were calculated using the combined Rietveld/MEM method, with the goal to propose possible mechanisms for the two observed transitions. The examination of the electron density maps, and also electron density difference distributions, revealed that the low pressure transition is related to an enhanced charge transfer of Ba atoms to the Si framework, while the 17 GPa transition is a result of a sudden change in the electron density topology of the Si-Si bonds. As the pressure is increased, the electrons in the Si-Si bonds are displaced from the bonding region into the interstitial region, leading to a weakening of the Si-Si bonds, which explains the large volume reduction accompanying this transition.