Structure and dynamics of complex metallic alloys studied by numerical simulation
Final Report Abstract
The intent of the project was to study structure and dynamics of complex metallic alloys by numerical simulation. Three systems were considered: Atomistic structure of ε-Al-Mn-Pd compounds: Starting from an experimentally determined average structure of Boudard et al. a detailed numerical structure renement was performed rst for the ξ- and ε6-phases. Criterion was the deviation of the enthalpy of formation from the convex hull of stable phases in the Al-Mn-Pd composition diagram, which was calculated by density-functional methods and molecular dynamics. The phases are built from columns of icosahedral Pseudo-Mackay clusters (PMI) whose projections along their axes form parallel or staggered hexagons. We identied the inner shell of the PMI as tricapped trigonal prism (TTP) of nine Al-atoms. Breaking the symmetry of the environment it possesses a threefold symmetry axis, which can dynamically reorient along the vefold axes of the icosahedral environment. Each PMI hexagon embraces two Large Bicapped Pentagonal Prisms (LBPP), where some occupations had to be modied. The structure renement was extended to the higher - phases, to decagonal Al-Mn-Pd and nally to the cores of metadislocations. This was done by cluster decoration of the hexagon-star-boat tiling by the PMI in their original form, by interpenetrating PMI and by LBPP with variable number of atoms. Dynamics of Mg-Zn compounds: Here the inuence of complexity on the vibrational properties was studied, both by inelastic neutron and x-ray scattering and by numerical simulations. First, the dynamical structure factor and the generalized vibrational density of states for the simple MgZn2 phase with 12 atoms in the hexagonal unit cell were determined. The data were compared with those for the mini-Bergman phase Mg2 Zn11. It contains 44 atoms Pauling triacontahedra forming the inner shell of the 104 atoms Bergman clusters and thus is a step towards the highly complex Bergman phase Mg42 (Al,Zn)49 with 162 atoms in the unit cell. But it also contains Tsai-type clusters, where a symmetry breaking polyhedron is surrounded by shells of icosahedral symmetry. Complexity showed up in many localized excitations. The work was then extended to Tsai-compounds cubic ScZn6 , i-ZnMgSc and i-ZnAgSc, where the rotational dynamics of inner-shell tetrahedra was illuminated. Clathrates: For these cage compounds the vibrational properties were calculated mainly by molecular dynamics with empirical potentials. It was found that the guest atoms in the cages cause rattling modes, which become evident in low lying optic bands. These hybridize with the heat carrying acoustic phonons, transforming them from propagating to localized excitations. In this way the heat conductivity κ of lled clathrates is reduced and the gure of merit for thermopower is increased, which was proved by explicit calculations of κ. All the goals of the project were achieved. The project turned out to be central both for the Paketantrag and for the European Network C-MAC, as evident from publications.
Publications
- Anomalous vibrational dynamics in the Mg2 Zn11 phase. Phys. Rev. B 83, 144202 (2011)
H. Euchner, M. Mihalković, F. Gähler, M. R. Johnson, H. Schober, S. Rols, E. Suard, A. Bosak, S. Ohhashi, A.-P. Tsai, S. Lidin, C. P. Gomez, J. Custers, S. Paschen, and M. de Boissieu
- Lattice Dynamics of Complex Metallic Alloys. Doctoral Thesis, University of Stuttgart, 11 October 2011
Holger Euchner
- Low-temperature structure of Al-Pd-Mn optimized by ab initio methods. Phys. Rev. B 84, 184203 (2011)
B. Frigan, A. Santana, M. Engel, D. Schopf, H.-R. Trebin, and M. Mihalković
- Embedded atom method potentials for Al-Pd-Mn phases. Phys. Rev. B 85, 054201 (2012)
D. Schopf, P. Brommer, B. Frigan, and H.-R. Trebin
(See online at https://doi.org/10.1103/PhysRevB.85.054201) - Ordering and dynamics of the central tetrahedron in the 1/1 Zn6 Sc periodic approximant to quasicrystal. Journal of Physics: Condensed Matter 24, 415403 (2012)
H. Euchner, T. Yamada, H. Schober, S. Rols, M. Mihalković, R. Tamura, T. Ishimasa, and M. de Boissieu
- Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba8 Ge40+x Ni6−x. Phys. Rev. B 86, 224303 (2012)
H. Euchner, S. Pailhès, L. T. K. Nguyen, W. Assmus, F. Ritter, A. Haghighirad, Y. Grin, S. Paschen, and M. de Boissieu
(See online at https://doi.org/10.1103/PhysRevB.86.224303) - Effective Potentials for Numerical Investigations of Complex Intermetallic Phases. Doctoral Thesis, University of Stuttgart, 27 March 2013
Daniel Schopf
- Effective potentials for simulations of the thermal conductivity of type-i semiconductor clathrate systems. Phys. Rev. B89, 214306 (2014)
D. Schopf, H. Euchner, and H.-R. Trebin
(See online at https://doi.org/10.1103/PhysRevB.89.214306)
