Vol 61, No 1 (2019)

Half-metallic properties of SrYO₂ (Y = Sc, Ti, V, and Cr) full-Hensler compounds were studied using full-potential linearized augmented plane wave method based on density functional theory. The negative formation energies of SrYO₂ (Y = Sc, Ti, V, and Cr) alloys confirmed that they can be synthesized experimentally. Total energy calculations showed that AlCu₂Mn-type structure was the ground state structure in all compounds. In both structures, SrYO₂ (Y = Ti, V, and Cr) alloys were half-metallic ferrromagnets, while SrScO₂ was a non- magnetic metal. The origin of half-metallicity was verified for SrCrO₂. SrYO₂ (Y = Ti, V, and Cr) alloys in both structures were half-metals in a wide range of lattice constants indicating that they are quite robust against hydrostatic strains. The magnetization of SrYO₂ (Y = Ti, V, and Cr) alloys was mainly originated from the 3d electrons of Y (= Ti, V, and Cr) atoms and followed the Slater–Pauling rule: M_tot = Z_tot – 12. Generally, It is expected that SrYO₂ (Y = Ti, V, and Cr) alloys are promising and interesting candidates in the future spintronic field.

The MF₂ (M = Ca, Sr, Ba, Pb) crystals undergo a structural phase transition from the cubic to the orthorhombic phase in a range of 1–10 GPa. Although the cubic fluorites are well studied experimentally, there has been almost no information on their orthorhombic phase to date. Despite a few data on the Raman spectra available in the literature, there is still a lack of IR spectra of these structures. In this respect, this work aims to study the crystal structure and the phonon spectrum of MF₂ crystals (M = Ca, Sr, Ba, Pb) in both the cubic and orthorhombic phases, using the MO LKAO approach with hybrid DFT functionals that take the contribution of nonlocal exchange in the Hartree—Fock formalism into account. The frequencies and types of fundamental IR and Raman vibrations, as well as the frequencies of “silent” modes, are found. The calculations of elastic constants for the orthorombic phase have been performed at the first time. The program CRYSTAL14 intended for simulation of periodic structures within the MO LKAO approach was used for the calculations.

On the basis of ab initio calculations of the phonon frequencies of compressed rare-gas crystals in the model of deformable and polarizable atoms, dynamic instability of the fcc lattice of these crystals is studied. In addition to the earlier-considered three-body interaction, which is associated with the overlapping of the electron shells of atoms, the short-range potential includes three-body forces caused by the mutual deformation of the electron shells of neighboring atoms. It is shown that the allowance for the deformation of the dipole-type electron shells of atoms in the pair and three-body approximations leads to softening of the critical vibrations and absolute instability of the fcc lattice at pressures higher than the critical values, p > p_c. For light crystals of Ne and Ar under compressions of 0.76 (p_c = 422 GPa) and 0.71 (p_c = 405 GPa), respectively, the softening of the longitudinal mode is observed at the boundary of the Brillouin zone at the point L; for heavy crystals of Kr and Xe under compressions of 0.686 (p_c = 240 GPa) and 0.605 (p_c = 88 GPa), the transverse mode T₁ is softened in the direction Σ. The behavior of second-order Fuchs elastic moduli of compressed rare-gas crystals is discussed.

The estimations made within the fractal analysis have shown that 2D-nanofiller sheets in polymer medium form clusters (tactoids) that are fractal objects. This conclusion is confirmed by means of the irreversible aggregation model. The major factor determining the dimension of these clusters is the dimension of the random walk of initial nanoparticles. The structure of the 2D-nanofiller tactoids definitely determines the elastic modulus of the nanocomposite at the fixed nanofiller content.

Two approaches are proposed to the problem of the coupling of adsorbed particles with atoms of a zigzag edge of graphene formed on a metal substrate. The first approach is based on the Kalkstein and Soven scheme, which makes it possible to determine the electronic structure of a semi-infinite graphene sheet. The second approach is based on a cluster model of a zigzag edge. Analytical expressions are obtained for the local densities of the system’s states and the occupation numbers of a carbon adatom and an adparticle. The case of an isolated adparticle is considered in detail, and a method of taking into account the dipole–dipole interaction of particles aligned along the edge is proposed.

Crystalline samples of the (La_1 – yEu_y)_0.7Pb_0.3MnO₃ (y = 0, 0.2, 0.4, 0.5, 0.6, and 0.8) manganite solid solutions have been grown. Temperature dependences of the specific heat, thermal expansion, and high-intensity magnetocaloric effect have been investigated. A comparative analysis of the effect of isovalent cationic substitution on the thermal and caloric parameters of the ferromagnetic phase transition has been made. It has been shown that the growth of the Eu atom concentration leads to a decrease in the phase transition entropy and an increase in the baric coefficient dT/dp. Field and temperature dependences of the magnetocaloric effect have been established. It is demonstrated that, varying the ratio of cations, one can obtain the solid solutions with the maximum magnetocaloric effect value in fields of up to 6 kOe in a temperature range of 90–340 K. Owing to the similar reduced relative cooling powers, the investigated solid solutions can be used as reference solid-state cooling media in designing the cascade cooling systems.