This thesis describes the design, assembly, operation and behaviour of a nuclear magnetic resonance spectrometer of the Pound-Knight type built for investigation of Knight shift in metals and alloys. A brief introduction to nuclear magnetic resonance is given followed by a short discussion of the Knight shift. A short description of the various arrangements for observing nuclear magnetic resonance is given and after pointing out the requirements for observing the Knight shift in metals, our choice of the spectrometer is justified. The working of the spectrometer as a whole unit is explained and this is followed by a description of its various components. The technique of determining frequency is described in detail by giving a typical example of A127 signal from a solution of AlCl3. The behaviour of the spectrometer is discussed and its use in detection of various signals is given. The necessary conditions for obtaining good signals are summarised. The temperature variation and also the homogeneity of the magnetic field are studied with the aid of the spectrometer. The accuracy and usefulness of the equipment is tested by measuring the Knight shift in aluminium metal.

Abstract not available.

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.

Measurements of the evolution of the magnetic moment of nonideal (i.e. pinning rich) type II superconductors in stationary magnetic fields H a as a function of time at constant temperatures T < Tc or as the temperature is varied to or from Tc provide insight into the dynamics of flux lines and their interactions with the pinning sites. Three concentric magnetic moments coexist in these materials when the isothermal sweep of Ha causes the locus of the magnetization ⟨M⟩ to migrate along any path joining the upper with the lower envelopes of the major hysteresis curves. The "outermost" magnetic moment is always diamagnetic and is generated by the field opposing Meissner surface current. The two other magnetic moments fill the bulk of the specimen, currents circulating in one direction in the outer annular volume embrace an inner volume occupied by counter-rotating persistent currents. In semi-reversible type II superconductors the magnitude of the Meissner magnetic moment is important compared to that arising from the currents in the bulk whereas in very hysteretic specimens it is negligible. In both types of material it is a fairly straightforward procedure to establish configurations in various static Ha where the opposing magnetic moments exactly cancel each other so that the macroscopic net magnetization is zero. However the internal pattern of persistent currents and magnetic flux density profiles which prevail in these special ⟨M⟩ = 0 states is complicated, fascinating and provides a full crucible for the investigation of flux line behaviour. In this thesis I present the results of our experimental study of the evolution of the magnetization of these initial ⟨M⟩ = 0 states during warming to Tc as a function of the static applied field H a. The phenomena encountered are seen to depend dramatically on the direction of the change of Ha causing ⟨M⟩ to migrate from one envelope of the major hysteresis loop to the opposite. Further we find major differences in the behaviour exhibited by the semi-reversible type II superconductors in comparison with that manifested by the highly hysteretic samples. We also see that the low Tc and high Tc semi-reversible specimens display very similar but highly intricate behaviour. Finally we show that a simple model which exploits the critical state concept and applies the principle of conservation of flux lines can account for the major features of all our observations.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.