On the Possibility of Describing Lattice Properties of Iridium in Terms of Pseudopotential Theory

Abstract

The possibility is discussed of describing lattice properties of iridium in terms of pseudopotential perturbation theory with allowance for second and third order. Values of pseudopotential parameters are proposed which provide a good description of the lattice properties of iridium under zero pressure, the contributions of the three-ion forces being small. The total energy, pressure, elastic moduli, Debye temperature, low-temperature Gruneisen parameter, pairwise potentials, and also effective solid sphere packing parameters are calculated. In addition, by invoking molecular dynamic methods, the vacancy formation and migration energies and the stacking fault energies are computed. The electron energy spectrum and group velocity calculations suggest that the electron density of states curve of Ir exhibits no van Hove singularities near the Fermi level, a fact which apparently accounts for the successful description of lattice properties in terms of the pseudopotential perturbation theory.

Peculiarities of phonon spectra and lattice heat capacity in Ir aned Rh

“…This situation would be absolutely impossible if in these metals the Fermi level were at the peak of the density of states E F , as, say in Ni or Pd, because this peak affects substantially the elastic moduli and phonon frequencies in the noticable part of the Brillouin zone . The band calculations show, however (Greenberg et al 1990) that not only peak of N (E) does not exist but also there are no relatively weak van Hove singularities near E F in Ir. Of course in itself it cannot be taken as a direct evidence of the applicability of the psudopotential model (in W, say, N (E) near E F is also a smooth function but a reasonable model pseudopotential cannot be constructed).…”

“…2 shows the maxima of χ(q) at the relevant values of wave vectors (in the case of the spherical Fermi surface the maxima should have manifested themselves only in dχ/dq). At the same time in the calculations of the phonon spectra in terms of the pseudopotential model in the second order of the perturbation theory (Greenberg et al 1990, Ivanov et al 1994 the character of the Kohn singularities corresponds to the assumption about the sphericity of the Fermi surface and, therefore, the phonon spectrum in the relevant region of the q space, naturally, proves to be too rigid. If we want to improve the description of phonon spectra in terms of the simple pseudopotential approach, the softening of the phonon frequencies due to the non-sphericity of the Fermi surface can be simulated and described by means of changing the form of the Fourier image of pseudopotential V ps (q) at 0.6(2π/a) < ∼ q < ∼ (2π/a) (a is the lattice constant), determined from the band calculations results.…”

“…The peculiarities of mechanical properties (at least those inherent to high pure single crystals) should be eventually determined by the specific features of interatomic interactions in metal. Greenberg et al 1990 andIvanov et al 1994 succeded in constructing of a pseudopotential model of pair forces for these metals wich describes surprisingly well (for transition metals) their elastic properties. In terms of this model the peculiarities of the Ir and Rh defect structure (Ivanov et al 1994, Gornostyrev et al 1994 could be analyzed, and the qualitative explanation of some of their peculiar mechanical properties, primarily, the possibility of brittle failure of single crystals after a long-term stage of plastic deformation could be given.…”

“…In the calculation of phonon spectra, elastic moduli, and the interatomic interaction potential in papers by Greenberg et al 1990, Ivanov et al 1994 of the common pseudopotential models (Heine and Weaire 1970;Browman and Kagan 1974;Vaks and Trefilov 1977) was used. The Fourier component of the local pseudopotential of the electron-ion interaction was taken in the form of the Animalu-Heine pseudopotential…”

Thermal expansion and the equation of state of Ir and Rh

“…However a slight change in the form of V ps (q) was made in I to describe better the phonon spectra. The screening function was taken in both cases in the Geldart and Taylor (1970) approximation (see, in more detail, Greenberg et al 1990). The temperature dependence of the volume per atom Ω(T ) ≡ Ω 0 + ∆Ω(T ) (Ω 0 = Ω(T = 0)), the coefficient of thermal expansion…”