A microscopic formalism based on computing many-particle densities is applied to the analysis of the diffusion-controlled kinetics of pattern formation in oppositely charged molecules on surfaces or adsorbed at interfaces with competing long-range Coulomb and short-range Lennard-Jones interactions. Particular attention is paid to the proper molecular treatment of energetic interactions driving pattern formation in inhomogeneous systems. The reverse Monte Carlo method is used to visualize the spatial molecular distribution based on the calculated radial distribution functions (joint correlation functions). We show the formation of charge domains for certain combinations of temperature and dynamical interaction parameters. The charge segregation evolves into quasicrystalline clusters of charges, due to the competing long- and short-range interactions. The clusters initially co-exist with a gas phase of charges that eventually add to the clusters, generating "fingers" or line of charges of the same sign, very different than the nanopatterns expected by molecular dynamics in systems with competing interactions in two dimensions, such as strain or dipolar versus van der Waals interactions.

The role of stochastization of magnetic field lines is analyzed in fast reconnection phenomena occurring in magnetized fusion plasma during various conditions in the ASDEX Upgrade tokamak. The mapping technique is applied to trace the field lines of toroidally confined plasma where perturbation parameters are expressed in terms of experimental perturbation amplitudes determined from the ASDEX Upgrade tokamak. It is found that fast reconnection observed during amplitude drops of the neoclassical tearing mode instability in the frequently interrupted regime can be related to stochastization. It is also shown that stochastization can explain the fast loss of confinement during the minor disruption. This demonstrates that stochastization can be regarded as a possible cause for different MHD events in ASDEX Upgrade.

The first-principles (ab initio) computations of the structural, electronic, and phonon properties have been performed for cubic and low-temperature tetragonal phases of BaTiO3 and SrTiO3 perovskite crystals, both stoichiometric and non-stoichiometric (with neutral oxygen vacancies). Calculations were performed with the CRYSTAL17 computer code within the linear combination of atomic orbitals approximation, using the B1WC advanced hybrid exchange-correlation functional of the density-functional-theory (DFT) and the periodic supercell approach. Various possible spin states of the defective systems were considered by means of unrestricted (open shell) DFT calculations. It was demonstrated that oxygen reduction leads to the appearance of new local vibrational modes associated with oxygen vacancies and new first-order peaks in the Raman spectra, which could be used for defect identification. The calculated Raman spectra for different vacancy positions and spins of the system, as well as other properties of defective crystals, are compared with the relevant experimental data.

An enhancement of the piezoelectric properties of lead-free materials, which allow conversion of mechanical energy into electricity, is a task of great importance and interest. Results of first-principles calculations of piezoelectric/electromechanical properties of the BaSrTiO (BSTO) ferroelectric solid solution with a perovskite structure are presented and discussed. Calculations are performed within the linear combination of atomic orbitals (LCAO) approximation and periodic-boundary conditions, using the advanced hybrid functionals of density functional theory (DFT). A supercell model allows the investigation of multiple chemical compositions x. In particular, three BSTO solid solutions with x = 0, 0.125, 0.25 are considered within the experimental stability domain of the ferroelectric tetragonal phase of the solid solution (x < 0.3). The configurational disorder with x = 0.25 composition is also investigated explicitly considering the seven possible atomic configurations corresponding to this composition. It is predicted that Sr-doping of BaTiO makes it mechanically harder and enhances its electromechanical/piezoelectric properties, which are important for relevant applications.

The results of experimental and theoretical ab initio study of structural and piezoelectric properties of (Ba,Sr)TiO3 perovskite solid solutions are discussed and compared. Experimentally, plate-like (Ba,Sr)TiO3 particles were synthesized by the topochemical conversion in the molten salt from Bi4Ti3O12 template plates. All dimensions (side length ≈ 1 µm, thickness ≈ 200-400 nm) were well above the critical size necessary for observation of piezo-and ferroelectricity. The first-principles computations of the structural and electromechanical properties of solid solutions were performed with CRYSTAL14 computer code within the linear combination of atomic orbitals (LCAO) approximation, using three advanced hybrid functionals of the densityfunctional-theory (DFT). Different chemical compositions are considered for the ferroelectric and paraelectric phases. Calculated structural properties of solid solutions in tetragonal and cubic phases are in a very good agreement with experimental data. Experimentally obtained and calculated band gaps are compared for cubic SrTiO3 and tetragonal BaTiO3. BaTiO3/SrTiO3 heterostructures were considered theoretically for different chemical compositions. The calculated piezoelectric properties of solid solutions and heterostructures in ferroelectric phase are compared. It is predicted that both solid solutions and heterostructures improve the piezoelectric properties of the bulk BaTiO3, but solid solutions are more preferable for equal Sr concentrations.