Golovina I. Oxygen-octahedral ferroelectrics: structure and properties of defects, size effects, MW resonators

Dissertation deals with the investigation of the ferroelectrics of oxygen-octahedral type (КТаО3, КNbО3, КТа1-xNbxО3, КNb1-xFexО3, a-RbTiOPO4). There were studied the nature and structure of defects, as well as size effects caused by the decrease of crystal dimensions down to the nanoscale level. On the basis of the studied materials, new MW resonators were developed and testified. Monitoring of defects in КТаО3 was carried out by EPR method, X-ray diffraction and elemental analysis. Based on the monitoring results, a new technology for high-quality crystal growth was developed. For the a -RbTiOPO4 crystal, the structure and radiospectroscopic parameters of V4+ centers were determined, a model of the center was proposed. With crystal sizes decrease, the appearance of a new phase transition in KTaO3 and shift of the phase transition temperatures in KNbO3 and KTa1-xNbxO3 were found out. A new effect in the nanosize crystals of KTaO3 and KNbO3 - the appearance of surface cluster ferromagnetism - was found out and explained. A doping of KTaO3 nanocrystals with manganese leads to the increase of the symmetry of Mn2+ center comparing to the macrocrystals, and doping with iron results in the formation of Fe3+ centers only with axial and rhombic symmetries, while the centers with all symmetries, from cubic to rhombic ones, exist in the macrocrystals. We showed that a MW resonator and the devices based on it, made from the material, having high dielectric permittivity and low losses, allow to increase the sensitivity of magnetic resonance methods (EPR, NMR and MRI), to perform a speed analysis by EPR method, to create the EPR minispectrometer.