The electrical and magnetic properties of barium manganite solid solutions Ba_4-xA_xMn₃O₁₀, Ba_6-xA_xMn₅O₁₆, (A = La, Sr and Nd), Ba_4-xSr_xMn_3-yFe_yO₁₀ and Ba₆Mn_5-yFe_yO₁₆ have been investigated.  Initially, Ba₄Mn₃O₁₀ and Ba_3.9La_0.1Mn₃O₁₀ were synthesised.  XPS¹ measurements suggested that the latter phase was substoichiometric in oxygen.  A.C. impedance showed that the La-doped sample displayed insulator-like behaviour, whereas the pure one exhibited a metallic to insulator transition.  ND² results showed both samples are antiferromagnetic (with T_N = 170 K for Ba₄Mn₃O₁₀).  In addition, a decrease of the ordered Mn magnetic moment was observed in the La-doped sample. SEM³ and a.c. impedance data for Ba_4-xSr_xMn₃O₁₀ revealed that conductivity drops as grain size decreases.  It has also been shown that doping with a smaller ion causes a decrease in the Mn magnetic moment and can increase the resistivity of the sample.  Doping with a higher valence ion prompts a change in the conduction mechanism leading to semiconductor behaviour above room temperature.  A loss of oxygen induces a creation of Mn³⁺ - O – Mn⁴⁺ interactions which may affect the magnetic properties. Comparison between the properties of Ba_3.9La_0.1Mn₃O₁₀ and Ba_3.9Nd_0.1Mn₃O_10­ was one of the major contributions of this thesis.  The former is antiferromagnetic, whilst the latter became ferromagnetic below 100 K. Both compounds exhibit semiconductor behaviour above room temperature.  However, Ba_3.9La_0.1Mn₃O₁₀ has lower resistivity than Ba_3.9Nd_0.1Mn₃O₁₀.

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A new procedure in the preparation of polycrystalline, structurally sound, doped manganites was developed. This method is called "flashing." U sing the new preparation technique, several batches of samples of Lao.7Cao.3Mn03 and Lao.7Cao.3 (Mn(1_x)Fex)03 (with x = 0.05 and 0.07) were produced under different pelletizing pressures and annealing conditions. In the case of the Ca doped manganites, electrical resistivity as a function of temperature measurements, with and without an applied magnetic field of 350 mT, show a negative magnetoresistive effect, and a practically field-independent metal-insulator transition that depends upon heat treatment and annealing conditions. Similar measurements in Ca and Fe doped manganites reveal the same behavior with relatively large resistivity values and lower transition temperatures that depend on concentration. Maximal values for electrical resistivity in Fe doped samples are at least one order of magnitude higher compared to the maximal values for Ca doped specimens. X-ray diffraction, electrical resistivity, and electron-microscope analyses indicate a correlation between grain size, annealing environment, conductivity, and magneto-resistance. Magnetic susceptibility measurements indicate that the metal-insulator transition occurs approximately at the Curie temperature in all the cases studied.

The aims of this thesis are concerned with the synthesis of layered double hydroxide nanoparticles with controlled morphology and particle size distribution and an investigation of their physical properties. An introduction of layer double hydroxide chemistry, especially existing synthetic approaches, is reviewed in Chapter 1. Structural investigations, characterisation techniques, the properties and the applications of LDHs are discussed consecutively. The first successful synthesis of lithium aluminium nanorods using the hydrothermal treatment of a gibbsite precursor with a rod-like morphology is described in Chapter 2. The rod morphology is depicted using electron microscopy and confirmed by comparing refined X-ray diffraction patterns to a standard sample. Chapter 3 describes the application of reverse microemulsion method to prepare Co-Al and Ni-Al LDH nanoplatelets. The LDH particle sizes can be effectively controlled, and the structures of the nanoplatelets are investigated. The magnetic properties of the LDH nanoplatelets are dependent on the size of the nanoplatelets. A novel single component microemulsion system for the synthesis of LDHs is developed in Chapter 4. Mg-Al LDH nanoplatelets were successfully synthesised with precise particle size control. The factors affecting the formation of the microemulsions and the mechanism of the synthesis are discussed. Chapter 5 focuses on the applications of the novel single component microemulsion methods to prepare a range of LDHs with different metal combinations including Co-Al, Ni-Al, Zn-Al, Li-Al, Ca-Al, and Ni-Fe. This method proves very effective at controlling the particle sizes. The magnetic properties of the LDHs containing paramagnetic transition metal centres have been studied in detail. In Chapter 6, the DIFFaX program has been used to simulate the XRD patterns of layered structures. The factors influencing the XRD patterns in these materials have been systematically investigated including the effects of particle size, stacking faults, and disorder. The XRD patterns of materials described in previous chapters are simulated using the latest DIFFaX+ code in order to estimate the particle sizes and stacking sequences. The characterising techniques and the experimental details are listed in Chapter 7.

A number of different aspects of the chemistry of layered double hydroxides and similar materials have been explored in this thesis. The intercalation chemistry of these compounds, in both aqueous and mixed solutions, has been explored to synthesise over forty new hybrid layered materials by ion-exchange and rehydration-reconstruction methods. The host used include both previously-reported layered hydroxide compounds, and novel host materials synthesised by the extension of heterogeneous reactions of solid oxides. The synthesis of these novel materials provides insights into the factors that control the compatibility of the metal ions that may form hydrotalcite-like structures, in addition to providing compounds that may act as the precursors of catalysts and other functional materials. The new intercalation compounds synthesised show interlayer distances ranging from 7.1Å to 23.6Å, and demonstrate a variety of interlayer alignments as determined by powder X ray diffraction, dependent upon both the metallic content and the degree of hydration of the host compound. The anionic contents of the interlayer regions have been further characterised through infra-red spectroscopy, elemental microanalysis, solid-state NMR and thermogravimetric analysis techniques. The mechanism and kinetics of the release of the intercalated anions from the hosts have been studied in situations approximating their real-world applications, and the degree of release quantified by UV/Visible spectroscopy. Both the rates and mechanisms of anion release have been found to be dependent on the anion, the host, the temperature and the solution into which the anions are released; as such, the release timescales of the anions may be fined tuned through modifications of the host materials. In addition, an explanation has been suggested for the observed change in mechanism observed in instances of release in which the host compound is in only partial contact with the release medium.