Single crystals of 8 and 12 mole % yttria stabilized zirconia (YSZ) have been studied to elucidate the relationship between the presence of point defects in the material and its electrical and mechanical properties, in particular its tendency to blacken and disintegrate on carrying an electrical current. The thesis introduces the material and its uses and describes how the presence of point defects is explained. X-ray and density measurements confirmed that most of the defects are oxygen vacancies but suggested that cation interstitials are also present. It was found that the crystals turned white due to cracking on heat treatment but did so more rapidly in on oxygen atmosphere than in, an argon atmosphere. Up to three types of electron spin resonance line were observed at 35 GHz depending on how the samples had been treated. One of these (for which g = 2.003 and g = 1,880) was attributed to an electron trapped in an oxygen vacancy which was associated with a yttrium ion. This complex (minus the electron) had been used to explain the temperature dependence of the conductivity of poly-crystalline YSZ, Measurements of the conductivity of single crystal 8 mole % YSZ gave an oxygen vacancy activation energy of 1,9 x 10(^-19)J and a complex association energy of 1.9 x 10(^-19)J which were similar to published values for polycryetalline material, but maintaining the sample as a single crystal was found to be difficult. Blackening by current passage was compared in the tivo compositions, but no difference was observed. Ultrasonic measurements confirmed that blackening was not associated with a phase change. Preliminary relative permittivity measurements gave a value of 32
