Electronic and dynamical aspects of the stability of metallic structures

Abstract: The structural phase stability of several transition metalsystems have been studied, using first-principles electronicstructure calculations. Frozen phonon techniques have been usedto study the details of structural phase transformations e.g.the transition paths, while linear response calculations of theharmonic phonon frequencies have been performed in order tostudy the vibrational properties and dynamical stability ofsystems. Thermodynamical properties of liquid copper have beenstudied using molecular dynamics simulations withsemi-empirical interatomic potentials. Simulation techniqueshave further been used to study the structure and dynamics of amodel quasicrystal.The main results in the thesis are:    A variational higher order interpolation scheme forBrillouin zone integration of the band structure energy issuggested.    The constant pressure heat capacity, Cp,for liquid copper is essentially independent of thetemperature, while the constant volume heat capacityCv,decreases with temperature and extrapolates toapproximately 2kB/atom well above the meltingtemperature.    An approach to generate optimally soft and very accuratepseudopotentials for use with plane-wave basedelectronic-structure methods is developed.    The cubic-tetragonal transition in Nb3Sn is due to theinstability of the cubic structure with respect to sublatticedistortions, discovered by Shirane and Axe [Phys. Rev.B4,2957 (1971)]. Electronic excitations play an importantrole for stabilizing the cubic phase at highertemperatures.    The bcc W is dynamically very stable, but at higherpressures incipient instabilities occur at certain phononmodes. The fcc W is dynamically unstable under all shear atnormal pressure. The region of instability in the BZ, isquite extended. Increasing pressure stablizes the fcc W.    The bcc Fe undergoes a pressure-induced martenisticbcc-hcp transition, at the pressure 50 GPa, well above thecalculated transition pressure 10.3 GPa. We suggest thatmagnetism stabilizes the bcc structure.    The long-wavelength prepeak appearing in the structurefactor of a simple monatomic liquid with icosahedralshort-range order arises from the ordering of structuralvoids.    A quasicrystal cluster subjected to external stressresponds, due to phason dynamics, in a plastic manner withoutbreaking its long-range dodecagonal symmetry.Key words:Electronic structures, pseudopotentials,density functional theory, linear response, phonons, transitionmetal, quasicrystals, glasses, liquids, martensitictransformation.