Understanding high pressure molecular hydrogen with a hierarchical machine-learned potential

The hydrogen phase diagram has several unusual features which are well reproduced by density functional calculations. Unfortunately, these calculations do not provide good physical insights into why those features occur. Here, we present a fast interatomic potential, which reproduces the molecular hydrogen phases: orientationally disordered Phase I; broken-symmetry Phase II and reentrant melt curve. The H2 vibrational frequency drops at high pressure because of increased coupling between neighbouring molecules, not bond weakening. Liquid H2 is denser than coexisting close-packed solid at high pressure because the favored molecular orientation switches from quadrupole-energy-minimizing to steric-repulsion-minimizing. The latter allows molecules to get closer together, without the atoms getting closer, but cannot be achieved within in a close-packed layer due to frustration. A similar effect causes negative thermal expansion. At high pressure, rotation is hindered in Phase I, such that it cannot be regarded as a molecular rotor phase.

Микроскопическое описание механизма перехода между политипами 2H и 4H карбида кремния

Abstract—The mechanism of displacement of one close-packed SiC layer from one minimum position to another on the example of SiC polytype transition 2 H → 4 H has been studied by ab initio methods. It has been shown that the intermediate state with monoclinic symmetry Cm greatly facilitates this displacement breaking it into two stages. Initially, the Si atom chiefly moves, only then—mainly the C atom. In this case, the Si–C bond is significantly tilted in comparison with the initial position, which allows the reducing of the compression of the SiC bonds in the ( $$11\bar {2}0$$ ) plane. Two transition states of this process, which also possess the Cm symmetry, have been computed. It has been found that the height of the activation barrier of the process of moving the close-packed layer of SiC from one position to another is equal to 1.8 eV. The energy profile of this movement has been calculated.

Equilibrium acoustic velocity in vapor-liquid mixture in layer of spherical particles

Theoretical and experimental study of acoustic velocity in vapor-liquid mixture that contained close-packed layer of spherical particles was performed with respect to nonstationary heat transfer between mixture and particles in compression half-wave. Theoretical model allows to explain the decay of equilibrium acoustic velocity compared to its adiabatic value in case of increasing of void fraction in the mixture. Calculated results are in agreement with experimental data, obtained in vertical channel with vapor-liquid filtration with close-packed layer of spherical particles of borosilicate glass, steel and lead.

SIMULATION STUDIES OF THE IMPURE ATOMS DIFFUSION ON Cu(001) SURFACE

Based on the vacancy-assisted diffusion mechanism, the impure atomic moving processes on the close-packed layer of a Cu (001) surface are studied by kinetic Monte Carlo simulations. Our simulation results show that the diffusivity of the Indium atom agrees quite well with the exact lattice solution. In addition, it is found that in the case of muti-impure atoms, there exist a critical point near the value of ∊=0, and the diffusivity of In atom depends on the number of impure atoms only when ∊ is positive.

Electron density of KNiF3: analysis of the atomic interactions

The topological analysis of the electron density in the perovskite KNiF3, potassium nickel trifluoride, based on the accurate X-ray diffraction data, has been performed. The topological picture of the atomic interactions differs from that resulting from the classic crystal chemistry consideration. The shapes of atoms in KNiF3 defined by zero-flux surfaces in the electron density are, in general, far from spherical. At the same time, their asphericity in the close-packed layer is very small. The topological coordination numbers of K and Ni are the same as the geometrical ones, whereas topological coordination for the F atom (6) differs from the geometrical value. The latter results from a specific shape of the Ni-atom basin preventing the bond-path formation between F atoms in the same atomic close-packed layer, in spite of the fact that the closest F—F distance is the same as K—F. Judging by the electron density value and curvature at the bond critical points, the K—F interaction in KNiF3 can be considered ionic, while the Ni—F bond belongs to the polar covalent type. No correlation of the topological ionic radii with crystal or ionic radii was found in KNiF3. Critical points in the electrostatic potential have also been studied.

Determination of Static Displacements of Atoms by Means of Large-Angle CBED

The extinction distance ξ g for electrons becomes large with the increase in an index of excited reflection g. The quasi-kinematical treatment would be applicable if the crystal thickness t is smaller than ξ g/π . When large-angle convergent electron beam is illuminated on a small area of crstal, many reflections may simultaneously satisfy the Bragg conditions and the thickness of the area is considered to be uniform. We can record the intensities of the many reflections on a negative film. If the crystal is tilted so that the high index reflections may fulfill the Bragg conditions, their intensities are sensitive to atom positions in the crystal. In the present paper we will show our recent study on the determination of the static displacement of Al atom in α Al2O3. Our interest is placed on the applicability and accuracy of the quqsi-kinematical treatment.In αAl2O3 (corundum structure) oxygen atoms ideally form a hexagonal close packed layer and aluminum atoms occupy 2/3 of centers of octahedra formed by oxygen atoms.

Recrystallisation of Cadmium Sulphide Powder Films with a CO2 Laser

Thin films (1–2μm) of cadmium sulphide, deposited by electrophoresis, consist of a close packed layer of randomly oriented cubic-phase microcrystalline particles with an average diameter of 30nm. A CW CO2 laser, operating at 10.6μm has been used to convert this into a polycrystalline structure with columnar crystals, of hexagonal phase, which extend through the thickness of the film and whose c-axis is perpendicular to the substrate. The recrystallised regions comprise aligned grains up to 300nm in diameter whose cathodoluminescence spectrum exhibits a narrow peak centred at 2.42eV with a half width identical to that for evaporated CdS (0.1eV).