An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic-or higher order-potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.Funding Agencies|Swedish Research Council (VR)|

We describe an efficient first-principles method that can be used to calculate mixing enthalpies of transition metal nitrides with B1 structure and substitutional disorder at the metal sublattice. The technique is based on the density functional theory. The independent sublattice model is suggested for the treatment of disorderinduced local lattice relaxation effects. It supplements the description of the substitutional disorder within the coherent potential approximation. We demonstrate the excellent accuracy of the method by comparison with calculations performed by means of the projector augumented wave method on supercells constructed as special quasirandom structures. At the same time, the efficiency of the technique allows for total energy calculations on a very fine mesh of concentrations which enables a reliable calculation of the second concentration derivative of the alloy total energy. This is a first step towards first-principles predictions of concentrations and temperature intervals where the alloy decomposition proceeds via the spinodal mechanism. We thus calculate electronic structure, lattice parameter, and mixing enthalpies of the quasibinary alloy c-Ti 1−x Al x N. The lattice parameter follows Vegard's law at low fractions of AlN but deviates increasingly with increasing Al content. We show that the asymmetry of the mixing enthalpy and its second concentration derivative is associated with substantial variations of the electronic structure with alloy composition. The phase diagram is constructed within the mean-field approximation.

We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.

We employ state-of-the-art ab initio simulations within the dynamical mean-field theory to study three likely phases of iron (hcp, fcc, and bcc) at the Earth's core conditions. We demonstrate that the correction to the electronic free energy due to correlations can be significant for the relative stability of the phases. The strongest effect is observed in bcc Fe, which shows a non-Fermi-liquid behavior, and where a Curie-Weiss behavior of the uniform susceptibility hints at a local magnetic moment still existing at 5800 K and 300 GPa. We predict that all three structures have sufficiently high magnetic susceptibility to stabilize the geodynamo.

The electronic state and transport properties of hot dense iron are of the utmost importance for the understanding of Earth's interior. Combining state-of-the-art density functional and dynamical mean field theories we study the impact of electron correlations on the electrical and thermal resistivity of hexagonal close-packed ò-Fe at Earth's core conditions and show that the electron-electron scattering in ò-Fe exhibit a nearly perfect Fermi-liquid (FL) behavior. Accordingly, the quadratic dependence of the scattering rate, typical of FLs, leads to a modification of the Wiedemann-Franz law and suppresses the thermal conductivity with respect to the electrical one. The consequence is a significant increase of the electron-electron thermal resistivity, which is found to be of comparable magnitude to the electron-phonon one.