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In Chapter 1, an account of the current research that has been carried out in the area of phosphorus-nitrogen systems with particular emphasis on cyclodiphosph(III)azanes is presented. The discussion considers the synthetic and structural differences between these compounds and focuses on their coordination chemistry. The major types of cyclic phosph(III)azanes are discussed along with the effect of the R group substituent and the synthetic procedure employed upon their structural type. Chapter 3, concerns the synthesis of new dichlorocyclodiphosph(III)azane species, [CIP(m-NR)]₂, and relates the synthetic and structural details of these to previously structurally characterised dichlorocyclodiphosph(II)azanes. These compounds are of particular interest because of their usefulness as precursor compounds to symmetrically heterosubstituted cyclodiphosph(III)azanes, and especially to macrocycles. Chapter 4 concerns the investigation of various substitution reactions that were undertaken with chloro-substituted cyclodiphosph(III)azanes, with the aim of producing amino-substituted species. It then turns to the reactions of these species with organo alkali metal reagents, such as ⁿBuLi, to obtain the anion derivatives. An interesting discovery is the existence of complexes containing sterically-uncongested [(RN)₂P]^- monoanions stabilised by coordination. The work presented in Chapter 5 discusses studies on the formation of macrocycles and gives insight into the mechanistic pathways to these species. This chapter initially addresses the formation of tetrameric and pentameric macrocycles constructed from [{P(m-N^tBu)}₂NH] units and pinpoints a divergent mechanism in their formation. This work leads to a proposed ‘twist’ mechanism that provides an understanding of steric factors governing the formation of different types of cyclodiphoph(III)azanes and the size of the macrocycles produced. In total, seventeen new compounds are described in this thesis, all of which have been structurally characterised.

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Double perovskite oxides have attracted much interest since the discovery of spin-polarized conduction and large low field magnetoresistances in Sr₂FeMoO₆.  This behaviour exists to high temperatures, making Sr₂FeMoO₆ and related materials candidates for application in data storage devices. This thesis describes the synthesis, structures and properties of some new double perovskite oxides. These can be divided into the (LaM)BNbO₆ (M = Ca, Sr, Ba; B = Mn, Co) and LaSrFeNbO₆ materials that have one magnetic transition metal (TM) cation, and the (La_1+xM_1-x)CoRuO₆ (M = Ca, Sr; -0.25 £ x £ 0.25 x = 1) materials that have two magnetic TM cations. The (LaM)CoNbO₆ double perovskites crystalize with the monoclinic P2_1/2 (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, which have rocksalt ordered arrangements of Co²⁺ (3d⁷) and Nb⁵⁺ (4d⁰), with almost no Co/Nb inversion (<4%). All three phases are anti-ferromagnetic with Neel temperatures of 17 K (M = Ca), 16 K (M = Sr) and 10K (M = Ba). The magnetic ordering is described by the magnetic propagation vector k = ( ½ 0 ½ ) for all materials. For M = Ca and Sr, two “non-collinear” and two “collinear” magnetic structures are found. These are experimentally indeterminate because of the small monoclinic angle (b = 89.9⁰).  The M = Ba material has a collinear antiparallel magnetic structure. The saturated cobalt moment decreases from 2.97(2) m_B to 2.52 (3) m_B to 1.85 (5) m_B for M = Ca, Sr and Ba, respectively. The frustration due to competing 90⁰ exchange interactions. The (LaK)MnNbO₆ materials also have the monoclinic P2₁/n (M = Ca, Sr) and tetragonal 14/m (M = Ba) superstructures, with <3% inversion of Mn²⁺ (3d⁵) and Nb⁵⁺ (4d⁰). All materials are antiferromagnetic with Neel temperatures of 9 K (M = Ca), 8 K (M = Ba) and 6.5 K (M = Ba). The magnetic Bragg reflections are indexed by k = (0 ½ ½ ), and the possible magnetic models are identical to those for (LaM)CoNbO₆. The manganese saturation moment decreases from 4.77(3) m_B (M = Ca) to 4.43 (4) m_B (M = Sr) to 41.9 (4) m_B (M = Ba). (LaSr)FeNbO₆ has the monoclinic P2₁/n superstructure with 14% inversion of Fe²⁺ (3d⁶) and Nb⁵⁺(4d⁰). This material is antiferromagnetic (T_N = 22 K), and has a ( ½ 0 ½) magnetic propagation vector (m_Fe = 3.45(3) m_B). The lattice parameters, antisite disorder, Neel temperatures and resistivities of the (La_1-xM_1-x)CoRuO₆ double perovskites are found to vary in a systematic manner. The Co/Ru antisite disorder is minimal for x = 0, showing that this disorder is driven by the mixed Co or Ru electronic states at x ≠ 0 rather than by La/M cation mixing. The x = 0 materials have T_N = 96K (M = Ca) and T_N = 85 K (M = Sr), and are variable range hopping semiconductors. Hole doping )x < 0) results in a rapid reduction in the resistivity while the Neel temperature increases only moderately, whereas electron doping (x > 0) results in a significant reduction in T_N but has little effect on the resistivity. X-ray absorption near edge spectroscopy shows that for x < 0, the Co²⁺ states are selectively hope doped while for x > 0 only the Ru⁵⁺ states are electron doped, which is in keeping with the strongly asymmetric doping response.

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A novel method of forming pure calcite seed materials by nucleation from an alkaline solution was developed. Batch experiments were conducted to investigate the growth and agglomeration rates of the calcite seed particles under varying agitation rates. Stirrer speeds between 700 and 2000 rpm were studied. The experiments were all carried out in a saline solution of ionic strength 0.153 M, with an initial supersaturation of approximately 4.0. The data from the experiments were analysed using the program Batch (Hounslow, 1990). This program extracts growth and agglomeration rates from the changes observed in particle size distributions with time. It is found that the growth rate is independent of the crystal size and the agitation rate. Growth rates from all experiments can be expressed by a single equation with a second order dependence of the growth rate on the supersaturation. This dependence, and the magnitude of the growth rate constant are consistent with other reports in the literature. The agglomeration rate is best described by a size-independent agglomeration kernel. At all except the lowest stirrer speed, the agglomeration rate is found to be proportional to the growth rate. The agglomeration rate is found to decrease as the stirrer speed increases. Correlation of the agglomeration rate with the stirrer speed shows that the agglomeration rate is proportional to the stirrer speed raised to the (-1.5) power. A comparison with estimates for the average shear rate within a stirred vessel suggest that the agglomeration rate is inversely proportional to the average shear rate within the crystalliser. Microscopic examination of many calcite samples was undertaken with SEM systems. Several hundred images of calcium carbonate particles were obtained. High resolution images were used to identify the phases present in preliminary trials, and to confirm that the calcite was the main product of the batch growth experiments. Images of samples taken from the batch growth experiments were used to confirm the decrease in the agglomeration rate at higher stirrer speeds, and the constancy of the growth rate. No evidence was found for any preferential alignment of the crystallites within the agglomerates produced by this method.

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