No abstract

Among a wide variety of point defects, crowdions can be distinguished by their high energy of formation and relatively low migration barriers, which makes them an important agent of mass transfer in lattices subjected to severe plastic deformation, irradiation, etc. It was previously shown that complexes and clusters of crowdions are even more mobile than single interstitials, which opened new mechanisms for the transfer of energy and mass in materials under intense external impacts. One of the most popular and convenient methods for analyzing crowdions is molecular dynamics, where the results can strongly depend on the interatomic potential used in the study. In this work, we compare the characteristics of a crowdion in an fcc lattice obtained using two different interatomic potentials — the pairwise Morse potential and the many-body potential for Al developed by the embedded atom method. It was found that the use of the many-body potential significantly affects the dynamics of crowdion propagation, including the features of atomic collisions, the evolution of energy localization and the propagation path.

Molecular dynamics study of discrete breathers in fcc Ni crystal is undertaken. The initial conditions for the excitation of breathers are constructed by imposing a spherically symmetric function, exponentially decreasing with the distance from the center of the sphere, on the delocalized vibrational normal mode, the frequency of which lies above the phonon spectrum of the crystal. This method allows to obtain a three-dimensional discrete breather of a new type, in the core of which the atoms oscillate like in the normal mode, and the amplitude of these oscillations decreases exponentially with distance from the center of the breather. A detailed discussion of the properties of the normal mode on which the breather is built is presented. It is shown that in the two extreme cases, constant volume and zero pressure, the mode shows hard type of non-linearity in a wide range of amplitudes, which ensures the hard type of the nonlinearity of the breather and its frequency lying above the phonon spectrum of the crystal. The spatial localization parameter of the breather is determined as the function of breather amplitude so that the breather life time is maximal. Found discrete breathers have relatively long lifetime of the order of 10 ps, which substantially exceeds the lifetime of thermal fluctuations.The proof of the existence of three-dimensional breathers in fcc Ni, presented in this paper, is an interesting and important result in the theory of discrete breathers.

The fullerene family, whose most popular members are the spherical C60 and C70 molecules, has recently added a new member, the cube-shaped carbon molecule C8 called a cubene. A molecular crystal based on fullerenes is called fullerite. In this work, based on relaxational molecular dynamics, two fullerites based on cubenes are described for the first time, one of which belongs to the cubic system, and the other to the triclinic system. Potential energy per atom, elastic constants, and mechanical stress components are calculated as functions of lattice strain. It has been established that the cubic cubene crystal is metastable, while the triclinic crystal is presumably the crystalline phase in the ground state (the potential energies per atom for these two structures are −0.0452 and −0.0480 eV, respectively).The cubic phase has a lower density than the monoclinic one (volumes per cubene are 101 and 97.7 Å3). The elastic constants for the monoclinic phase are approximately 4% higher than those for the cubic phase. The presented results are the first step in studying the physical and mechanical properties of C8 fullerite, which may have potential for hydrogen storage and other applications. In the future, the influence of temperature on the properties of cubenes will be analyzed.