Kovalenko O. PECULIARITIES OF THE FORMATION OF ISO- AND ANISOTROPIC BARIUM TITANATE NANOSTRUCTURES

The thesis work is devoted to the synthesis of nano-sized particles based on BaTiO3 of different structure and morphology, as well as to the study of factors that affect the physical and chemical properties of the material and determine the relationship between the chemical composition, structure, morphology, and properties of this material. The relationship between the precipitation conditions of precursors in the Pechini method, the composition of the precipitated intermediate compound, the nature of thermal decomposition, and the physicochemical properties of the final BaTiO3 were established. The optimal conditions for the formation of oxalate complexes with a minimum number of hydroxo groups, the difference in the mechanism of their thermal decomposition, as well as their advantages from the point of view of the formation of a single-phase compound BaTiO3, minimization of the thermal decomposition temperature of the intermediate complex, as well as the particle sizes of the final BaTiO3 are shown. The dependences of the phase composition and aggregate sizes on the Ba/Ti ratio, are shown. The dependence of crystallite and particle sizes on precursor deposition conditions and Ba/Ti ratio is shown. The synergistic effect of pH and concentration of reagents on the formation of aggregates based on hexagonal two-dimensional nanosized polycrystals is shown, and the nature of the hexagonal shape of these aggregates is assumed. The influence of hydrothermal synthesis parameters (nature of the initial precursor and solvent, concentration of reagents, pH of the medium, temperature, and process duration) on the development of the iso- and anisotropic structure of the crystal based on BaTiO3, and its intermediate compounds, as well as their morphology, is shown. The influence of the nature, polarity, and concentration of surfactant molecules on the selectivity of inhibition of crystal faces, and the nature of its growth is shown. The supersaturation range for obtaining single crystalline BaTiO3 nanorods with c/a= 1.013 and particle aspect ratio of 6–9 was established. Possible reasons for the formation of a one-dimensional single crystalline BaTiO3 nanoparticle are assumed. A negative sloped linear dependence of the width and length of the particles on the degree of supersaturation was established in the range of SR = 19 – 39. A change in the dominant crystallization mechanism from in-situ topotactic transformation to dissolution-precipitation above 24 SR = 19 was revealed. It was found that the preservation of morphology occurs mainly through the in-situ mechanism and requires a lower rate of particle deposition and stresses in the crystal. The development of morphology from a lamellar intermediate precursor (SR = 6 – 9) to a rod-like product with an aspect ratio of 6–9 (SR = 19–29) and to nanoparticles with an undefined shape and a centrosymmetric structure (SR = 39) is shown. The possibility of obtaining mesocrystalline two-dimensional BaTiO3 nanoparticles with high orientation of crystallites (misorientation less than 2 degrees) with grain sizes of 22 nm is shown. This is done by combining hydrothermal conditions (200 °C, 10 bar) to form the plate-shaped templates based on metal glycolate and subsequent heat treatment at standard pressure and temperature of 730 °C until the intermediate compound is completely transformed into BaTiO3. The composition of the precursor and the features of its transformation into a BaTiO3 mesocrystal are shown. For the first time, the electrical properties of nanoscale plates based on BaTiO3 and its intermediate compounds were directly measured. For the first time, the resistive properties of plate-shaped samples of the precursor and BaTiO3 were revealed, which indicates the potential use of these samples as a resistive switching layer in the memristors. Keywords: nanostructures, BaTiO3, perovskite, single crystalline, polycrystal, crystallization mechanism, oxalate complex, deposition, hydrothermal synthesis, thermal decomposition, morphology, anisotropy, and resistive properties.