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A novel powder preparation technique involving the use of a high temperature plasma spray was employed to produce homogeneous YBCO powder suitable for melt texturing. It was found that the well mixed, un-reacted starting powder would produce material with a fine distribution of secondary phase inclusions. Melt textured samples were produced from this powder using differing thermal cycles in an attempt to ascertain the optimum conditions for producing the desired microstructure. In an attempt to improve the flux pinning properties of single grain Y123 undoped and samples doped with lithium underwent irradiation with thermal neutrons. Magnetic measurements were then used to illustrate the enhancement due to this process. In order to improve the electrical properties of Sm123 it is desirable to be able to control the amount of Sm/Ba substitution. Already grown samples of Sm123 were processed in a reduced oxygen atmosphere in a manner that was shown to control the degree of Sm/Ba substitution. For many potential applications large, shaped samples of YBCO are required. Due to the properties of the material this is very difficult. Using a liquid phase infiltration method single grain samples of YBCO have been re-joined without the introduction of a grain boundary. R(T) and I-V dependencies close to Tc and in zero magnetic field were obtained from single grain samples of YBCO in the nV region. For T < Tc, the lower parts of the resulting curves were interpreted in the frame of the J-M model of current induced unbinding of thermally generated vortex/anti-vortex pairs, allowing us to obtain the temperature dependence of the characteristic current, Ic₁(T), close to Tc. In addition, the current value for a given temperature above which the J-M model becomes inadequate has been measured and interpreted.

PrBa2Cu3O6+x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa2Cu3O6+x:it is the only compound which has the same structure as YBa2Cu3O6+x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa2Cu3O6+x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O2 planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa2Cu3O6+x, I have used instead the compounds PrO2 and Y0.3Pr0.7Ba2Cu4O8, which contain praseodymium in a chemically similar environment. Y0.3Pr0.7Ba2Cu4O8 shows the same broadening effects as PrBa2Cu3O6+x; PrO2 shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.