Electron-phonon coupling (EPC) in the three high-pressure phases of Ba is investigated using a pseudopotential plane-wave method based on density functional perturbation theory. The calculated values of superconducting critical temperature T c of Ba-I and Ba-II under pressure are consistent well with the trends observed experimentally. Moreover, Ba-V is found to be superconducting with a maximum T c exceeding 7.8 K at 45 GPa. With the increase of pressure, the values of T c increase in Ba I and Ba-II but the value of T c decreases in Ba-V. For Ba-I at pressures below 2 GPa, the increases of logarithmic average frequency ω log and electron-phonon coupling parameters λ both contribute to the enhancement of T c . For all the three phases at pressures above 2 GPa, T c is found to be primarily determined by λ . Further investigation reveals that for all the three phases, the change in λ with pressure can be explained mainly by change in the phonon frequency. Thus for Ba-II and Ba-V, although they exhibit completely different superconducting behaviors, their superconductivities have the same origin; the pressure dependence of T c is determined finally by the pressure dependence of phonon frequency.
First principles calculations are preformed to systematically investigate the electronic structures, elastic and thermodynamic properties of the monoclinic and orthorhombic phases of Si C2N4 under pressure. The calculated structural parameters and elastic moduli are in good agreement with the available theoretical values at zero pressure. The elastic constants of the two phases under pressure are calculated by stress–strain method. It is found that both phases satisfy the mechanical stability criteria within 60 GPa. With the increase of pressure, the degree of the anisotropy decreases rapidly in the monoclinic phase, whereas it remains almost constant in the orthorhombic phase. Furthermore, using the hybrid density-functional theory, the monoclinic and orthorhombic phases are found to be wide band-gap semiconductors with band gaps of about 2.85 e V and 3.21 e V, respectively. The elastic moduli, ductile or brittle behaviors, compressional and shear wave velocities as well as Debye temperatures as a function of pressure in both phases are also investigated in detail.
The geometries, stabilities, and electronic properties of FSin(n = 1 ~ 12) clusters are systematically investigated by using first-principles calculations based on the hybrid density-functional theory at the B3LYP/6-311G level. The geometries are found to undergo a structural change from two-dimensional to three-dimensional structure when the cluster size n equals 3. On the basis of the obtained lowest-energy geometries, the size dependencies of cluster properties, such as averaged binding energy, fragmentation energy, second-order energy difference, HOMO–LUMO(highest occupied molecular orbital–lowest unoccupied molecular orbital) gap and chemical hardness, are discussed. In addition, natural population analysis indicates that the F atom in the most stable FSincluster is recorded as being negative and the charges always transfer from Si atoms to the F atom in the FSinclusters.
By the particle-swarm optimization method,it is predicted that tetragonal P42mc,I41md,and orthorhombic Amm2phases of vanadium nitride(VN)are energetically more stable than NaCl-type structure at 0 K.The enthalpies of the predicted three new VN phases,along with WC,NaCl,AsNi,CsCl type structures,are calculated each as a function of pressure.It is found that VN exhibits the WC-to-CsCl type phase transition at 256 GPa.For the considered seven crystallographic VN phases,the structures,elastic constants,bulk moduli,shear moduli,and Debye temperatures are investigated.Our calculated equilibrium structural parameters are in very good agreement with the available experimental results and the previous theoretical results for the NaCl phase.The Debye temperatures of VN predicted three novel phases,which are all higher than those of the remaining structures.The elastic constants,thermodynamic properties,and elastic anisotropies of VN under pressure are obtained and the mechanical stabilities are analyzed in detail based on the mechanical stability criteria.Moreover,the effect of metallic bonding on the hardness of VN is also investigated,which shows that VNs in P42mc,I41md,and Amm2 phases are potential superhard phases.Further investigation on the experimental level is highly recommended to confirm our calculations presented in this paper.
