RESUMO
Density functional theory was used to study structural and dynamical changes related to the magnetostructural phase transition in MnAs. The soft mode inducing the transition from the high-symmetry hexagonal to the low-symmetry orthorhombic phase was revealed. A giant coupling between the soft mode and magnetic moments was found and its crucial role in the magnetostructural transition was established. The estimated phonon contribution to the total entropy change has the opposite sign to the magnetic entropy change.
RESUMO
The structure and stability properties of wadsleyite II as the new phase of Mg(2)SiO(4) has been studied at high pressure by the DFT method. The pressure range corresponds to the transition zone in the Earth. At zero pressure the calculated lattice parameters of the wadsleyite II structure are a=5.749 Å, b=28.791 Å and c=8.289 Å with the density ρ=3406 kg m(-3). The third order Birch-Murnaghan equation of state has been determined for the structure with isothermal bulk moduli K(T)=160.1 GPa and K(T)'=4.3 at a pressure range up to 50 GPa. The elasticity tensor coefficients C(ij)(P), as well as the compressional and shear wave velocities and their pressure derivatives, have been calculated using the deformation method at a range of pressures up to 25 GPa. The results agree with the experimental data and structure properties of the wadsleyite II model. The properties of the wadsleyite II phase are very close to the wadsleyite phase.
RESUMO
The phonon dispersion curves of the superconductor PuCoGa(5) were studied by inelastic x-ray scattering at room temperature. The experimental data agree well with ab initio lattice dynamics calculations. An accurate description of the phonon spectrum is obtained only when a local Coulomb repulsion U approximately equal 3 eV among 5f electrons is taken into account.