Semkiv I. Structure doping modification of Ag8SnSe6 argyrodites for resistive memory elements

The thesis is devoted to the physical properties, physical and chemical technology of synthesis Ag8SnSe6 argyrodite polycrystals and thin films and creation of resistive switching cells based on them. The technology of crystal growth by direct melting of elemental Ag, Sn and Se was improved. It was developed the effective selenization method for synthesis Ag8SnSe6 argyrodite thin films According to X-ray analysis the polycrystals and thin film characterized by orthorhombic space group Pmn21 with similar lattice parameters. First time calculated energy-band structure of Ag8SnSe6 argyrodite shows the genesis of the conduction and valence bands. Energy band gap obtained by local density functional theory is 0.75 eV. By projector augmented wave method obtained energy band gap value is 0.66 eV. A group-theoretical classification of phonon modes in Ag8SnSe6 argyrodite carried out. Activity and frequency of modes in Raman scattering and IR spectra are presented. Raman scattering spectra shows one peak, which is a combination of peaks at 119,08, 168,43, 200,05, 216,73 cm-1, which correspond to high symmetric mode A1 and peaks at 150,73 and 212,75 см-1, which correspond to modes В1 and В2. In the infrared region of absorption spectra measured fundamental absorption edge of Ag8SnSe6 argyrodite locates at 1512 nm and corresponds to interband electronic transition with the energy band gap of 0.82 eV. First time investigated low temperature photoluminescence of Ag8SnSe6 argyrodite. The spectra are characterized by one asymmetric peak, which consists of two recombination peaks at 0.85 eV and 0.74 eV. Peak at 0.74 eV refer to the donor-acceptor recombination, while the peak at 0.85 eV is associated with radiative recombination between the charges carriers in conduction and valence bands. First time fabricated solid state electrochemical cell based on the Ag8SnSe6 argyrodite thin films with silver (active to Ag+ ions) and blocking (to Ag+ ions) graphite electrodes. In the cell Ag/Ag8SnSe6/C found resistive switching effect by the voltammetry measurement. Fabricated cell characterization by Impedance Spectroscopy is carried out. Electrical equivalent circuit of the cell process is identified.