Fomichov Y. Influence of the flexoeffect and surface screening on the functional properties of nanosized ferroics

The purpose of the dissertation is to determine the impact of the flexo effect and surface screening on polar properties, domain structure and phase diagrams of nanosized ferroics, in particular ferroelectric nanoparticles and thin films. The phase diagrams of thin films and nanoparticles of different sizes and shapes ferroelectrics were calculated numerically to succeed achieving of the goal of the work. The analytical expressions for the dependence of the temperature of the ferroelectric-paraelectric phase transition from the conditions of incomplete screening of polarization and sizes were derived and compared with similar dependences, calculated numerically and experimentally measured for nanosized ferroelectrics. The influence of flexo-electrochemical coupling and surface screening were studied on phase diagrams, the distribution of spontaneous polarization and the properties of the domain structure of nanoparticles and thin films of ferroelectrics. Object of study – ferroics, in particular ferroelectric nanoparticles and thin films. Subject of study – phase diagrams, distribution of spontaneous polarization and properties of the domain structure of nanoparticles and thin films of ferroelectrics. Research methods – the theory of phase transformations of Landau-Ginzburg-Devonshire, classical thermodynamics and electrostatics, theory of elasticity and semiconductor theory, variational calculations, analytic and numerical methods of solving boundary value problems for nonlinear differential equations in partial derivatives. The dissertation consists of introduction, three sections, conclusions and list of sources used. The introduction substantiates the relevance of the topic, general provisions, formulation of goals and objectives for achieving it, describes the used theoretical methods of research and approaches, there is a scientific novelty and practical significance of the results of the conducted researches. The scientific novelty of the obtained results is that for the first time we calculated numerically and analyzed phase diagrams and domain structure of ferroelectric nanoparticles SrBi2Ta2O9, СuInP2S6, Sn2P2S6 and thin films PbTiO3 for different temperatures, film thicknesses and sizes of nanoparticles under conditions of incomplete screening of spontaneous polarization. The emergence of a polydomain region in the tri-critical point of a phase diagram of ferroelectric nanoparticles and its extension with increasing of surface screening length for a particle radius that is more than critical. The developed analytical description of the morphology of the domain structure and phase diagrams of ferroelectric nanoparticles. The analytical expressions for the dependence of the temperature of the ferroelectric-paraelectric transition on surface screening length and the radius of the nanoparticle are derived. Established the influence of flexo-electrochemical coupling and surface screening on the temperature of ferroelectric phase transition, distribution of spontaneous polarization and properties of the domain structure of nanoparticles and thin films of ferroelectrics. It is shown that the internal electric field has a strong influence on the polar and elastic properties of deformed films due to the connection of the heterogeneous Vegard stresses and the flexoelectric effect. The obtained results point to the attractive possibilities of controlling the polar, dielectric and electromechanical properties of nanoparticles and thin films of ferroics due to the selection of incomplete screening conditions and the flexo-coupling coefficient that can be very promising for their applications in nanoelectronics, improving existing ones and creating new nanoscale memory elements based on ferroics for the latest nanotechnology.