The well-known lanthanide cuprates exist in two principal forms, T and T´, which behave as p-type and n-type conductors, respectively. In order to understand the structural properties and crystal chemistry from the T to T´ phase, the Nd₁.₈₋ₓLaₓPr₀.₂CuO₄ (NLPCO) system was studied varying the La substitution ratio (0≤x≤1.8) and then characterised using high temperature X-ray powder diffraction. From analysis of the X-ray diffraction patterns obtained at room temperature, there are clearly five distinguishable regions for the NLPCO system. They are, (1) monophasic T´ solid–solution (2) two phase mixture T´ + T´´ (3) monophasic T´´solid–solution (4) two phase mixture T´´ + O and finally (5) monophasic O phase solid–solution. The T´´ form has previously been suggested as an ordered form of T´; however here we show via high temperature X-ray diffraction studies that it is a non-transformable metastable phase formed on quenching of the T phase via an orthorhombically distorted variant. Also neutron diffraction and selected area electron diffraction (SAED) studies confirmed that the T ´´phase is 4- fold Cu coordinated. The structural, magnetic and electrical properties of this NLPCO series have been investigated for the selected compositions using X-ray diffraction, magnetization measurements, thermal analysis and conductivity measurements. The aim of the second half of this work was to discover the basic high temperature electrical characteristics of Nd₂₋ₓPrₓCuO₄ and investigate how this matches with those required for components on the SOFC cathode side to identify which dopant level shows highest conductivity and whether it is stable at different temperatures. The idea was to make a new concept in SOFC cathodes and current collector development, using n-type conductors instead of p- type conductors and to try to produce a high conductivity material which is stable under the chemical and thermal stresses that exist while under load that can be used in cathode or current collector applications. The Nd₂₋ₓPrₓCuO₄ (NPCO) series has been studied over a range of dopant levels (x=0.15 - 0.25) and maximum conductivity of 86.7 Scm⁻¹ has been obtained for the composition where x = 0.25. Also NPCO shows n-type semiconductor behaviour which gives operational advantages when operating at mild oxygen deficiency. AC impedance studies have been carried out on symmetrical cells to investigate the performance of NPCO as a cathode material. These studies mainly focused on polarization resistance and the activation energies of the cells. Low Rp values and low activation energies are obtained for a composite cathode compared to pure cathode material. Two configurations of NPCO as cathode materials were tested, pre-fired and in-siu fired. Pre-fired NPCO exhibited better performance than in-situ fired NPCO. Both in-situ and pre-fired current collecting NPCO still showed lowest activation energies which suggest good catalytic activity. From all of these studies, it is evident that the praseodymium doped neodymium cuprate material shows considerable promise as a potential cathode material for solid oxide fuel cell applications.

The production of stable solid crystalline material is an important issue in the pharmaceutical industry and the challenge to control the desired active pharmaceutical ingredient (API) with the specific chemical and physical properties has led to more development in the drug industry. Increasing the solubility and the dissolution of the drug will increase its bioavailability; therefore the solubility can be improved with the change in the preparation method. The formation of co-crystals has emerged as a new alternate to the salts, hydrates and solvate methods since the molecules that cannot be formed by the usual methods might crystallise in the form of co-crystals. Co-crystals are multicomponent crystals which can be known as supramolecules and are constructed by the non covalent bonds between the desired former and co-former. Therefore the synthon approach was utilised to design co-crystals with the specific properties, this involves the understanding of the intermolecular interactions between these synthons. These interaction forces can be directed to control the crystal packing in the design of the new crystalline solid with the desired chemical and physical properties. The most familiar synthon was the amide group with its complementary carboxylic group, in this work isonicotinamide and benzoic acid were chosen to design co-crystal and much literature exist that introduce the determination of co-crystal growth from these two compounds. The growth of co-crystals was carried out in water, ethanol and ethanol / water mixed solvent (30 - 90 % ethanol) by utilising the Cryo-Compact circulator. Co-crystals (1:1) and (2:1) were grown in ethanol and water respectively and a mixture of both phases were grown in the mixed solvent. All the phases were examined by powder X-ray diffraction (PXRD), Raman, Infrared and 1H-NMR spectroscopy. The solubility of isonicotinamide, benzoic acid, co-crystals (1:1) and (2:1) in water, ethanol and ethanol/water mixed solvent (30 - 90 % ethanol) were determined at 25 °C, 35 °C and 40 °C by utilising the React-Array Microvate. It was important to understand some of the thermodynamic factors which control the formation of these polymorphs such as the change in the enthalpy and the change in the entropy. Also it was important to study the pH behaviour during dissolution of the former, co-former and co-crystals in water, ethanol and ethanol/water mixed solvent (30 - 90 % ethanol) in-order to examine the affect of the solvent composition on the solubility and to identify if some ions were formed during the dissociation and how this could affects the formation of co-crystals. A discussion has been introduced in this research of how similar solubility of the compounds maps the formation of the typical ternary phase diagram of the mixture of 1:1 while compounds with different solubility maps the formation of skewed phase diagram as shown in section 1.6.2.3. In this project an isotherm ternary phase diagram at 20 °C and 40 °C was constructed to map the behaviour of benzoic acid and isonicotinamide and to show all possible phases formed and the regions where all phases are represented in the ternary phase diagram were determined by the slurry method. The ternary phase diagram was used to design a drawn out and cooling crystallisation at 100 cm3 solution of 50 % ethanol / water mixed solvent and a study of the impact of seeds of co-crystals 1:1 on the cooling crystallisation method.