Structure and dynamics in liquid iron at high pressure and temperature. A first principles study

Producción CientíficaWe have studied the evolution of structural and dynamic properties of liquid Fe as a function of pressure for 11 thermodynamic states close to the melting line. The pressure range considered goes from ambient pressure to 323 GPa, and the study has been carried out by using the ab-initio molecular dynamics technique. The agreement between the calculated static structure and the available experimental data is very good, including details like an asymmetric second peak, which remains over most of the whole pressure range and suggests a significant local icosahedral short-range order in the liquid. The dynamical structure is studied through the characteristics of the propagating density fluctuations and the associated longitudinal and transverse particle currents. The transverse dispersion relations expose two branches of modes for all pressures, whose range of appearance is analyzed and put in connection with the double-peak structure of the Fourier spectra of velocity autocorrelation functions. We have also investigated the existence of fingerprints of transverse acoustic excitation modes in the dynamic structure factor for the high pressure states similar to those observed in the inelastic X-ray scattering intensity data of liquid Fe at ambient pressure. The calculated electronic density of states shows that with increasing pressure there is a widening of the conduction band along with a decreasing significance of spin polarization. Finally, we also report results for transport coefficients like self-diffusion, shear viscosity and adiabatic sound velocity, which are compared with the available experimental data.Ministerio de Economía y Competitividad junto con el Fondo Europeo de Desarrollo Regional (Project PGC2018-093745-B-I00

Depth-dependent dynamics of liquid metal surfaces with first principles simulations

Producción CientíficaLiquid metal surfaces have gained increased interest over the last decade due to new applications in synthesis of 2D materials, catalysis, or fusion reactors. Static properties such as the reflectivity and density profile have been determined, both experimentally and computationally, for numerous liquid metals and alloys. However, the characterization of the dynamic properties has remained a challenging task and only one experimental study by Reichert et al. has evaluated the depth-dependence of different dynamic properties in the liquid indium (l-In) surface. In this paper, we present an ab inito molecular dynamics study of the collective dynamic properties of this same system at different depths, obtaining very good agreement with the experimental data. In addition, we are able to compute the properties much closer to the surface than experimentally attainable, and have discovered that at these shallower depths, the properties drastically differ from those deeper in the slab. Therefore, this study sheds light into the behavior of dynamic properties at the atomic interface and highlights the ability of ab initio molecular dynamics to study such unknown dynamic behavior of liquid metals surfaces at depths not yet attainable experimentally but of crucial importance for liquid surface physics.Junta de Castilla y León (Ref. project VA124G18)Ministerio de Economía, Industria y Competitividad (Project PGC2018-093745-B-I00) and FEDE

First principles study of liquid uranium at temperatures up to 2050 K

Producción CientíficaUranium compounds are used as fissile materials in nuclear reactors. In present day reactors the most used nuclear fuel is uranium dioxide, but in generation-IV reactors other compounds are also being considered, such as uranium carbide and uranium mononitride. Upon possible accidents where the coolant would not circulate or be lost the core of the reactor would reach very high temperatures, and therefore it is essential to understand the behaviour of the nuclear fuel under such conditions for proper risk assessment. We consider here molten metallic uranium at several temperatures ranging from 1455 to 2050 K. Even though metallic uranium is not a candidate for nuclear fuel it could nevertheless be produced due to the thermochemical instability of uranium nitride at high temperatures. We use first principles techniques to analyse the behaviour of this system and obtain basic structural and dynamic properties, as well as some thermodynamic and transport properties, including atomic diffusion and viscosity.Junta de Castilla y León (Ref. project VA124G18)Ministerio de Economía, Industria y Competitividad (Project PGC2018-093745-B-I00) and FEDE

Structure and dynamics of the liquid 3d transition metals near melting. An ab initio study

Producción CientíficaThe static and dynamic properties of several bulk liquid 3d transition metals at thermodynamic conditions near their respective melting points have been evaluated by using ab initio molecular dynamics simulations. The calculated static structure factors show an asymmetric second peak followed by a more or less marked shoulder which points to a sizeable amount of icosahedral local order. Special attention is devoted to the analysis of the longitudinal and transverse current spectral functions and the corresponding dispersion of collective excitations. For some metals, we have found the existence of two branches of transverse collective excitations in the second pseudo-Brillouin zone. Finally, results are also reported for several transport coefficients.Junta de Castilla y León (Ref. project VA124G18)Ministerio de Economía, Industria y Competitividad (Project PGC2018-093745-B-I00) and FEDE

Structural, dynamic, and electronic properties of liquid tin: An ab initio molecular dynamics study

Producción CientíficaWe report on a study of several structural, dynamic, and electronic properties of liquid Sn at a thermodynamic state close to the triple point (573 K) and another one at a higher temperature (1273 K). This study has been performed by ab initio molecular dynamics simulations using 205 atoms and around 20 ps of simulation time. The calculated static structures show a good agreement with the available experimental data. The dynamic structure factors fairly agree with their experimental counterparts obtained by inelastic x-ray scattering experiments, which display inelastic side peaks. The calculated dispersion relations exhibit a positive dispersion, although not so marked as suggested by the experiment; moreover, its slope at the long-wavelength limit compares favorably with the experimental sound velocity. Electron densities near selected triplets of atoms are similar to those appearing in the solid phases, but these features have an extremely short lifetime, so they should not be considered as solid remnants in the melt

Microscopic dynamics in the liquid Li-Na alloy: An ab initio molecular dynamics study

Producción CientíficaWe present results for several structural and dynamical properties of the liquid Li1-xNax alloy. The study has been carried out by means of the orbital-free ab initio molecular dynamics method, combined with local ionic pseudopotentials constructed within the same framework. We obtain good agreement with the available experimental data, reproducing accurately, the strong homocoordinating tendencies exhibited by this alloy. The calculated partial dynamic structure factors exhibit clear side peaks whose frequencies, for q<~0.25Å-1, correspond to the hydrodynamic sound dispersion of the binary alloy, whereas for larger q values fast and slow sound modes are identified. The mass ratio in this system, mNa/mLi≈3, is the smallest one so far for which the fast mode is observed

Properties of bulk liquid Pd and Pt and their free liquid surface studied with first principles techniques

Producción CientíficaWe have performed first principles computer simulations in order to study the structural and dynamic properties of bulk liquid Pd and Pt near their melting points. We find good agreement with the available experimental static structure and transport properties, and furthermore we provide more detailed information that is not available from experiments. Additional simulations have also been undertaken so as to study the free liquid surface of both liquid metals. The calculated longitudinal ionic density profile exhibits an oscillatory behavior whose properties have been analyzed. For both metals, the associated intrinsic surface structure factor presents a marked maximum related to surface layeringJunta de Castilla y León (Ref. project VA124G18)Ministerio de Economía, Industria y Competitividad (Project PGC2018–093745-B-I00) and FEDE

First principles determination of some static and dynamic properties of the liquid 3d transition metals near melting

Producción CientíficaWe report an ab initio molecular dynamics simulation study of several static and dynamic properties of the liquid 3d transition metals. The calculated static structure factors show qualitative agreement with the available experimental data, and its second peak displays an asymmetric shape which suggests a signi1cant local icosahedral short-range order. The dynamical structure reveals propagating density 2uctuations whose dispersión relation has been evaluated; moreover, its long wavelength limit is compatible with their respective experimental sound velocity. Results are reported for the longitudinal and transverse current spectral functions as well as for the respective dispersion relations. We also analyze the possible appearance of transverse-like low-energy excitations in the calculated dynamic structure factors. Several transport coeWcients have been evaluated and compared with the available experimental data.Junta de Castilla y León (Ref. project VA124G18)Ministerio de Economía, Industria y Competitividad (Project PGC2018-093745-B-I00) and FEDE

Collective ionic dynamics in the liquid Na-Cs alloy: An ab initio molecular dynamics study

Producción CientíficaWe present results for several structural and dynamical properties of the liquid Na-Cs alloy. The study has been carried out by means of the orbital-free ab initio molecular dynamics method, combined with local ionic pseudopotentials constructed within the same framework. The results show good agreement with the available experimental data, reproducing the homocoordinating tendency exhibited by this alloy

Neutron Brillouin scattering and ab initio simulation study of the collective dynamics of liquid silver

Producción CientíficaWe present a thorough investigation of the collective dynamics of liquid Ag combining neutron Brillouin scattering and ab initio molecular dynamics (AIMD) determinations of the dynamic structure factor S(Q,ω). The main scope of this work is not only to provide experimental results for some important dynamical properties of this liquid metal in the wave-vector range 4<Q<16nm−1, but also to inquire about the scarce detectability of shear waves apparently characterizing two elements of group IB, differently from other metals. In fact, as in the case of Au, a transverse-like dynamics is not deducible from the experimental S(Q,ω) of Ag, despite the indisputable quality of the neutron data collected on the BRISP spectrometer at the Institut Laue Langevin in Grenoble. However, the significant agreement between experiment and AIMD calculations allowed for an in-depth study of the simulated S(Q,ω) in a Q range overlapping and extending the experimental one. A multimode analysis, already proven very successful in the description of various dynamical properties of fluid systems, is shown to be extremely effective also to analyze the intermediate scattering function predicted by AIMD at the various Q values, and eventually enables a reliable determination of both longitudinal and transverse branches in the dispersion curve of this liquid. Throughout the paper we highlight the importance of referring to theoretically well-founded models for S(Q,ω) and of imposing physical constraints in a fit-based analysis: These ensure that the used models obey fundamental properties of the dynamic structure factor.Ministero dell’Istruzione dell’Università e della Ricerca Italiano (grant PRIN2017-2017Z55KCW)Ministerio de Economía, Industria y Competitividad (project PGC2018-093745-B-I00)Junta de Castilla y León (project VA124G18