In the rigid band approximation the electronic structure of W-Re and Mo-Re alloys is calculated and their band contribution to the elastic moduli Cik. Critical Re concentrations are determined at which electronic topological transitions occur. An analysis of the computational results for C, is invoked to discuss variations in the degree of "covalency" and "metallicity" of the chemical bonds in rhenium-doped tungsten. Consideration is given to the role of the elastic anisotropy constant of W-Re and Mo-Re alloys and to its effect on the individual dislocation mobility and plastic properties of these alloys.In der Naherung starrer Bander wird die elektronische Struktur von W-Re-und Mo-Re-Legierungen und ihr Bandbeitrag zum Elastizitatsmodul Ci , berechnet. Kritische Re-Konzentrationen werden bestimmt, bei denen topologische elektronjsche Ubergange auftreten. Eine Analyse der Computerergebnisse fur Ca wird eingeschlossen, um Anderungen im Grad der ,,Kovalenz" und ,,Metallizitit" chemischer Bindungen in Rhenium-dotiertem Wolfram zu diskutieren. Berucksichtigt werden die Rolle der Konstante der elastischen Anisotropie von W-Re-und Mo-Re-Legierungen und ihr EinfluB auf die individuelle Versetzungsbeweglichkeit und plastischen Eigenschaften dieser Legierungen.

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.

Band-structure andshear-constant (C,,) calculations have been performed for the Li, Na and Ba BCC phase and the Cr, Ca and Sr BCC and FCC phases within a broad range of mmpressions(~fromOto40-55~.Todescribethe'baiidwntributionstoC,.,useismade of calculations based on the linear muffin-tin orbital/atomic sphere approximation (LMTO-ASA) method; the other contributions to C,. are described by using the semiempirical 'electrostatic' model. At U = 0 the calculated C,. values are usually close to the observed ones. We have revealed pronounced effects of softening for the shear constants, up to the loss of stability (C,, < 0). when the Fermi level approaches the maximum points of the density of states n(~), as well as under certain changes of shape of " ( P ) . The compression values, at which this softening takes place, are in good agreement with the position of structural phase transitions under pressure. observed in the metals considered. A number of anomalies in the C'(W and C, ( U) dependences are predicted. in particular a sharp drop of C'(LJ'J near the phase transition points U = U, in BCC Li and Ba and FCCCS. Ca and Sr. as well as a significant decrease of C d U ) with rising U in FCC Cs and BCC Sr at U a 0.5.