Дисертації з теми "Novel Oxides"

This thesis describes the search for new mixed conductors as potential anodes and cathodes for applications in Solid Oxide Fuel Cells. Several compounds have been synthesised and characterised by means of different diffraction techniques (X-Rays, neutrons and electrons), high resolution electron microscopy and electrical measurements (AC Impedance Spectroscopy and DC resistivity measurements).

This work is based on the study of new synthetic paths to obtain thioimidate N-oxides (TINOs) from D-ribose and to study their reactivity with the purpose to obtain ketonitrones. TINOs, aren’t well known molecules, but these enantiomerically pure backbones could be valuable intermediates in the synthesis of novel ketonitrones which are key intermediates in the synthesis of iminosugars. TINOs were discovered from the study of glucoraphanin, a particular glucosinolate, that unexpectedly cyclized into a TINO after desulfatation, by a spontaneous intramolecular Michael addition. The first part of this work was to synthetize the TINO 3 from D-ribose 1. The key step was the desilylative cyclisation of a suitably functionalized thiohydroximate 2. Based on precedent work developed in the laboratory, we could obtain the thiohydroximate from D-ribose. We then focused our studies on the cyclisation step trying to find the suitable substituents that could give the TINO in good yield by desilylative cyclisation. The second part of the project is to obtain ketonitrones 4 by palladiumcatalyzed coupling reaction.

Metal oxide materials could offer earth-abundant, non-toxic alternatives to existing lightabsorber materials in thin-film photovoltaic and photoelectrochemical cells. However, efficiency of these devices based on existing metal oxides is typically low due to poor material properties. In this research, novel Sb:SnO2 nanorod and nanotube electron collectors have been synthesized, investigated and were used to improve the photo-conversion efficiency of top-performing BiVO4 photoelectrochemical cell. The performance of Sb:SnO2/BiVO4 photoanode achieved a new record for the product of light absorption and charge separation efficiencies (ηabs × ηsep) of ~ 57.3% and 58.5% under front- and back-side illumination at 0.6 VRHE and Sb:SnO2/BiVO4 PV cell achieved 1.22% solar power conversion efficiency. In addition, a new promising metal oxide material (CuBiW2O8) has been synthesized and its optoelectronic properties have been investigated to make photovoltaic cell which has potential to achieve over 30% solar power conversion efficiency.

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.

Transition metal compounds, especially the oxides, containing dn (0 ≤ n ≤ 10) electronic configuration, constitute the backbone of solid state/materials chemistry aimed at realization of novel materials properties of technological importance. Some of the significant materials properties of current interest are spin-polarized metallic ferromagnetism, negative thermal expansion, second harmonic nonlinear optical (NLO) susceptibility, fast ionic and mixed electronic/ionic conductivity for application in solid state batteries, and last but not the least, high-temperature superconductivity. Typical examples for each one of these properties could be found among transition metal oxides. Thus, alkaline-earth metal (A) substituted rare-earth (Ln) manganites, Lnı.xAxMnΟ3, are currently important examples for spin-polarized magnetotransport, ZrV2O7 and ZrW2O8 for negative thermal expansion coefficient, KTiOPO4 and LiNbO3 for second harmonic NLO susceptibility, (Li, La) TiO3 and LiMn2O4 for fast-ionic and mixed electronic/ionic conductivity respectively, and the whole host of cuprates typified by YBa2Cu3O7 for high Tc superconductivity. Solid state chemists constantly endeavour to obtain structure-property relations of solids so as to be able to design better materials towards desired properties. Synthesis coupled with characterization of structure and measurement of relevant properties is a common strategy that chemists adopt for this task. The work described in this thesis is based on such a broad-based chemists' approach towards understanding and realization of novel materials properties among the family of metal oxides. A search for metallic ferro/ferrimagnetism among the transition metal perovskite oxides, metallicity and possibility of superconductivity among transition-metal substituted cuprates and second order NLO susceptibility among metal oxides containing d° cations such as Ti(IV), V(V) and Nb(V) - constitute the main focus of the present thesis. New synthetic strategies that combine the conventional ceramic approach with the chemistry-based 'soft1 methods have been employed wherever possible to prepare the materials. The structures and electronic properties of the new materials have been probed by state-of-the art techniques that include powder X-ray diffraction (XRD) together with Rietveld refinement, electron diffraction, thermogravimetry, measurement of magnetic susceptibility (including magnetoresistance), Mossbauer spectroscopy and SHG response (towards 1064 nm laser radiation), besides conventional analytical techniques for determination of chemical compositions. Some of the highlights of the present thesis are: (i) synthesis of new mixed valent [Mn(III)/Mn(IV)] perovskite-type manganites, ALaMn2O6-y (A = K, Rb) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr) that exhibit ferromagnetism and magnetoresistance; (ii) investigation of a variety of ferrimagnetic double-perovskites that include ALaMnRuO6 (A = Ca, Sr, Ba) and ALaFeVO6 (A = Ca, Sr) and A2FeReO6 (A = Ca, Sr, Ba) providing new insights into the occurrence of metallic and nonmetallic ferrimagnetic behaviour among this family of oxides; (iii) synthesis of new K2NiF4-type oxides, La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe, Ru) and investigation of Cu-O-M interaction in two dimension and (iv) identification of the structural rnotif(s) that gives rise to efficient second order NLO optical (SHG) response among d° oxides containing Ti(IV), V(V), Nb(V) etc., and synthesis of a new SHG material, Ba2-xVOSi2O7 having the fresnoite structure. The thesis consists of five chapters and an appendix, describing the results of the investigations carried out by the candidate. A brief introduction to transition metaloxides, perovskite oxides in particular, is presented in Chapter 1. Attention is focused on the structure and properties of these materials. Chapter 2 describes the synthesis and investigation of two series of anion-deficient perovskite oxides, ALaMn2O6-y (A = K, Rb, Cs) and ALaBMn3O9_y (A = Na, K; B = Ca, Sr). ALaMn2O6-y (A = K, Rb, Cs) series of oxides adopt 2 ap x 2 ap superstructure for K and Rb phases and √2 av x √2 ap x 2 ap superstructure (ap = perovskite subcell) for the Cs phase. Among ALaBMn3O9-y phases, the A = Na members adopt a new kind of perovskite superstructure, ap x 3 ap, while the A = K phases do not reveal an obvious superstructure of the perovskite. All these oxides are ferromagnetic (Tc ~ 260-325 K) and metallic exhibiting a giant magnetoresistance behaviour similar to alkaline earth metal substituted lanthanum manganites, Lai_xAxMnO3. However, unlike the latter, the resistivity peak temperature Tp for all the anion-deficient manganites is significantly lower than Tc. In Chapter 3, we have investigated structure and electronic properties of double-perovskite oxides, A2FeReO6 (A = Ca, Sr and Ba). The A = Sr, Ba phases are cubic (Fm3m) and metallic, while the A = Ca phase is monoclinic (P2yn) and nonmetallic. All the three oxides are ferrimagnetic with Tcs 315-385 K as reported earlier. A = Sr, Ba phases show a negative magnetoresistance (MR) (10-25 % at 5 T), while the Ca member does not show an MR effect. 57Fe Mossbauer spectroscopy shows that iron is present in the high-spin Fe3+ (S = 5/2) state in Ca compound, while it occurs in an intermediate state between high-spin Fe2+ and Fe3+ in the Ba compound. Monoclinic distortion and high covalency of Ca-O bonds appear to freeze the oxidation states at Fe+3/Re5+ in Ca2FeRe O6, while the symmetric structure and ionic Ba-O bonds render the FeReO6 array highly covalent and Ba2FeReO6 metallic. Mossbauer data for Sr2FeReO6 shows that the valence state of iron in this compound is intermediate between that in Ba and Ca compounds. It is likely that Sr2FeReO6 which lies at the boundary between metallic and insulating states is metastable, phase-seperating into a percolating mixture of different electronic states at the microscopic level. In an effort to understand the occurrence of metallicity and ferrimagnetism among double perovskites, we have synthesized several new members : ALaMnFeO6 (A = Ca, Sr, Ba), ALaMnRuO6 (A = Ca, Sr, Ba) and ALaVFeO6 (A = Ca, Sr) (Chapter 3). Electron diffraction reveals an ordering of Mn and Ru in ALaMnRuO6 showing a doubling of the primitive cubic perovskite cell, while ALaVFeO6 do not show an ordering. ALaMnRuOs are ferrimagnetic (Tcs ~ 200-250 K) semiconductors, but ALaVFeO6 oxides do not show a long range magnetic ordering . The present work together with the previous work on double perovskites shows that only a very few of them exhibit both metallicity and ferrimagnetism, although several of them are ferrimagnetic. For example, among the series Ba2MReO6 (M = Mn, Fe, Co, Ni), only the M = Fe oxide is both metallic and ferrimagnetic, while M = Mn and Ni oxides are ferrimagnetic semiconductors. Similarly, A2CrMoO6 (A = Ca, Sr), A2CrRe06 (A = Ca, Sr), and ALaMnRuO6 (A = Ca, Sr, Ba) are all ferrimagnetic but not metallic. While ferrimagnetism of double perovskites arise from an antiferromagnetic coupling of B and B' spins through the B-O-B' bridges, the occurrence of metallicity seems to require precise matching of the energies of d-states of B and B' cations and a high covalency in the BB'O6 array that allows a facile electron-transfer between B and B', Bn++B’m+↔B(n+1)++B’(m-1)+ without an energy cost, just as occurs in ReO3 and other metallic ABO3 perovskites. In an effort to understand the Cu-O-M (M = Ti, Mn, Fe, Ru) electronic interaction in two dimension, we have investigated K2N1F4 oxides of the general formula La2-2xSr2XCui.xMxO4 (M = Ti, Mn, Fe or Ru). These investigations are described in Chapter 4. For M = Ti, only the x = 0.5 member could be prepared, while for M = Mn and Fe, the composition range is 0 < x < 1.0, and for M = Ru, the composition range is 0 < x ≤ 0.5. There is no evidence for ordering of Cu(II) and M(IV) in the x = 0.5 members. While the members of the M = Ti, Mn and Ru series are semiconducting/insulating, the members of the M = Fe series are metallic, showing a broad metal-semiconductor transition around 100 K for 0 < x ≤ 0.15 that is possibly related to a Cu(II)-O-Fe(IV) < > Cu(III)-O-Fe(III) valence degeneracy. Increasing the strontium content at the expense of lanthanum in La2-2xSr2XCui.xFexO4 for x ≤ 0.20 renders the samples metallic but not superconducting. In a search for inorganic oxide materials showing second order nonlinear optical (NLO) susceptibility, we have investigated several borates, silicates and phosphates containing /ram-connected MO6 octahedral chains or MO5 square-pyramids, where M = d°: Ti(IV), Nb(V) or Ta(V). Our investigations, which are described in Chapter 5, have identified two new NLO structures: batisite, Na2Ba(TiO)2Si4O12, containing trans-connectd TiO6 octahedral chains, and fresnoite, Ba2TiOSi2O7, containing square-pyramidal T1O5. Investigation of two other materials containing square-pyramidal TiO5, viz., Cs2TiOP2O7 and Na4Ti2Si8O22. 4H2O, revealed that isolated TiO5 square-pyramids alone do not cause a second harmonic generation (SHG) response; rather, the orientation of T1O5 units to produce -Ti-O-Ti-O- chains with alternating long and short Ti-0 distances in the fresnoite structure is most likely the origin of a strong SHG response in fresnoite. Indeed, we have been able to prepare a new fresnoite type oxide, Ba2.xVOSi2O7 (x ~ 0.5) that shows a strong SHG response, confirming this hypothesis. In the Appendix, we have described three synthetic strategies that enabled us to prepare magnetic and NLO materials. We have shown that the reaction CrO3 + 2 NH4X > CrO2 + 2 NH3 + H2O + X2 (X = Br, I), which occurs quantitatively at 120-150 °C, provides a convenient method for the synthesis of CrO2. Unlike conventional methods, the method described here does not require the use of high pressure for the synthesis of this technologically important material. For the synthesis of magnetic double perovskites, we have developed a method that involves reaction of basic alkali metal carbonates with the acidic oxides (e.g. Re2O7) first, followed by reaction of this precursor oxide with the required transition metal/transition metal oxide (e.g. Fe/Fe2O3). By this method we have successfully prepared single-phase perovskite oxides, A2FeReO6, ACrMoO6 and ALaFeVO6. We have prepared the new NLO material Ba2_xV0Si207 from Ba2VOSi2O7 by a soft chemical redox reaction involving the oxidation of V(IV) to V(V) using Br2 in CH3CN/CHCI3. Ba2V0Si207 + 1/2 Br2 > Bai.5V0Si207 + 1/2 BaBr2. The work presented in this thesis was carried out by the candidate as part of the Ph.D. training programme. He hopes that the studies reported here will constitute a worthwhile contribution to the solid state chemistry of transition metal oxides and related materials.

This thesis is comprised of three parts. The first part concerns the investigation of the topochemical reduction of LaSrNiRuO₆ in order to prepare LaSrNiRuO₄ via anion deintercalation. The second part discusses the oxide-for-hydride anion exchanges performed in SrV_1-xTi_xO₃, and the resulting SrV_1-xTi_xO_2-yH_1+y reduction products. Finally, the results from redox-neutral topochemical cation exchange reactions conducted in the three-dimensional perovskite structure of NaTaO₃ are presented along with the characterisation of a novel product of composition Ni_0.5TaO₃. The topochemical reduction of LaSrNiRuO₆ using CaH2 was carried out to produce a novel extended oxide phase with composition LaSrNiRuO₄. This phase is composed of sheets of apex-linked Ni¹⁺O₄ and Ru²⁺O₄ squares in a checkerboard ordered arrangement. To the best of our knowledge, this material is the first example of a B-cation ordered infinite-layer oxide phase. The low oxidation states of the transition-metal cations are confirmed by DFT calculations from which a spin moment S = ½ is determined for the nickel while the ruthenium centres adopt an intermediate-spin S = 1 configuration. LaSrNiRuO4 behaves paramagnetically at room temperature. However, upon cooling (T < 250 K) a phase transition is observed in which the nickel spins interact ferromagnetically, while the ruthenium cations appear to undergo a change in spin configuration to a diamagnetic spin state. A possible explanation is given for this observation based on an ordered arrangement of local Jahn-Teller distortions. While investigating the preparation of LaSrNiRuO₄, it was observed that different samples of the LaSrNiRuO₆ starting materials exhibited markedly different reactivity. The observed differing reactivity is inconsistent with the crystal structure and composition of the LaSrNiRuO₆ samples, from which all the materials are identical. Careful investigation of the X-ray diffraction data collected from the LaSrNiRuO6 materials revealed that the reactivity of the samples is a consequence of the microstructure. By quenching or slow-cooling the materials during their synthesis, the size of the crystalline domains formed is affected and this in turn is observed to define the extent to which the topochemical deintercalation of oxide anions takes place. A mechanism to explain this effect is presented in which the greater 'plasticity' of small crystalline domains helps to limit the influence of lattice strain during the reaction. Similar with the observations for the LaSrNiRuO₆ phases, it was found that the reactivity of SrV_0.95Ti_0.05O₃ samples towards topochemical oxide-for-hydride exchange is also determined by the characteristics of the starting materials. The cooling rate can lead to phase segregation in SrV_0.95Ti_0.05O₃ samples which in turn affects the reduction behaviour. A modification of the energy profile for the oxide-for-hydride exchange in SrV_1-xTi_xO₃ phases is proposed on the basis of the electronic configuration that the transition-metal cations adopt upon reduction (d²,V³⁺ and d¹,Ti³⁺). Finally, topochemical exchange reactions can also be carried out between cations in complex transition metal oxides when the mobility of the species to be exchanged is sufficiently greater with respect to the host lattice. The preparation of Ni_0.5TaO₃ from exchange of Na⁺ by Ni²⁺ in NaTaO3 represents a synthetic approach not yet widely explored in the long-standing challenge that the preparation of magnetoelectric multiferroic materials represents. The topochemical reactions studied in this work highlight the possibility of directing and modifying the product phases, by tuning features of the reagents. This is in contrast with the limited control available in thermodynamic processes.

This thesis is divided into two parts. The first part concerns the synthesis and characterisation of substituted metallocene complexes of the transition and main group metals. The second part describes the preparation and characterisation of inorganic fullerene (IF)-related materials. Chapter 1 reviews the chemistry of dialkyl- and diaryl-phosphino substituted cyclopentadienyl complexes of the transition metals. Chapter 2 describes the synthesis and characterisation of the new ligands [M(C₅Me₄)CH₂PMe₂] (M = H, Li, Na and K) via the precursors [HC₅Me₄)CH₂PHMe₂][X] (X = Cl and PF₆). The synthesis, characterisation and chemical reactivity of the compounds [Zr{{η-C₅Me₄)CH₂PMe₂}₂Cl₂ and [Mn{η-C₅Me₄)CH₂PMe₂}₂] is reported together with supporting evidence for the synthesis of the bimetallic complex [Zr{U+03B7-C₅Me₄)CH₂PMe₂}₂Cl₂PtI₂] and the complex [Mn{η-C₅Me₄)CH₂PMe₂B(C₆F₅)₃}₂]. Chapter 3 provides a brief introduction into the field of main group metallocenes and describes the synthesis and characterisation of the new main group metallocenes [M{η- C₅H₄)CMe₂PMe₂}₂] (M = Pb and Sn). The B(C₆F₅)_{3 adduct [Pb{η-C5H4)CMe2PMe2(B(C6F5)3}2] has also been synthesised. The main group - transition metal bimetallic complex [Pb{η-C5H4)CMe2PMe2}2PtI2 in which the substitutedplumbocene acts as a bidentate ligand, has been characterised by mass spectrometry and ³¹P{¹H}NMR spectroscopy. Chapter 4 provides an introduction into the field of IF-related materials and an overview of the analytical techniques used in their characterisation. Chapter 5 describes the preparation and characterisation of IF-MoS2 and IF-(Nb,W)S2 materials from MoC and the binary oxides Nb8 W9O47 and Nb4W13O47 respectively. A powder X-ray diffraction study of the conversion of WC to WO3-X and the subsequent sulfidisation by H2S to form novel IF-WS2 morphologies is also described. Chapter 6 describes the preparation of amorphous Group 5 metal (V, Nb and Ta) oxides using the metal vapour synthesis technique. These amorphous precursors have been annealed in a reducing atmosphere to form the oxide phases U+03B1-V2O3, NbO2 and TT-Ta2O5 which have been identified by powder X-ray diffraction. Upon reaction with H2S, the crystalline oxides afford layered sulfides of the form MS2 (M = V, Nb and Ta), some of which exhibit behaviour typical of IF-like materials and have been characterised by HRTEM, powder X-ray diffraction and EDX analysis. Chapter 7 outlines the experimental details for the synthesis, characterisation, reactions and compounds described in the preceding chapters. Chapter 8 presents the characterising data for the new compounds described in chapters 2 and 3. Appendix A contains details of the crystallographic data for the structurally characterised compound [Mn{η-C5Me4)CHPMe2}2].}

The family of phases Li3M2XO6 (M = Mg, Co, Ni; X = Nb, Ta, Sb) has a novel rock salt superstructure in which the X cations occupy one set of octahedral sites with the Li and M cations distributed over three octahedral sites in a non-random manner. The non-random site occupancies vary depending on M and X and appear to be an equilibrium feature of the structure. Li3Ni2NbO6 undergoes a continuous order-disorder transition and can be doped with Cr3+ by a number of mechanisms to give either ordered or disordered rock salt structures. The phase diagram of the solid solutions of general formula Li3-xNi2-xCr xNbO6 has been determined and shows solid solutions for both ordered and disordered polymorphs with a transition zone that is 200-300A°C wide. Li3Ni2TaO6 and Cr-doped Li3Ni 2NbO6 compositions are modest semiconductors. The alpha and beta structures of polymorphic Li2CuZrO 4 have been determined. Both structures are ordered rock salt with unique sites for Zr and one set of Li atoms; in the alpha polymorph, Cu and the other set of Li atoms are partially ordered over two sites, but in beta-Li 2CuZrO4 all cations are uniquely ordered. In the phase transition from alpha-Li2CuZrO4 to the low temperature beta polymorph, the CuO6 octahedra, which predominantly corner-share in alpha, become edge-sharing. Magnetic susceptibility data indicate that, in both polymorphs, there are both ferromagnetic and antiferromagnetic interactions. The predominant interactions in the alpha phase are antiferromagnetic whereas in the beta polymorph, they are ferromagnetic. Conductivity measurements show that beta-Li2CuZrO4 is a modest semiconductor.

The thesis is focused on the study of high temperature water gas shift catalysis, the identification of new improved catalysts and the study of the kinetics and mechanism of reaction over these catalysts. Rh-promoted Fe2O3-Cr2O3 was found to offer best performance which was significantly better than unpromoted catalyst over wide temperatures range. An extensive literature survey is first reported. Guidelines to develop new WGS catalysts are developed. As a result, the activities of precious metals supported on various oxides for high temperature WGS reaction have been tested. Rh(1wt%) doped Fe2O3/Cr2O3, exhibits the highest activity for WGS over a wide temperature range. 5wt%CuO/Fe2O3-Cr2O3, 1wt%Pt/Cr2O3, 1wt%Pt/Fe2O3-Cr2O3, 1wt%Pt/U3O8, 1wt%Pt/10%U3O8-Al2O3 and 1wt%Pt/5%V2O5-TiO2 fall into the second most active catalysts group, with an improved activity compared to commercial Fe2O3-Cr2O3 catalyst. It is clear that both the support/catalyst and the promoter can affect the efficiency of the WGS, leading to the obvious inference that the reaction rate is controlled at the promoter ??? support interface. Further kinetic studies and characterisation (TPR, TPD, pulse-adsorption (reaction)) on Rh/Fe3O4/Cr2O3 have been conducted. The study, conducted under conditions without inhibition from products of both forward and backward reactions, shows that the overall reaction rate expression is described as: 2 22 ??? =0.0041exp(???4042.6 ) 0.64 0.5 ???0.024 exp(???6022.9 ) 0.46 0.73 CO CO H O CO H r PP P P T T . Kinetics studies carried out under fuel reforming gas compositions shows that reaction rate expression changed when the temperature of reaction varied. The reaction rate equations at temperatures of 573K, 623K and 673K are derived as: 573K: 2 2 2 - 2.84 10-6 0.6 0.12 - 9.08 10-7 0.09 0.52 rCO = ?? PCO PH O ?? PCO PH 623K: 2 2 2 - 1.45 10-6 0.99 0.40 - 7.12 10-7 0.11 0.73 rCO = ?? PCO PH O ?? PCO PH 673K: -6 2 2 2 - = 4.37 ?? 10 0.86 0.41 -1.83 ??10-6 0.28 0.66 rCO PCO PH O PCO PH , The apparent activation energy was 61.7??2.5 kJmol-1 . TPR, TPD, TPO characterisation studies and reoxidation of catalysts by CO2 or H2O show that the active site for high temperature WGS reaction on Rh/Fe2O3/Cr2O3 is reduced magnetite Fe3O4 which dissociatively breaks down the H2O to form H* and OH* and adsorbs CO2. The deposited metal, Rh, acts as a promoter by facilitating the uptake of hydrogen (H2) and carbon monoxide (CO), desorption of H2 (at high temperature) and CO2.

Platinum group metals (PGMs) are well established and widely used for catalytic processes. It has been demonstrated that PGMs can be superior catalysts for hydrocarbon conversion reactions compared to the industry standard. However, the findings in academic labs cannot always translate directly to industrial usage. One limitation to using PGMs for large scale processes is sometimes cost. Access to an inexpensive and highly efficient catalyst could be a step towards using recovered hydrocarbons, in the form of natural gas and shale gas, more widely for energy production.

While platinum group metal species have been intensively examined, less is know about reactivity associated with ionic PGMs in oxides. Using ionic species could be a route to achieving more efficient conversion of mixed hydrocarbon feedstocks to fuels and commodity chemicals. The work presented in this dissertation focuses on Pd–substitution in binary and complex oxides along with model compounds containing noble metals, with the aim of preparing an inexpensive and robust C–H bond activation catalyst. With an emphasis on preparation methods and careful characterization, it has been a goal of this work to establish structure–property relationships in oxide catalysts.

Initial work on Pd–substitution in CeO₂ has lead to the development of ultrasonic spray pyrolysis (USP) as a method for preparing substituted oxides having relatively high surface area. Phase pure Pd–substituted perovskites were also prepared using this technique. Methane partial oxidation reactions on Pd–substituted CeO₂ provided some understanding of Pd substitution in oxides. It was determined that ionic Pd when substituted in CeO₂ is readily reduced to fcc-Pd. Investigation of more complex oxides that could stabilize ionic Pd under reducing conditions through inductive effects became the target of subsequent research.

Pd-substituted LnFeO₃ (Ln = Y, La) showed promising results for increased stabilization of Pd ions under methane partial oxidation conditions. Microwave-assisted heating methods were employed to prepare these materials very rapidly. With just several minutes of microwave-assisted heating, Pd–substituted perovskite materials were prepared for characterization and testing. With the help of our collaborators, Pd–substituted LaFeO₃ was applied as a catalyst precursor material for aryl chloride coupling under mild conditions.

The focus was shifted to model compounds, noble metal complex oxides, La₂BaPdO₅ and La₂BaPtO₅, already having unique noble metal sites. The thermal stability of these complex oxides compared to binary oxides was both an attractive property for probing ionic PGM catalysis and a fascinating feature, worthy of further investigation. Using density functional theory, the electronic structures of La₂BaPdO ₅ and La₂BaPtO₅ were compared to those of the binary PdO and PtO oxides. It was determined that a shift in the O 2p band is responsible for the increased stability in complex oxides.

Through this study of Pd ions, we have developed two novel methods for preparation of substituted and stiochiometric oxide materials. A combination of characterization methods, including X-ray diffraction and X-ray photoelectron spectroscopy, are required to understand the structure of these complicated materials. Often times, more advanced characterization, such as neutron diffraction and extended X-ray absorption fine structure measurements, provide the necessary insights to understanding the structures and properties of oxide catalysts. In this dissertation, preparation and characterization are emphasized for ionic PGM and oxide catalysts.

This thesis investigates the interstitial oxide ion accommodation, induced local deformation around interstitial defects and increase in ionic conductivity in doped La3Ga5GeO14 materials with a langasite structure. The choice of langasites as a target system for oxide interstitial doping was motivated by their structural resemblance with apatite and melilite materials, which by the accommodation of up to 3.7 and 4.6 % of extra oxygen respectively increased the ionic conductivity up to 5.6 ⨯ 10-3 S∙cm-1 in La9.75Sr0.25(SiO4)6O2.875 and 4.1 ⨯ 10-2 S∙cm-1 in La1.54Sr0.46Ga3O7.27 at 700 °C. The good ionic conducting properties shown by these materials lights the way for the development of novel oxide ionic conducting materials toward the doping of tunnelled structures with large voids for oxide interstitials and away from the more traditional approach based on the generation of oxygen vacancies by aliovalent doping of close-packed systems such as fluorite (.. YSZ, GDC) and perovskite (.. LSGM) type materials. Here we show that the La3Ga5GeO14 langasite is able to accommodate 5.36 % of extra oxygen in La3Ga3.75Ge2.5O14.75 by a carefully developed Pechini route. Elemental analysis in various La3Ga5-xGe1+xO14+x/2 compositions presented in this work demonstrated the successfully substitution of Ga for Ge at the desired ratio with the consequent incorporation of extra oxygen. AC conductivity measurements were carried out on dense pellets prepared by conventional and fast Spark Plasma-assisted (SPS) sintering methods revealing an increase in the conductivity of two orders of magnitude (~4 × 10-3 S∙cm-1 at 700 °C in La3Ga5GeO14.25) when compared to the un-doped langasite (La3Ga5GeO14, ~1 × 10˗5 S∙cm-1). The location of the extra oxygen and the induced local deformation was studied by 71Ga and 17O Solid-State NMR and high-resolution neutron powder diffraction (NPD) techniques. The extra oxygen incorporated by doping was found to form a (Ga/Ge)2O8 unit predicted by DFT calculations. Two neighbouring tetrahedra sites bridged by oxygen are relaxed into two pseudo squarebased pyramid like polyhedral sharing one edge. This unit shows a high resemblance to the Ge2O8 unit found in La3GaGe5O16. Two additional doping mechanisms were studied involving the partial and complete substitution of La/Ga in La3Ga5-xGe1+xO14+x/2 for isovalent cations: La3˗yLnyGa5˗xGe1+xO14+x/2 where Ln = Pr, Nd, Sm and Gd and La3Al5-y-xGayGe1+xO14+x/2. Neutron powder diffraction studies carried out in these compositions revealed a (Ge/M)2O8 environment for the extra oxygen analogous to the one determined for La3Ga3.5Ge2.5O14.75 and La3Ga4Ge2O14.5. The conductivity analysis revealed a decrease in the bulk conductivity at 500 °C with the decrease in size of the large lanthanide atoms from ~2.44 × 10-5 S∙cm-1 in La3Ga4.5Ge1.5O14.25 to 4.51 × 10-6 S∙cm-1 in La1.5Nd1.5Ga4.5Ge1.5O14.25. The isovalent substitution of Ga3+ for Al3+ causes a decrease in conductivity by one order of magnitude, where the highest conductivity value of 2.94 × 10-6 S∙cm-1 is reached by La3Al4.8Ge1.2O14.1 at 500 °C.

In this thesis two sensitive absorption techniques are used for the detection of trace concentrations of two different nitrogen oxides, which are of interest for atmospheric chemistry. These techniques are both based upon the interaction of mid-infrared laser radiation, provided by Quantum Cascade Lasers, with the molecules being studied. The Optical Feedback-Cavity Enhanced Absorption spectroscopy (OF-CEAS) technique combines the path length benefits of an optical cavity with injection locking of the Quantum Cascade Laser with light returned from the optical cavity. This optical feedback creates a comb of intense modes with spacing dependent on the length of the cavity whose intensity variations can be used to retrieve an absorption spectrum of an analyte gas. This thesis provides a novel method for the improvement of the spectral resolution of the spectrometer: by mounting one of the cavity mirrors upon a moveable stage the cavity mode comb can be translated. By interleaving a series of these spectra a higher resolution spectrum can be created . This is shown to give an improvement in the precision of both the concentration and linewidth retrieved from OF-CEAS measurements. The spectrometer showed the capability of detecting nitrous oxide at concentrations found in ambient air and with pressures as low as 10 Torr. The second technique is Faraday Modulation Spectroscopy. This technique which uses the magnetic splitting of ro-vibrational transitions of paramagnetic molecules to produce a sensitive . absorption technique. The magnetic field is modulated at a frequency much greater than that being used for the laser frequency sweeping and the signal is detected at multiples of the modulation frequency. This spectrometer was used for the detection of nitric oxide and showed an improvement in sensitivity relative to direct absorption measurements.

The crystal growth conditions of compounds of the series Srn+1IrnO3n+1 (n=1, 2 and ∞) are investigated. It was found that the ratio of IrO2:SrCO3 in the starting mixture is the most important variable in determining the phase formed. Good quality samples of Sr3Ir2O7 were found to have a sharp change in gradient at the Néel temperature of 287.5 K and no secondary T* transition between 230 K and 260 K. All crystals of Sr3Ir2O7 grown were found to be heavily oxygen deficient by EPMA regardless of the crystal growth conditions used with an average stoichiometry of Sr2:87Ir2O6:27. Adding more electrons via replacing strontium with lanthanum causes (Sr(1-x)Lax)3Ir2O7 to become metallic by x=0.072, which also fully quenches the long-range antiferromagnetic order. Heat capacity and resistivity measurements show that metallic (Sr(1-x)Lax)3Ir2O7 is a weakly correlated Fermi-liquid metal. Given that there are only subtle changes to the structure upon lanthanum doping, the metal-insulator transition is a result of electron doping rather than structural distortions. No structural phase transitions were found up to a temperature of 800°C and no additional evidence was found to support the Bbcb space group model of the structure of Sr3Ir2O7. Using crystals five times better in quality than those reported in the literature, SrIrO3 was found to be a Fermi-liquid metal, rather than a non-Fermi liquid metal as previously reported, and no superconductivity was found down to temperatures of 20 mK. A known Pt(III) compound, CaPt2O4, was found to be a weakly correlated metal down to 2 K and a novel Pt(III) based compound, K2CaPt3-δ O6 (δ ≈ 0.4), was discovered. K2CaPt3δ-O6 has a structure consisting of monolayers of edge-sharing PtO6 octahedra separated by layers of ordered K+ and Ca2+ ions in a 2:1 ratio. The structure of K2CaPt3-δO6 was found to be flexible to doping with copper, causing the magnetic properties to change from temperature independent to paramagnetic.

The materials synthesised in this study were designed to have novel morphology coupled with a tightly controlled surface composition that could be varied depending on a application. Preparations with simple single metal oxides (i.e. TiO2, SiO2, Al2O3 and ZrO2) were used as the starting point with the latter stages involving multi-metal oxide coatings and materials. The research was divided into three interconnected areas; i) biotemplating, ii) alternative synthetic morphologies to biotemplating and iii) the synergy between microparticles and insecticides. For the investigation into biotemplating pollen was chosen as the main example due to its ubiquity. Here, good replication of its structure with metal oxides can be achieved by two-dimensional solgel chemistry. Such materials can be further modified to have tunable surface chemistry through dopants and optical properties (i.e. fluorescence) through the use of dyes. Materials were extensively characterised using primarily spectroscopy (UV and IR) and microscopy (i.e. SEM coupled with EDX elemental analysis). These were considered for several applications and examples investigated here included as a taggant technology and photocatalytic removal of methyl orange in an aqueous environment (TiO2-pollen only). For the latter, results have been compared with those of a commercially available alternative (P25) where the preliminary results are very promising. The method of overcoating was also shown to be transferrable to other flora and fauna biotemplates. Synthetic alternatives for the biotemplated pollen were considered in the second investigative area where solution sol-gel processes such as the Stöber method were considered in addition to other suspension based precipitation methods (i.e. refluxes and microemulsions). Processes developed in the biotemplating research were applied here and analysed again using spectroscopy and microscopy as the main techniques. As part of this aspect, a novel fast-drying water-in-oil microemulsion delivery and preparative system was also developed using low boiling point solvents such as isopentane and ethanol and low toxicity sucrose ester surfactants. Hollow oxide shells could be prepared in these using a novel low-temperature route that were comparable in thickness (but significantly smaller in size) to hollow pollen replicas. In this second area attention was shifted to more focus on oxides of Si and Al (as opposed to TiO2 that used extensively in biotemplating) to broaden the scope of the research and investigate other potential applications, such as nanoabrasives (surface roughness and ability to cleave DNA). The third and final area of interest used the materials from the previous two aspects in coatings that were applied to investigating the knockdown (KD) and total mortality (TM) of selected arthropods. Here mosquitoes of the A. Gambiae and S. Aegypti genus were considered with particular focus on synergistic effects with existing commercial insecticides (using mainly CDC bottle tests). Microscopy was used as the primary characterisation technique here to determine particle transfer after each assay. In these tests %TM suggested SiO2 microspheres were particularly effective at in enhancing mortality of the commercial l-cyhalothrin insecticide. Additionally, novel methods of recording mosquito behaviour was investigated through optical and thermographic stills and videos.

In this thesis, preliminary growths of SrCu0₂ epitaxial films were undertaken. Despite none of the growths yielding the desired stoichiometry or oxidation, the difficulties that were encountered, such as properly oxidizing the film, are demonstrated to be surmountable. The structure of the films was probed with RHEED, and found to display growth patterns that are consistent with epitaxial growth. The desire to perform future ARPES studies on this material imposes strict sample quality requirements and the need for the most accurate measurements of stoichiometry possible. This warranted an investigation of the properties of the SPECS PHOIBOS 150 hemispherical electron analyzer used to perform XPS measurements of films chemistry. The degree of non-linearity of the electron detection system was determined and a method introduced that could correct for it. Correcting for the transmission function of the analyzer was found to be a necessary step in quantitative chemical analysis. Therefore, transmission functions were measured using two techniques and were found to be in good agreement with calculated transmission functions provided by SPECS. While correcting for detector non-linearity and measuring the actual transmission function did not drastically alter the stoichiometry measurements for scans taken using reasonable analyzer settings for XPS (1% for the films grown in this thesis, mostly from detector non-linearity), it is possible to drive the analyzer into regimes where the corrections will be necessary. The methods used to first correct for the intensity response of the analyzer, and then the transmission function would be more applicable in these instances. The work performed in this thesis will ensure that future growths of SrCu0₂ films will yield samples of adequate quality to perform an ARPES study.
Science, Faculty of
Physics and Astronomy, Department of
Graduate

In this thesis I present the synthesis and characterization of materials exhibiting frustrated magnetism. In Chapter 1 I describe magnetic frustration and some of the magnetic states that can arise from it followed by the background on iridates and platinates with honeycomb structures and rare earth pyrochlores.

In Chapter 3 I discuss my work on the synthesis and properties of ternary sodium iridates with formulas Na_xM_2/3Ir_1/3O ₂ and Na_xM_1/3Ir_2/3O₂ (M = Mn, Fe, Co, Ni, Cu, Zn). The ternary iridates are based on the honeycomb compound Na₂IrO₃ but show more disorder in the honeycomb layer than the parent. The six new compounds are all spin glasses but show distinct magnetic properties from one another.

In Chapter 4 I continue my work on honeycombs by exploring new ternary sodium platinates. These three new compounds with formulas Na₃MPt ₂O_6+x (M = Mg, Cu, Zn) are structurally very similar to the iridates discussed in Chapter 2 but have non-magnetic Pt⁴⁺ in place of magnetic Ir⁴⁺. The Mg and Zn variants are non-magnetic while the Cu variant is paramagnetic at 2 K.

Chapter 5 is a synchrotron X-ray diffraction study of the magnetically frustrated rare earth pyrochlores Ho₂Ti₂O₇, Er₂Ti₂O₇ and Yb₂Ti₂O ₇. Previous neutron scattering studies have shown reflections that are forbidden by the assigned space group Fd-3m, therefore high intensity, high resolution X-ray diffraction data was collected to determine if the reflections are present. Slight variations in sample stoichiometry were studied to account for possible sample variation. The forbidden reflections are absent from the X-ray diffraction patterns, providing strong evidence that the extra reflections in neutron scattering experiments are not structural in origin.

Thesis submitted in fulfilment of the requirements for the degree of Masters of Technology: Chemical Engineering in the Faculty of Engineering Cape Peninsula University of Technology, South Africa Supervisor: Prof TV Ojumu Cape Town Campus September 2013
Jarosite formation is regarded as undesirable in the bioleaching processes as it depletes ferric reagent; a critical reagent for the oxidation of most sulphide minerals, from bioleach solution. It creates kinetic barriers and clogs on mineral surfaces, thereby retarding leach rates of most minerals. However, jarosite has also been shown to serve as support for the attachment of bioleaching microbes, facilitating a high ferric-iron generation rate. In this study, a series of experiments on microbial ferrous-iron oxidation by a mesophilic microbe were carried out in a novel packed-column bioreactor with a view to investigating the potential use of solution pH to manage jarosite accumulation in the bioreactor. The kinetics of the oxidation was also investigated to establish base case data for the novel bioreactor. The bioreactor was packed with glass balls 15 mm in diameter. The experiments were conducted at a constant temperature of 38.6 °C, residence time of 18 hrs, airflow rate of 20 mL.s-1 and at desired solution pHs (1.3, 1.5 and 1.7). The results showed that the amount of jarosite accumulation is proportional to the operating solution pH and also to the duration of operation of the bioreactor. Jarosite precipitate of 4.95, 5.89 and 7.08 g.L-1 were obtained after 10 days of continuous operation at solution pH of 1.3, 1.5 and 1.7 respectively, while after 15 days the precipitate concentration increased to 5.50, 7.90 and 9.98 g.L-1respectively. The results also showed that a 33% and 52% reduction in jarosite accumulation could be achieved by a gradual decrease of the bioreactor solution pH after being continuously operated for 10 days from pH 1.7 to 1.5 and pH 1.7 to 1.3, respectively, for an additional five days of continuous operation. The results of the ferrous-iron biooxidation kinetics investigated at pH 1.3 show a maximum ferrous oxidation rate ( max 2 Fe r ) of 6.85 mmol.L-1.h-1 and apparent affinity kinetics constants (   2 Fe K , 2 Fe K ) of 0.001 mmol Fe2+.L-1 and 0.006 (dimensionless) using Hansford and Monod equations, respectively. Although a direct relationship exists between jarosite formation and solution pH, the results of this study may be relevant in bioleach heaps, or at least in column bioreactors, to manage and control jarosite accumulation, thereby improving leach kinetics of sulphide minerals.

Three methods for the generation of pyranosyl nitrile oxide have been utilised: firstly isocyanate-induced dehydration of pyranosylnitromethanes (the Mukaiyama approach); secondly hypochlorite-mediated oxidation of pyranosyl aldoximes, and finally base-catalysed dehydrohalogenation of pyranosyl hydroxyimoyl chlorides. The pyranosyl aldoximes derived from D-mannose, D-glucose, D-xylose and L-fucose were synthesised via stannate (II) mediated reduction of the corresponding pyranosylnitromethane (80-90%), whilst direct chlorination of the aldoximes gave the hydroxyimoyl chlorides (80-95%). The efficiency of nitrile oxide generation by all three methods was illustrated by the high levels of 3,4-(dipyranosyl)-1,2,5-oxadiazole-2-oxides (furoxans) obtained by dimerisation on generation in the absence of a dipolarophile. For example, 3,4-(2,3,4,6-tetra-O-acetyl-β-D-mannopyranosyl)-1,2,5-oxadiazole-2-oxide was prepared from 2,3,4,6-tetra-O-acetyl-β-D-mannopyranosylnitromethane in a 90% yield. The D-glucose, D-xylose and L-fucose derived analogues were synthesised in high yields (79-96%) utilising all three methods. Where compatible, the Mukaiyama approach gave cycloadducts in good yields (>75%) (e.g. on reaction with methylenecyclohexane and styrene). However for low boiling or thermally unstable dipolarophiles or for dipolarophiles with functional groups incompatible groups incompatible with the isocyanate dehydrating agent (e.g. dimethyl acetylenedicarboxylate, allyl alcohol), cycloadditions were carried out successfully using pyranosyl aldoximes and pyranosyl hydroximoyl chlorides as precursors. The cycloaddition reactions with achiral alkenes proceeded with low levels of diastereoselectivity (d.e. < 10%), in contrast to the cycloaddition of chiral alkenes where significant p-facial selectivity was observed (d.e. 40-80%). The chemistry of the dipyranosyl furoxans has also been investigated. Hydrogenation of 3,4-(2,3,4-tri-O-acetyl-β-D-xylopyranosyl)-furoxan in the presence of Raney nickel yielded the corresponding dipyranosyl-1,2-dioxime in 77% yield. Preliminary studies of the chemistry of this class of dioxime have been carried out, for example dehydration to the 1,2,5-oxadiazole (furazan) and re-oxidation to the dipyranosyl furoxan. Attempts at complexation of these novel chiral ligands with nickel show promising results.

This thesis details the synthesis and characterisation of novel silica coated Fe3O4-ZnO core-shell nanostructures for applications in target-specific photodynamic cancer therapy. While the silica shell was outfitted with folic acid for promoting cell specificity, dansyl chloride for fluorescent imaging and polyethylene glycol for biocompatibility and stability, the photoactive core showed significant in-vitro photo-killing ability. The methods and results established in this thesis provide researchers of cancer therapeutics new insights into nanoparticle design and modification schemes.

Metal oxide nanoparticles, both magnetic and nonmagnetic, have a multitude of applications in gas sensors, catalysts and catalyst supports, airborne trapping agents, biomedicines and drug delivery systems, fuel cells, laser diodes, and magnetic microwaves. Over the past decade, an inexpensive, simple, recyclable, and environmentally friendly large, scale synthesis method for the synthesis of these metal oxide nanoparticles has been sought. Many of the current techniques in use today, while good on the small, laboratory bench scale, suffer from drawbacks that make them unsuitable for the industrial scale. The aminolytic method, developed by Dr. Man Han while working for Dr. Zhang, fits industrial scale-up requirements. The aminolytic method involves a reaction between metal carboxylate(s) and oleylamine in a non-coordinating solvent. This system was shown to produce a range of spinel ferrites. Dr. Lisa Vaughan showed that this method can be recycled multiple times without degrading the quality of the produced nanoparticles. The purpose of this thesis is to test the versatility of the aminolytic method in the production of a wide range of metal oxides as well as various core/shell systems. Chapter 2 explores the effect of precursor carboxylates chain length on the aminolytic synthesis of cobalt ferrite, and manganese ferrite nanoparticles. In Chapter 3, a series of CuxMn1-xFe₂O₄, (x ranges from 0.0 to 0.2), nanoparticles were synthesized via the aminolytic method. This series allows for the investigation of the effects of orbital Jahn-Teller distortion as well as orbital angular momentum on the magnetic properties of this ferrite. The quantum couplings of magnetic ions in spinel ferrites govern their magnetic properties and responses. An understanding of the couplings between these metal ions allows for tailoring magnetic properties to obtain the desired response needed for various applications. Chapter 4 investigates the synthesis of MnO and Mn₃O₄ nanoparticles in pure single phase with high monodispersity. To the best of our knowledge, the range of sizes produced for MnO and Mn₃O₄ is the most extensive, and therefore a magnetic study of these systems shows some intriguing size dependent properties. The final part of this chapter investigates the applicability of the aminolytic method for building a MnO shell on a CoFe₂O₄ core. Chapter 5 explores the synthesis of another metal oxide, ZrO₂ in both the cubic and monoclinic phases with no impurities. The use of the aminolytic method here removes the need for dangerous/expensive precursors or equipment and eliminates the need for extensive high temperature heat treatments that destroy monodispersity which is required for most techniques. The creation of a core/shell system between CoFe₂O₄ and ZrO₂ using the aminolytic method was also tested. This core/shell system adds magnetic manipulation which is especially useful for the recovery of zirconia based photocatalyst. Chapter 6 studies the application of the aminolytic method in the synthesis of yttrium iron garnet (YIG) and yttrium iron perovskite (YIP) nanoparticles. Current synthesis techniques used to produce YIG and YIP nanoparticles often requires high temperatures, sensitive to contamination, which could be eliminated through the use of our method

Functional materials, particularly metal oxides, have been the focus of much attention in solid state chemistry for many years and impact every aspect of modern life. The approach adopted in this thesis to access desirable functionality for enhanced fundamental understanding is via modifying existing materials by deploying reducing synthetic procedures. This work spans several groups of inorganic crystalline materials, but is unified by the development of new properties within host compounds of particular relevance to solid oxide fuel cell technology, which allow interstitial oxide ion conduction at elevated temperatures. The Ca₁₂Al₁₄O₃₂e₂ electride was successfully synthesized by replacing the mobile extra-framework oxygen ions with electrons acting as anions. The high concentration of electrons in the C12A7 electride gives rise to an exceptionally high electronic conductivity of up to 245 S cm⁻¹ at room temperature. Making use of the high density of electrons in Ca₁₂Al₁₄O₃₂e₂ electride, the strong N-N bonds in N₂ was found to be broken when heating Ca₁₂Al₁₄O₃₂e₂ in a N₂ atmosphere. A reaction between silicate apatites and the titanium metal yielded another completely new electride material La₉.₀Sr₁.₀(SiO₄)₆O₂.₄e₀.₂ which was found to be a semiconductor. To fully understand the role of oxygen interstitials in silicate apatites, high-resolution transmission electron microscopy (HRTEM) was employed as the main technique in probing how the oxygen nonstoichiometry is accommodated at the atomic level. Atomic-resolution imaging of interstitial oxygen in La₉.₀Sr₁.₀(SiO₄)₆O₂.₅ proved to be a success in this thesis. Substitution of oxygen in 2a and interstitial sites with fluoride ions in La[subscript(8+y)]Sr[subscript(2- z)](SiO₄)₆O[subscript(2+(3y-2z)/2)] (0