The structure and physical properties of layered oxides and carbon materials were studied. Two layered carbon materials were studied: carbon nanotubes (CNTs) synthesized by electron irradiation from amorphous carbon in situ in a transmission electron microscope (TEM) and a carbon and silver nanocomposite consisting of graphitic carbon nanospheres encapsulating Ag nanoparticles. In the CNT experiments, the effect of electron irradiation in the TEM is shown to alter drastically their structure and properties, even being able to transform amorphous carbon into a CNT. This suggests a possible alternative synthesis technique for the production of CNTs, in addition to providing a method for tailoring their properties. The structure and magnetic properties of the carbon and silver nanocomposite was characterized with x-ray diffraction, scanning and transmission electron microscopy techniques, and magnetic susceptibility measurements with a superconducting quantum interference device (SQUID) magnetometer. While the sp² bonding gives a grapheme sheet its mechanical properties, the p[subscript pi] electrons are responsible for its electronic and magnetic properties. In a flat graphene sheet the p[subscript pi] electrons are itinerant, but in a narrow p[subscript pi] band. The introduction of curvature to the graphene sheets that encapsulate the Ag nanoparticles is demonstrated to narrow the p[subscript pi] band sufficiently to result in "ferromagnetic" behavior. A model that is able to explain spin localization and ferrimagnetic spin-spin interactions in graphitic materials with positive curvature is presented. Layered oxides from the family of the P2 Na[subscript x]CoO₂ structure were synthesized and their properties studied. Na[subscript x]CoO₂ has a rich phase diagram ranging form a promising Na-rich thermoelectric composition to the hydrated Na-poor composition Na[subscript 0.33]CoO₂· 1.3H₂O that is superconductive. Intermediate to these two Na compositions exists an insulating phase with x [approximately equal to] 0.5 that presents a variety of interesting structural, magnetic, thermoelectric, and electronic behavior. Investigations of Na[subscript x]CoO₂ that probe the role of H₂O in the superconductive Na[subscript 0.33]CoO₂· 1.3H₂O are presented and conclude that H₂O plays a more active role than a passive lattice spacer. The relationship between Na ordering and an interesting magnetic behavior observed with [chi](T) measurements of annealed NaxCoO₂ and Sr[subscript x/2]CoO₂ samples is determined and found to correspond to a (2a x 2a) superstructure. The properties of NaxCoO₂ (x [approximately equal to] 0.5) are reviewed and thermoelectric S(T) measurements are made in order to develop a model that is able to explain the salient features of the NaxCoO₂ (x [approximately equal to] 0.5) phase. / text

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Two groups of ferroelectric oxides have been studied using a.c. impedance techniques. These were donor-doped BaTiO₃ ceramics showing the positive temperature of resistance, PTCR, effect and single crystal LiTaO₃. Existing theories of the PTCR effect in BaTiO₃ ceramics assume that it is associated with the grain boundary regions. An in-depth analysis of a.c. data, using combined impedance and modulus spectroscopy revealed the presence of at least two components, both of which exhibited PTCR effects. These were attributed to bulk and grain boundary effects because of the different temperature dependence of their associated capacitances: grain boundary effects have temperature independent capacitances whereas bulk effects show a capacitance maximum at the Curie pont and Curie-Weiss behaviour above the Curie point. An explanation for the bulk PTCR effect is proposed. The a.c. data handling techniques used and developed here provide information regarding the inhomogeneous nature of the grain boundary and bulk components which cannot be obtained from d.c. measurements. An equivalent circuit to model the a.c. response of PTCR BaTiO₃ ceramics is presented. The influence of processing conditions on the various bulk and grain boundary PTCR effects, such as the sample cooling rate from the sintering temperature and low temperature anneals, < 400 °C, in various reducing and oxidising atmospheres is discussed. For quickly cooled samples, the PTCR response is dominated by the bulk impedance, whereas for slowly cooled samples, the grain boundary component dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas bulk PTCR effects are relatively insensitive to the atmosphere at low temperatures. Information regarding the conductive core of the grains and the behaviour of component resistances below the Curie point for slowly cooled samples is presented. A general model is proposed which explains the PTCR behaviour of BaTiO₃ ceramics. The effects of both cooling rate and atmosphere are incorporated in this model. A.c. impedance data for single crystal LiTaO₃ with the crystal c axis oriented parallel and perpendicular to the electric field were recorded above and below the Curie point, 590^oC. With the polar c axis parallel to the electric field, the following were measured: the charge polarisation associated with the ferroelectric domains, the intrinsic lattice polarisation, the resistance associated with domain re-orientation and the resistance due to lithium ion migration. The correct choice of equivalent circuit is crucial to the determination of these parameters. With the c axis parallel to the electric field, no ferroelectric behaviour was observed and the crystal was a modest electronic conductor at elevated tempeatures, 500-700^oC. The LiTaO₃ results presented indicate the potential of a.c. impedance techniques for probing the electrical properties of ferroelectric single crystals. Domain re-orientation may be characterised by macroscopic resistance and capacitance values. This allows the temperature dependence of domain re-orientation phenomena to be characterised as the crystal is heated through the Curie point. Such detailed characterisation cannot be obtained from existing fixed-frequency or d.c. measurements.