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Information × Registration Number 0205U002007, 0102U000219 , R & D reports Title Investigation of the regular and disordered ferroactive materials within the reference system approach popup.stage_title Head Levyts'kyj Roman Romanovych, Registration Date 17-02-2005 Organization Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine popup.description2 The present report contains results of investigations of various problems of thermodynamics and dynamics of regular and disordered pseudospin systems with essential short-range and long-range interactions within the framework of the reference approach. We consider several aspects of development of the theory of such systems; the developed methods and approaches are being used to calculate physical characteristicis of ferroelectric compounds with order-disorder phase transitions, including as well those with competing interactions, hydrogen bonded ferroelectric systems, regular and disordered magnets. Special attention is paid to development of microscopic theories for concrete ferroelectric materials, taking into account the real structures of the crystals and the major types of interactions in them. It was fundamentally important to explore the effects associated with piezoelectric and striction phenomena. Within the Mitsui model, modified by taking into account piezoelectric coupling with the shearstrain e4, the thermodynamic, dynamic, piezoelectric, and elastic characteristics of Rochelle salt are calculated in the mean field approximation. A satisfactory quantitative description of the corresponding experimental data is obtained. It is shown that the modified model qualitatively correctly describes the behavior of relaxation times and dynamic dielectric permittivity of Rochelle salt in the vicinity of the phase transition points. A theory of thermodynamic, dynamic, piezoelectric, and elastic properties of partially deuterated crystals of Rochelle salt is proposed. In the framework of the modified Mitsui model, which in a linear approximation takes into account the interactions with diagonal components of the strain tensor, the related to these strains piezoelectric and elastic characteristics of Rochelle salt are calculated. A satisfactory description of the corresponding experimental data is obtained. Within the framework of the four-particle cluster approximation for the extended proton ordering model of the ferroelectric crystals of the KH2PO4 family with taking into account the short-range and long-range interactions, piezoelectric coupling with strains u4, u5 and u6 as well as tunneling of protons on hydrogen bonds, the thermodynamic, dielectric, piezoelectric, and elastic characteristics of these crystals. A good quantitative description of the corresponding experimental data for the KH2PO4 ferroelectrics and NH4H2PO4 antiferroelectrics is obtained. A thorough experimental investigation of low-frequency (0,1-104 Hz) dielectric properties of KH2PO4 and RbH2PO4 ferroelectrics is performed. It is shown that the obtained results can be properly explained within the framework of the elastic domain walls theory, taking into account the interactions with fluctuations of crystal defects. In the two-particle cluster approximation, we study the influence of longitudinal magnetic field on the physical characteristics of the spin-1 Ising magnet with bilinear and quadruple interactions and of the XXZ-model withthe easy axis anisotropy. Within the symmetric replica approach in the two-particle cluster approximation, the model of a spin glass with competing interactions is studied. It is shown that weak fluctuations of internal fields lead to essential smearing out of the region of the phase transition to the spin glass state. The thermodynamic properties of the spin-1/2 XY chains with the Hamiltonian parameters (intersite exchange interactions, fields on sites) that regularly change along the chain are explored. The obtained exact results for the thermodynamic functions of the studied models allows to analyze the influence of the changes in the microscopic parameters on the magnetization processes, quantum phase transitions, and to clarify the influence of the exchange interaction anisotropy on the spin-Peierls structural instability. Gas-liquid phase transition in a fluid, adsorbed in random porous medium, is explored. The present report is written in Ukrainian on 203 pages. It consists of 11 sections and two appendices. The report contains 86 figures, 7 tables, 72 references to publications of the obtained results. Key works: pseudospin model, short range correlations, long-range interactions, cluster approximation, KH2PO4, NH4H2PO4, Rochelle salt, piezoelectric effect, Mitsui model, thermodynamic, dielectric and piezoelectric characteristics, hydrogen bond, domains, relaxation time, Blume-Emery-Griffits model, XXZ-model, continued fraction method, quantum phase transitions, fluid, porous medium. Product Description popup.authors popup.nrat_date 2020-04-02 Close